Source Generators, part 3: Another Refactor

- Update to new gaemstone.ECS' new handling of
  generic context with World<T>, Entity<T>, etc
- Completely rewrite the source generator
- Remove AutoAdd from [Singleton]
  [Add<Flecs.Core.Disabled>] can be used instead
- Replace some term attributes with generics
  For example, "[Has] T foo" is now "Has<T> foo"
- Move some Flecs types into Flecs.Core
- Remove IL generator related code
- Remove [Game] attribute and entity
- Turn Canvas and GameWindow into singletons
- Probably some stuff I forgot

src/Immersion/Immersion.csproj

@@ -9,7 +9,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <EmbeddedResource Include="Resources/*" />
+    <EmbeddedResource Include="Resources/**" />
   </ItemGroup>
 
   <ItemGroup>

src/Immersion/ManagedComponentTest.cs

@@ -1,5 +1,6 @@
 using System;
 using gaemstone.ECS;
+using gaemstone.ECS.Utility;
 
 namespace Immersion;
 
@@ -13,9 +14,9 @@ public partial class ManagedComponentTest
 	}
 
 	[System]
-	public static void CreateLotsOfGarbageData(World world)
+	public static void CreateLotsOfGarbageData<T>(World<T> world)
 	{
-		var game = world.LookupByPathOrThrow("/Game");
+		var game = world.LookupPathOrThrow("/Game");
 		game.Remove<BigManagedData>();
 		game.Set(new BigManagedData());
 		// This is to make sure the number of objects kept alive stays stable.

src/Immersion/ObserverTest.cs

@@ -1,7 +1,7 @@
 using System;
 using gaemstone.ECS;
-using gaemstone.Flecs;
 using static gaemstone.Bloxel.Components.CoreComponents;
+using static gaemstone.Flecs.Core;
 
 namespace Immersion;
 
@@ -10,7 +10,7 @@ namespace Immersion;
 [DependsOn<gaemstone.Client.Components.RenderingComponents>]
 public partial class ObserverTest
 {
-	[Observer<ObserverEvent.OnSet>]
+	[Observer<OnSet>]
 	[Expression("[in] Chunk, [none] (MeshHandle, *)")]
 	public static void DoObserver(in Chunk chunk)
 		=> Console.WriteLine($"Chunk at {chunk.Position} now has a Mesh!");

src/Immersion/Program.cs

@@ -5,7 +5,6 @@ using System.Numerics;
 using System.Threading;
 using gaemstone;
 using gaemstone.Bloxel;
-using gaemstone.ECS;
 using gaemstone.Utility;
 using Silk.NET.Windowing;
 using static gaemstone.Bloxel.Components.CoreComponents;
@@ -20,15 +19,16 @@ var culture = CultureInfo.InvariantCulture;
 Thread.CurrentThread.CurrentCulture     = culture;
 CultureInfo.DefaultThreadCurrentCulture = culture;
 
-var universe = new Universe();
-var game = universe.LookupByTypeOrThrow<Game>();
+var universe = new Universe<Program>();
+var world    = universe.World;
 
+// TODO: Figure out a nice way to get rid of "compile errors" here.
 universe.Modules.Register<gaemstone.Flecs.Systems.Rest>();
 universe.Modules.Register<gaemstone.Flecs.Systems.Monitor>();
 
 var window = Window.Create(WindowOptions.Default with {
 	Title = "gæmstone",
-	Size = new(1280, 720),
+	Size  = new(1280, 720),
 	PreferredDepthBufferBits = 24,
 });
 window.Initialize();
@@ -57,19 +57,24 @@ universe.Modules.Register<gaemstone.Client.Systems.EntityInspector>();
 universe.Modules.Register<gaemstone.Client.Components.CameraComponents>();
 universe.Modules.Register<gaemstone.Client.Systems.FreeCameraController>();
 
-game.Set(new Canvas(Silk.NET.OpenGL.ContextSourceExtensions.CreateOpenGL(window)));
-game.Set(new GameWindow(window));
+foreach (var module in universe.Modules)
+	if (!module.IsInitialized) throw new InvalidOperationException(
+		$"Module '{module.Entity.Path}' is not initialized");
 
-universe.New("MainCamera")
+// Initialize Canvas and GameWindow singletons with actual values.
+world.Entity<Canvas>().Set(new Canvas(Silk.NET.OpenGL.ContextSourceExtensions.CreateOpenGL(window)));
+world.Entity<GameWindow>().Set(new GameWindow(window));
+
+world.New("MainCamera")
 	.Set(Camera.Default3D)
 	.Set((GlobalTransform)Matrix4x4.CreateTranslation(0.0F, 2.0F, 0.0F))
 	.Set(new CameraController { MouseSensitivity = 12.0F })
 	.Build();
 
-var heartMesh = universe.New("/Immersion/Resources/heart.glb").Add<Mesh>().Build();
-var swordMesh = universe.New("/Immersion/Resources/sword.glb").Add<Mesh>().Build();
+var heartMesh = world.New("/Immersion/Resources/heart.glb").Add<Mesh>().Build();
+var swordMesh = world.New("/Immersion/Resources/sword.glb").Add<Mesh>().Build();
 
-var entities = universe.New("Entities").Build();
+var entities = world.New("Entities").Build();
 var rnd = new Random();
 for (var x = -12; x <= 12; x++)
 for (var z = -12; z <= 12; z++) {
@@ -86,13 +91,13 @@ universe.Modules.Register<gaemstone.Bloxel.Components.CoreComponents>();
 universe.Modules.Register<gaemstone.Bloxel.Systems.BasicWorldGenerator>();
 universe.Modules.Register<gaemstone.Bloxel.Client.Systems.ChunkMeshGenerator>();
 
-var texture = universe.New("/Immersion/Resources/terrain.png").Add<Texture>().Build();
+var texture = world.New("/Immersion/Resources/terrain.png").Add<Texture>().Build();
 
-var stone = universe.New("Stone").Set(TextureCoords4.FromGrid(4, 4, 1, 0)).Build();
-var dirt  = universe.New("Dirt" ).Set(TextureCoords4.FromGrid(4, 4, 2, 0)).Build();
-var grass = universe.New("Grass").Set(TextureCoords4.FromGrid(4, 4, 3, 0)).Build();
+var stone = world.New("Stone").Set(TextureCoords4.FromGrid(4, 4, 1, 0)).Build();
+var dirt  = world.New("Dirt" ).Set(TextureCoords4.FromGrid(4, 4, 2, 0)).Build();
+var grass = world.New("Grass").Set(TextureCoords4.FromGrid(4, 4, 3, 0)).Build();
 
-var chunks = universe.New("Chunks").Build();
+var chunks = world.New("Chunks").Build();
 var sizeH = 4; var sizeY = 2;
 for (var cx = -sizeH; cx < sizeH; cx++)
 for (var cy = -sizeY; cy < sizeY; cy++)
@@ -110,12 +115,12 @@ for (var cz = -sizeH; cz < sizeH; cz++) {
 // universe.Modules.Register<ObserverTest>();
 
 var stopwatch    = Stopwatch.StartNew();
-var minFrameTime = TimeSpan.FromSeconds(1) / 30;
+var minFrameTime = TimeSpan.FromSeconds(1) / 60;
 window.Run(() => {
 	var delta = stopwatch.Elapsed;
 	stopwatch.Restart();
 
-	if (!universe.Progress(delta))
+	if (!world.Progress(delta))
 		window.Close();
 
 	var requiredTime = stopwatch.Elapsed;

src/gaemstone.Bloxel/ChunkPaletteStorage.cs

@@ -168,9 +168,9 @@ public class ChunkPaletteStorage<T>
 	}
 
 	private int GetIndex(int x, int y, int z)
-		=> (x |
-		    y <<  Constants.ChunkBitShift |
-		    z << (Constants.ChunkBitShift * 2)) * _indicesLength;
+		=> (x | y <<  Constants.ChunkBitShift
+		      | z << (Constants.ChunkBitShift * 2))
+			* _indicesLength;
 
 
 	private struct PaletteEntry

src/gaemstone.Bloxel/Client/Systems/ChunkMeshGenerator.cs

@@ -2,7 +2,9 @@ using System;
 using System.Numerics;
 using gaemstone.Client.Systems;
 using gaemstone.ECS;
+using Silk.NET.OpenGL;
 using static gaemstone.Bloxel.Components.CoreComponents;
+using static gaemstone.Bloxel.Systems.BasicWorldGenerator;
 using static gaemstone.Client.Components.RenderingComponents;
 using static gaemstone.Client.Components.ResourceComponents;
 using static gaemstone.Client.Systems.Windowing;
@@ -10,6 +12,11 @@ using static gaemstone.Client.Systems.Windowing;
 namespace gaemstone.Bloxel.Client.Systems;
 
 [Module]
+[DependsOn<gaemstone.Bloxel.Components.CoreComponents>]
+[DependsOn<gaemstone.Bloxel.Systems.BasicWorldGenerator>]
+[DependsOn<gaemstone.Client.Components.RenderingComponents>]
+[DependsOn<gaemstone.Client.Components.ResourceComponents>]
+[DependsOn<gaemstone.Client.Systems.Windowing>]
 public partial class ChunkMeshGenerator
 {
 	private const int StartingCapacity = 1024;
@@ -20,29 +27,32 @@ public partial class ChunkMeshGenerator
 		new Vector3[]{ new(1,1,0), new(0,1,0), new(0,1,1), new(1,1,1) }, // Up    (+Y)
 		new Vector3[]{ new(1,0,1), new(0,0,1), new(0,0,0), new(1,0,0) }, // Down  (-Y)
 		new Vector3[]{ new(0,1,1), new(0,0,1), new(1,0,1), new(1,1,1) }, // South (+Z)
-		new Vector3[]{ new(1,1,0), new(1,0,0), new(0,0,0), new(0,1,0) }  // North (-Z)
+		new Vector3[]{ new(1,1,0), new(1,0,0), new(0,0,0), new(0,1,0) }, // North (-Z)
 	};
 
 	private static readonly int[] TriangleIndices
 		= { 0, 1, 3,  1, 2, 3 };
 
-
-	private ushort[]  _indices  = new ushort[StartingCapacity];
-	private Vector3[] _vertices = new Vector3[StartingCapacity];
-	private Vector3[] _normals  = new Vector3[StartingCapacity];
-	private Vector2[] _uvs      = new Vector2[StartingCapacity];
+	// TODO: Turn these into a component on the module.
+	private static ushort [] _indices  = new ushort [StartingCapacity];
+	private static Vector3[] _vertices = new Vector3[StartingCapacity];
+	private static Vector3[] _normals  = new Vector3[StartingCapacity];
+	private static Vector2[] _uvs      = new Vector2[StartingCapacity];
 
 	[System]
-	[Expression("[in] Chunk, ChunkStoreBlocks, HasBasicWorldGeneration, !(Mesh, *)")]
-	public void GenerateChunkMeshes(Universe universe, EntityRef entity,
-	                                in Chunk chunk, ChunkStoreBlocks blocks)
+	public static void GenerateChunkMeshes<T>(
+		World<T> world, Canvas canvas,
+		Entity<T> entity, in Chunk chunk, ChunkStoreBlocks blocks,
+		Has<BasicWorldGenerationDone> _1, Not<Mesh, Flecs.Core.Wildcard> _2)
 	{
-		if (Generate(universe, chunk.Position, blocks) is MeshHandle handle)
-			entity.Add<Mesh>(entity.NewChild("Mesh").Set(handle).Build());
-		else entity.Delete();
+		var result = Generate(world, canvas.GL, chunk.Position, blocks);
+		if (result is MeshHandle handle) {
+			var mesh = entity.NewChild("Mesh").Set(handle).Build();
+			entity.Add<Mesh>(mesh);
+		} else entity.Delete();
 	}
 
-	public MeshHandle? Generate(Universe universe,
+	public static MeshHandle? Generate<T>(World<T> world, GL GL,
 		ChunkPos chunkPos, ChunkStoreBlocks centerBlocks)
 	{
 		// TODO: We'll need a way to get neighbors again.
@@ -59,47 +69,45 @@ public partial class ChunkMeshGenerator
 		var indexCount  = 0;
 		var vertexCount = 0;
 		for (var x = 0; x < 16; x++)
-		for (var y = 0; y < 16; y++)
-		for (var z = 0; z < 16; z++) {
-			var block = universe.LookupAlive(centerBlocks[x, y, z]);
-			if (block == null) continue;
-
-			var blockVertex = new Vector3(x, y, z);
-			var textureCell = block.GetOrThrow<TextureCoords4>();
-
-			foreach (var facing in BlockFacings.All) {
-				if (!IsNeighborEmpty(storages, x, y, z, facing)) continue;
-
-				if (_indices.Length <= indexCount + 6)
-					Array.Resize(ref _indices, _indices.Length << 1);
-				if (_vertices.Length <= vertexCount + 4) {
-					Array.Resize(ref _vertices, _vertices.Length << 1);
-					Array.Resize(ref _normals , _vertices.Length << 1);
-					Array.Resize(ref _uvs     , _vertices.Length << 1);
-				}
-
-				for (var i = 0; i < TriangleIndices.Length; i++)
-					_indices[indexCount++] = (ushort)(vertexCount + TriangleIndices[i]);
-
-				var normal = facing.ToVector3();
-				for (var i = 0; i < 4; i++) {
-					var offset = OffsetPerFacing[(int)facing][i];
-					_vertices[vertexCount] = blockVertex + offset;
-					_normals[vertexCount]  = normal;
-					_uvs[vertexCount]      = i switch {
-						0 => textureCell.TopLeft,
-						1 => textureCell.BottomLeft,
-						2 => textureCell.BottomRight,
-						3 => textureCell.TopRight,
-						_ => throw new InvalidOperationException()
-					};
-					vertexCount++;
+			for (var y = 0; y < 16; y++)
+				for (var z = 0; z < 16; z++) {
+					var maybeBlock = world.LookupAliveOrNull(centerBlocks[x, y, z]);
+					if (maybeBlock is not Entity<T> block) continue;
+
+					var blockVertex = new Vector3(x, y, z);
+					var textureCell = block.GetOrThrow<TextureCoords4>();
+
+					foreach (var facing in BlockFacings.All) {
+						if (!IsNeighborEmpty(storages, x, y, z, facing)) continue;
+
+						if (_indices.Length <= indexCount + 6)
+							Array.Resize(ref _indices, _indices.Length << 1);
+						if (_vertices.Length <= vertexCount + 4) {
+							Array.Resize(ref _vertices, _vertices.Length << 1);
+							Array.Resize(ref _normals , _vertices.Length << 1);
+							Array.Resize(ref _uvs     , _vertices.Length << 1);
+						}
+
+						for (var i = 0; i < TriangleIndices.Length; i++)
+							_indices[indexCount++] = (ushort)(vertexCount + TriangleIndices[i]);
+
+						var normal = facing.ToVector3();
+						for (var i = 0; i < 4; i++) {
+							var offset = OffsetPerFacing[(int)facing][i];
+							_vertices[vertexCount] = blockVertex + offset;
+							_normals[vertexCount]  = normal;
+							_uvs[vertexCount]      = i switch {
+								0 => textureCell.TopLeft,
+								1 => textureCell.BottomLeft,
+								2 => textureCell.BottomRight,
+								3 => textureCell.TopRight,
+								_ => throw new InvalidOperationException()
+							};
+							vertexCount++;
+						}
+					}
 				}
-			}
-		}
 
-		// TODO: Should dynamically generating meshes require getting GL this way?
-		var GL = universe.LookupByTypeOrThrow<Game>().GetOrThrow<Canvas>().GL;
 		return (indexCount > 0)
 			? MeshManager.Create(GL,
 				_indices.AsSpan(0, indexCount), _vertices.AsSpan(0, vertexCount),

src/gaemstone.Bloxel/Systems/BasicWorldGenerator.cs

@@ -6,14 +6,15 @@ using static gaemstone.Bloxel.Constants;
 
 namespace gaemstone.Bloxel.Systems;
 
-[Public, Module]
+[Module]
 [DependsOn<gaemstone.Bloxel.Components.CoreComponents>]
 public partial class BasicWorldGenerator
 {
-	private readonly FastNoiseLite _noise;
-	private readonly Random _rnd = new();
+	// TODO: Turn these into a component on the module.
+	private static readonly FastNoiseLite _noise;
+	private static readonly Random _rnd = new();
 
-	public BasicWorldGenerator()
+	static BasicWorldGenerator()
 	{
 		_noise = new(new Random().Next());
 		_noise.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
@@ -23,16 +24,17 @@ public partial class BasicWorldGenerator
 	}
 
 	[Tag]
-	public struct HasBasicWorldGeneration {  }
+	public struct BasicWorldGenerationDone {  }
 
 	[System]
-	public void Populate(World world, EntityRef entity,
+	public static void Populate<T>(
+		World<T> world, Entity<T> entity,
 		in Chunk chunk, ChunkStoreBlocks blocks,
-		[Not] HasBasicWorldGeneration _)
+		Not<BasicWorldGenerationDone> _)
 	{
-		var stone = world.LookupByPathOrThrow("Stone");
-		var dirt  = world.LookupByPathOrThrow("Dirt");
-		var grass = world.LookupByPathOrThrow("Grass");
+		var stone = world.LookupPathOrThrow("Stone");
+		var dirt  = world.LookupPathOrThrow("Dirt");
+		var grass = world.LookupPathOrThrow("Grass");
 		for (var localX = 0; localX < ChunkLength; localX++)
 			for (var localY = 0; localY < ChunkLength; localY++)
 				for (var localZ = 0; localZ < ChunkLength; localZ++) {
@@ -43,6 +45,6 @@ public partial class BasicWorldGenerator
 					if (_noise.GetNoise(globalX, globalY, globalZ) > bias)
 						blocks[localX, localY, localZ] = _rnd.Pick(stone, dirt, grass);
 				}
-		entity.Add<HasBasicWorldGeneration>();
+		entity.Add<BasicWorldGenerationDone>();
 	}
 }

src/gaemstone.Bloxel/gaemstone.Bloxel.csproj

@@ -5,6 +5,7 @@
     <TargetFramework>net7.0</TargetFramework>
     <ImplicitUsings>disable</ImplicitUsings>
     <Nullable>enable</Nullable>
+    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles> <!-- -->
   </PropertyGroup>
 
   <ItemGroup>

src/gaemstone.Client/Systems/EntityInspector.cs

@@ -7,7 +7,6 @@ using gaemstone.ECS;
 using gaemstone.Flecs;
 using ImGuiNET;
 using static gaemstone.Client.Systems.ImGuiManager;
-using static gaemstone.Flecs.Core;
 using Icon = gaemstone.Client.Utility.ForkAwesome;
 using ImGuiInternal = ImGuiNET.Internal.ImGui;
 
@@ -22,7 +21,7 @@ public partial class EntityInspector
 	public struct InspectorWindow {  }
 
 	[Relation, Exclusive]
-	[Add<DeletionEvent.OnDeleteTarget, DeletionBehavior.Delete>]
+	[Add<Core.OnDeleteTarget, Core.Delete>]
 	public struct Selected {  }
 
 	[Tag]
@@ -44,17 +43,16 @@ public partial class EntityInspector
 			public Entry? Prev { get; set; }
 			public Entry? Next { get; set; }
 
-			public Entry(EntityRef entity, Entry? prev, Entry? next)
+			public Entry(Entity entity, EntityPath path, Entry? prev, Entry? next)
 			{
 				Entity = entity;
-				Path   = entity.GetFullPath();
+				Path   = path;
 				if ((Prev = prev) != null) Prev.Next = this;
 				if ((Next = next) != null) Next.Prev = this;
 			}
 		}
 	}
 
-
 	[Component]
 	public struct DocPriority { public float Value; }
 
@@ -64,10 +62,10 @@ public partial class EntityInspector
 
 	private const string DefaultWindowTitle = "Inspector Gadget";
 
-	public void Initialize(EntityRef module)
+	public static void Initialize<T>(Entity<T> module)
 	{
 		void SetDocInfo(string path, float priority, string icon, float r, float g, float b)
-			=> module.World.LookupByPathOrThrow(path)
+			=> module.World.LookupPathOrThrow(path)
 				.Add<Doc.DisplayType>()
 				.Set(new DocPriority { Value = priority })
 				.Set(new DocIcon { Value = icon[0] })
@@ -78,17 +76,18 @@ public partial class EntityInspector
 		SetDocInfo("/flecs/core/Observer"    , 2 , Icon.Eye          , 1.0f, 0.8f, 0.8f);
 		SetDocInfo("/gaemstone/Doc/Relation" , 3 , Icon.ShareAlt     , 0.7f, 1.0f, 0.8f);
 		SetDocInfo("/flecs/core/Component"   , 4 , Icon.PencilSquare , 0.6f, 0.6f, 1.0f);
+		// TODO: Handle tags like Flecs does.
 		SetDocInfo("/flecs/core/Tag"         , 5 , Icon.Tag          , 0.7f, 0.8f, 1.0f);
 		SetDocInfo("/flecs/core/Prefab"      , 6 , Icon.Cube         , 0.9f, 0.8f, 1.0f);
 	}
 
 
 	[System]
-	public void ShowUIButton(World world, ImGuiData _)
+	public static void ShowUIButton<T>(World<T> world, ImGuiData _)
 	{
 		var hasAnyInspector = false;
-		var inspectorWindow = world.LookupByTypeOrThrow<InspectorWindow>();
-		foreach (var entity in Iterator.FromTerm(world, new(inspectorWindow)))
+		var inspectorWindow = world.Entity<InspectorWindow>();
+		foreach (var entity in Iterator<T>.FromTerm(world, new(inspectorWindow)))
 			{ hasAnyInspector = true; break; }
 
 		if (ImGuiUtility.UIButton(0, Icon.Search, DefaultWindowTitle, hasAnyInspector))
@@ -96,7 +95,7 @@ public partial class EntityInspector
 	}
 
 	[System]
-	public void ShowInspectorWindow(EntityRef window, InspectorWindow _, History? history)
+	public static void ShowInspectorWindow<T>(Entity<T> window, InspectorWindow _, History? history)
 	{
 		var isOpen = true;
 		var fontSize = ImGui.GetFontSize();
@@ -105,7 +104,7 @@ public partial class EntityInspector
 		ImGui.SetNextWindowSize(new(fontSize * 40, fontSize * 25), ImGuiCond.Appearing);
 		ImGui.PushFont(ImGui.GetIO().Fonts.Fonts[1]);
 		var title = window.GetDocName() ?? DefaultWindowTitle;
-		if (ImGui.Begin($"{Icon.Search} {title}###{window.Id}",
+		if (ImGui.Begin($"{Icon.Search} {title}###{window.NumericId}",
 		                ref isOpen, ImGuiWindowFlags.NoScrollbar)) {
 			ImGui.PushFont(ImGui.GetIO().Fonts.Fonts[0]);
 
@@ -121,7 +120,7 @@ public partial class EntityInspector
 			ExplorerView(window, history, selected);
 			ImGui.EndChild();
 
-			void Tab(string name, Action<EntityRef, History?, EntityRef?> contentMethod)
+			void Tab(string name, Action<Entity<T>, History?, Entity<T>?> contentMethod)
 			{
 				if (!ImGui.BeginTabItem(name)) return;
 				ImGui.BeginChild($"{name}Tab", new(-float.Epsilon, -float.Epsilon));
@@ -150,16 +149,14 @@ public partial class EntityInspector
 		if (!isOpen) window.Delete();
 	}
 
-	[Observer<ObserverEvent.OnRemove>]
-	public void ClearStorageOnRemove(EntityRef _1, InspectorWindow _2)
+	[Observer<Core.OnRemove>]
+	public static void ClearStorageOnRemove<T>(Entity<T> _1, InspectorWindow _2)
 	{
 		// TODO: Clear out settings store for the window.
 	}
 
-	private void ActionBarAndPath(EntityRef window, History? history, EntityRef? selected)
+	private static void ActionBarAndPath<T>(Entity<T> window, History? history, Entity<T>? selected)
 	{
-		var world = window.World;
-
 		static bool IconButtonWithToolTip(string icon, string tooltip, bool enabled = true) {
 			if (!enabled) ImGui.BeginDisabled();
 			var clicked = ImGui.Button(icon);
@@ -176,9 +173,9 @@ public partial class EntityInspector
 		ImGui.TableSetupColumn("Entity", ImGuiTableColumnFlags.WidthFixed);
 
 		ImGui.TableNextColumn();
-		var hasExpanded = window.Has<Expanded, Wildcard>();
+		var hasExpanded = window.Has<Expanded, Core.Wildcard>();
 		if (IconButtonWithToolTip(Icon.Outdent, "Collapse all items in the Explorer View", hasExpanded))
-			window.Remove<Expanded, Wildcard>();
+			window.Remove<Expanded, Core.Wildcard>();
 
 		if (history != null) {
 			var hasPrev = ((selected != null) ? history.Current?.Prev : history.Current) != null;
@@ -201,9 +198,7 @@ public partial class EntityInspector
 
 		ImGui.TableNextColumn();
 		if (IconButtonWithToolTip(Icon.PlusCircle, "Create a new child entity", (selected != null)))
-			// FIXME: Replace this once Flecs has been fixed.
-			SetSelected(window, history, world.New().Build().ChildOf(selected));
-			// SelectAndScrollTo(windowEntity, windowData, selected!.NewChild().Build(), selected);
+			SetSelected(window, history, selected?.NewChild().Build());
 
 		ImGui.SameLine();
 		if (IconButtonWithToolTip(Icon.Pencil, "Rename the current entity", false && (selected != null)))
@@ -214,26 +209,26 @@ public partial class EntityInspector
 		var icon    = !isDisabled ? Icon.BellSlash : Icon.Bell;
 		var tooltip = $"{(!isDisabled ? "Disable" : "Enable")} the current entity";
 		if (IconButtonWithToolTip(icon, tooltip, (selected != null)))
-			{ if (isDisabled) selected!.Enable(); else selected!.Disable(); }
+			{ if (isDisabled) selected?.Enable(); else selected?.Disable(); }
 
 		ImGui.SameLine();
 		if (IconButtonWithToolTip(Icon.Trash, "Delete the current entity", (selected != null))) {
 			// TODO: Delete history for deleted entity?
-			SetSelected(window, history, selected!.Parent);
-			selected.Delete(); // TODO: Confirmation dialog?
+			SetSelected(window, history, selected?.Parent);
+			selected?.Delete(); // TODO: Confirmation dialog?
 		}
 
 		ImGui.EndTable();
 		ImGui.PopStyleVar();
 	}
 
-	private void PathInput(EntityRef window, History? history, EntityRef? selected, float availableWidth)
+	private static void PathInput<T>(Entity<T> window, History? history, Entity<T>? selected, float availableWidth)
 	{
 		var style = ImGui.GetStyle();
 		ImGui.AlignTextToFramePadding();
 		ImGui.PushStyleVar(ImGuiStyleVar.ItemSpacing, new Vector2(1, style.ItemSpacing.Y));
 
-		var path = selected?.GetFullPath() ?? null;
+		var path = selected?.Path ?? null;
 
 		if (path != null) {
 			var visiblePath = path.GetParts().ToList();
@@ -261,8 +256,10 @@ public partial class EntityInspector
 					ImGui.EndDisabled();
 					if (ImGui.IsItemHovered(ImGuiHoveredFlags.AllowWhenDisabled))
 						ImGui.SetTooltip(path[1..(numHiddenItems+2)].ToString());
-				} else if (ImGui.Button(visiblePath[i]))
-					SetSelected(window, history, window.World.LookupByPath(path[..(actualIndex+1)]));
+				} else if (ImGui.Button(visiblePath[i])) {
+					var toSelect = window.World.LookupPathOrNull(path[..(actualIndex + 1)]);
+					SetSelected(window, history, toSelect);
+				}
 				ImGui.SameLine();
 			}
 		}
@@ -275,12 +272,27 @@ public partial class EntityInspector
 		ImGui.PopStyleVar();
 	}
 
-	private class ExplorerEntry
-		: IComparable<ExplorerEntry>
+	private interface IExplorerEntry
+		: IComparable<IExplorerEntry>
 	{
-		private readonly EntityInspector _module;
+		public Entity Entity { get; }
+		public int NumChildren { get; }
+		public bool HasChildren { get; }
+		public bool IsExpanded { get; }
+		public bool IsDisabled { get; }
+
+		public string? Name { get; }
+		public string? DocName { get; }
 
-		public EntityRef Entity { get; }
+		public float Priority { get; }
+		public Entity? Type { get; }
+	}
+
+	private class ExplorerEntry<T>
+		: IExplorerEntry
+	{
+		public Entity<T> Entity { get; }
+		Entity IExplorerEntry.Entity => Entity;
 		public int NumChildren { get; }
 		public bool HasChildren => (NumChildren > 0);
 		public bool IsExpanded { get; }
@@ -292,65 +304,67 @@ public partial class EntityInspector
 		public string? DocName => (_docNameCached++ > 0) ? _docName : _docName = Entity.GetDocName();
 
 		private float? _priority;
-		private EntityRef? _type;
-		public float Priority  => EnsureTypeCached()._priority!.Value;
-		public EntityRef? Type => EnsureTypeCached()._type;
+		private Entity? _type;
+		public float Priority => EnsureTypeCached()._priority!.Value;
+		public Entity? Type   => EnsureTypeCached()._type;
 
-		public ExplorerEntry(EntityInspector module, EntityRef entity,
-		                     int numChildren, bool isExpanded, bool isDisabled)
+		public ExplorerEntry(Entity<T> entity,
+			int numChildren, bool isExpanded, bool isDisabled)
 		{
-			_module     = module;
 			Entity      = entity;
 			NumChildren = numChildren;
 			IsExpanded  = (isExpanded && HasChildren);
 			IsDisabled  = isDisabled;
 		}
 
-		private ExplorerEntry EnsureTypeCached()
+		private ExplorerEntry<T> EnsureTypeCached()
 		{
 			if (_priority != null) return this;
-			(_type, _priority) = _module.FindDisplayType(Entity);
+			(_type, _priority) = FindDisplayType(Entity);
 			return this;
 		}
 
-		public int CompareTo(ExplorerEntry? other)
+		public int CompareTo(IExplorerEntry? other)
 		{
 			if (other == null) return -1;
+
 			return Compare(Priority, other.Priority)
 				?? Compare(Name, other.Name)
 				?? Compare(DocName, other.DocName)
-				?? Compare(Entity.Id, other.Entity.Id)
+				?? Compare(Entity.NumericId, other.Entity.NumericId)
 				?? 0;
-			static int? Compare<T>(T x, T y) {
+
+			static int? Compare<TCompare>(TCompare x, TCompare y)
+			{
 				if (x is null) { if (y is null) return null; else return 1; }
 				else if (y is null) return -1;
-				var result = Comparer<T>.Default.Compare(x, y);
+				var result = Comparer<TCompare>.Default.Compare(x, y);
 				return (result != 0) ? result : null;
 			}
 		}
 	}
 
-	private void ExplorerView(EntityRef window, History? history, EntityRef? selected)
+	private static void ExplorerView<T>(Entity<T> window, History? history, Entity<T>? selected)
 	{
 		// For some reason, the analyzer thinks world can be
 		// nullable, so let's be explicit about the type here.
-		World world = window.World;
+		var world = window.World;
 
-		var Wildcard = world.LookupByTypeOrThrow<Wildcard>().Entity;
-		var Any      = world.LookupByTypeOrThrow<Any>().Entity;
-		var This     = world.LookupByTypeOrThrow<This>().Entity;
-		var Var      = world.LookupByTypeOrThrow<Variable>().Entity;
+		var Wildcard = world.Entity<Core.Wildcard>().Value;
+		var Any      = world.Entity<Core.Any>().Value;
+		var This     = world.Entity<Core.This>().Value;
+		var Variable = world.Entity<Core.Variable>().Value;
 		bool IsSpecialEntity(Entity entity)
 			=> (entity == Wildcard) || (entity == Any)
-			|| (entity == This) || (entity == Var);
-
-		var expId = world.LookupByTypeOrThrow<Expanded>().Id;
-		List<ExplorerEntry> GetEntries(Entity? parent) {
-			var result = new List<ExplorerEntry>();
-			using var rule = new Rule(world, new(
-				  $"(ChildOf, {parent?.Id ?? 0})"  // Must be child of parent, or root entity.
-				+ $",?{expId}({window.Id}, $This)" // Whether entity is expanded in explorer view.
-				+ $",?Disabled"                    // Don't filter out disabled entities.
+			|| (entity == This)     || (entity == Variable);
+
+		var expId = world.Entity<Expanded>().NumericId;
+		List<IExplorerEntry> GetEntries(Entity? parent) {
+			var result = new List<IExplorerEntry>();
+			using var rule = new Rule<T>(world, new(
+				  $"(ChildOf, {parent?.NumericId ?? 0})"  // Must be child of parent, or root entity.
+				+ $",?{expId}({window.NumericId}, $This)" // Whether entity is expanded in explorer view.
+				+ $",?Disabled"                           // Don't filter out disabled entities.
 			));
 			foreach (var iter in rule.Iter()) {
 				var isExpanded = iter.FieldIsSet(2);
@@ -358,8 +372,8 @@ public partial class EntityInspector
 				for (var i = 0; i < iter.Count; i++) {
 					var entity = iter.Entity(i);
 					var count  = IsSpecialEntity(entity) ? 0
-					           : IdRef.Pair<ChildOf>(entity).Count;
-					result.Add(new(this, entity, count, isExpanded, isDisabled));
+						: world.Pair<Core.ChildOf>(entity).Count;
+					result.Add(new ExplorerEntry<T>(entity, count, isExpanded, isDisabled));
 				}
 			}
 			return result;
@@ -373,8 +387,8 @@ public partial class EntityInspector
 
 		float GetIndent(int depth) => depth * (nodeHeight + spacingX);
 
-		bool RenderNode(ExplorerEntry entry, int? depth) {
-			var entity = entry.Entity;
+		bool RenderNode(IExplorerEntry entry, int? depth) {
+			var entity = Entity<T>.GetOrThrow(world, entry.Entity);
 			var startY = ImGui.GetCursorPosY();
 			var isVisible    = ImGui.IsRectVisible(new(nodeHeight, nodeHeight));
 			var isSelected   = (entity == selected);
@@ -384,7 +398,7 @@ public partial class EntityInspector
 				// Button for expanding the child entries.
 				if (entry.HasChildren && !entry.IsExpanded) {
 					ImGui.SetCursorPosX(GetIndent(depth!.Value));
-					if (ImGui.Button($"##Expand{entity.Id}", new(nodeHeight)))
+					if (ImGui.Button($"##Expand{entity.NumericId}", new(nodeHeight)))
 						window.Add<Expanded>(entity);
 				}
 
@@ -427,7 +441,7 @@ public partial class EntityInspector
 
 			var fullHeight = ImGui.GetCursorPosY() - startY - spacingY;
 			ImGui.SetCursorPos(new(GetIndent(depth!.Value), startY));
-			if (ImGui.Button($"##Expand{entity.Id}", new(nodeHeight, fullHeight)))
+			if (ImGui.Button($"##Expand{entity.NumericId}", new(nodeHeight, fullHeight)))
 				window.Remove<Expanded>(entity);
 
 			return true;
@@ -439,33 +453,33 @@ public partial class EntityInspector
 			RenderNode(child, 0);
 	}
 
-	private void ComponentsTab(EntityRef window, History? history, EntityRef? selected)
+	private static void ComponentsTab<T>(Entity<T> window, History? history, Entity<T>? sel)
 	{
-		if (selected == null) return;
-		var ChildOf = window.World.LookupByTypeOrThrow<ChildOf>();
+		if (sel is not Entity<T> selected) return;
+		var ChildOf = window.World.Entity<Core.ChildOf>();
 		foreach (var id in selected.Type) {
 			// Hide ChildOf relations, as they are visible in the explorer.
-			if (id.IsPair && (id.Id.RelationUnsafe == ChildOf)) continue;
+			if (id.IsPair && (id.Value.RelationUnsafe == ChildOf)) continue;
 			RenderIdentifier(window, history, id);
 		}
 	}
 
-	private void ReferencesTab(EntityRef window, History? history, EntityRef? selected)
+	private static void ReferencesTab<T>(Entity<T> window, History? history, Entity<T>? sel)
 	{
-		if (selected == null) return;
+		if (sel is not Entity<T> selected) return;
 		var world    = window.World;
-		var ChildOf  = world.LookupByTypeOrThrow<ChildOf>();
-		var Wildcard = world.LookupByTypeOrThrow<Wildcard>();
+		var ChildOf  = world.Entity<Core.ChildOf>();
+		var Wildcard = world.Entity<Core.Wildcard>();
 
-		if (ImGui.CollapsingHeader($"As {Icon.Tag} Entity", ImGuiTreeNodeFlags.DefaultOpen))
-			foreach (var iter in Iterator.FromTerm(world, new(selected)))
+		if (ImGui.CollapsingHeader($"As {Icon.Tag} Component", ImGuiTreeNodeFlags.DefaultOpen))
+			foreach (var iter in Iterator<T>.FromTerm(world, new(selected)))
 				for (var i = 0; i < iter.Count; i++)
 					RenderEntity(window, history, iter.Entity(i));
 
 		if (ImGui.CollapsingHeader($"As {Icon.ShareAlt} Relation", ImGuiTreeNodeFlags.DefaultOpen))
-			foreach (var iter in Iterator.FromTerm(world, new(selected, Wildcard))) {
+			foreach (var iter in Iterator<T>.FromTerm(world, new(selected, Wildcard))) {
 				var id = iter.FieldId(1);
-				if (id.AsPair() is not (EntityRef relation, EntityRef target)) throw new InvalidOperationException();
+				if (id.AsPair() is not (Entity<T> relation, Entity<T> target)) throw new InvalidOperationException();
 				if (relation == ChildOf) continue; // Hide ChildOf relations.
 
 				for (var i = 0; i < iter.Count; i++) {
@@ -476,9 +490,9 @@ public partial class EntityInspector
 			}
 
 		if (ImGui.CollapsingHeader($"As {Icon.Bullseye} Target", ImGuiTreeNodeFlags.DefaultOpen))
-			foreach (var iter in Iterator.FromTerm(world, new(Wildcard, selected))) {
+			foreach (var iter in Iterator<T>.FromTerm(world, new(Wildcard, selected))) {
 				var id = iter.FieldId(1);
-				if (id.AsPair() is not (EntityRef relation, EntityRef target)) throw new InvalidOperationException();
+				if (id.AsPair() is not (Entity<T> relation, Entity<T> target)) throw new InvalidOperationException();
 				if (relation == ChildOf) continue; // Hide ChildOf relations.
 
 				for (var i = 0; i < iter.Count; i++) {
@@ -489,7 +503,7 @@ public partial class EntityInspector
 			}
 	}
 
-	private void DocumentationTab(EntityRef _1, History? _2, EntityRef? selected)
+	private static void DocumentationTab<T>(Entity<T> _1, History? _2, Entity<T>? selected)
 	{
 		var hasSelected = (selected != null);
 
@@ -515,7 +529,7 @@ public partial class EntityInspector
 		if (!hasSelected) ImGui.BeginDisabled();
 		var name = selected?.GetDocName(false) ?? "";
 		if (ImGui.InputText("##Name", ref name, 256))
-			selected!.SetDocName((name.Length > 0) ? name : null);
+			selected?.SetDocName((name.Length > 0) ? name : null);
 		if (!hasSelected) ImGui.EndDisabled();
 
 		Column($"{Icon.Comment} Description",
@@ -523,7 +537,7 @@ public partial class EntityInspector
 		if (!hasSelected) ImGui.BeginDisabled();
 		var brief = selected?.GetDocBrief() ?? "";
 		if (ImGui.InputText("##Brief", ref brief, 256))
-			selected!.SetDocBrief((brief.Length > 0) ? brief : null);
+			selected?.SetDocBrief((brief.Length > 0) ? brief : null);
 		if (!hasSelected) ImGui.EndDisabled();
 
 		Column($"{Icon.FileText} Documentation", """
@@ -539,7 +553,7 @@ public partial class EntityInspector
 		// TODO: Needs wordwrap.
 		if (ImGui.InputTextMultiline("##Detail", ref detail, 2048,
 		                             new(-float.Epsilon, Math.Max(minHeight, availHeight))))
-			selected!.SetDocDetail((detail.Length > 0) ? detail : null);
+			selected?.SetDocDetail((detail.Length > 0) ? detail : null);
 		if (!hasSelected) ImGui.EndDisabled();
 
 		Column($"{Icon.Link} Link", """
@@ -549,7 +563,7 @@ public partial class EntityInspector
 		if (!hasSelected) ImGui.BeginDisabled();
 		var link = selected?.GetDocLink() ?? "";
 		if (ImGui.InputText("##Link", ref link, 256))
-			selected!.SetDocLink((link.Length > 0) ? link : null);
+			selected?.SetDocLink((link.Length > 0) ? link : null);
 		if (!hasSelected) ImGui.EndDisabled();
 
 		Column($"{Icon.PaintBrush} Color", """
@@ -561,15 +575,15 @@ public partial class EntityInspector
 		var hasColor   = (maybeColor != null);
 		var color      = maybeColor ?? Color.White;
 		if (ImGui.Checkbox("##HasColor", ref hasColor)) {
-			if (hasColor) selected!.SetDocColor(color.ToHexString());
-			else          selected!.SetDocColor(null);
+			if (hasColor) selected?.SetDocColor(color.ToHexString());
+			else          selected?.SetDocColor(null);
 		}
 		ImGui.SameLine();
 		if (!hasColor) ImGui.BeginDisabled();
 		ImGui.SetNextItemWidth(-float.Epsilon);
 		var colorVec = color.ToVector3();
 		if (ImGui.ColorEdit3("##Color", ref colorVec))
-			selected!.SetDocColor(Color.FromRGB(colorVec).ToHexString());
+			selected?.SetDocColor(Color.FromRGB(colorVec).ToHexString());
 		if (!hasColor) ImGui.EndDisabled();
 		if (!hasSelected) ImGui.EndDisabled();
 
@@ -582,71 +596,71 @@ public partial class EntityInspector
 	// == Utility Functions ==
 	// =======================
 
-	private EntityRef NewEntityInspectorWindow(World world)
+	private static Entity<T> NewEntityInspectorWindow<T>(World<T> world)
 		=> world.New().Add<InspectorWindow>().Set(new History())
 		        .Build().SetDocName(DefaultWindowTitle);
 
-	private void SetSelected(
-		EntityRef window,     // The InspectorWindow entity.
+	private static void SetSelected<T>(
+		Entity<T> window,     // The InspectorWindow entity.
 		History? history,     // InspectorWindow's History component, null if it shouldn't be changed.
-		EntityRef? entity,    // Entity to set as selected or null to unset.
+		Entity<T>? entity,    // Entity to set as selected or null to unset.
 		bool scrollTo = true) // Should entity be scrolled to in the explorer view?
 	{
-		if (entity != null) window.Add<Selected>(entity);
-		else                window.Remove<Selected, Wildcard>();
+		if (entity is Entity<T> e1) window.Add<Selected>(e1);
+		else window.Remove<Selected, Core.Wildcard>();
 
-		for (var parent = entity?.Parent; parent != null; parent = parent.Parent)
+		for (var p = entity?.Parent; p is Entity<T> parent; p = parent.Parent)
 			window.Add<Expanded>(parent);
 
 		if ((entity != null) && scrollTo)
 			window.Add<ScrollToSelected>();
 
 		if (history != null) {
-			if (entity != null) history.Current = new History.Entry(entity, history.Current, null);
+			if (entity is Entity<T> e2) history.Current = new(e2, e2.Path, history.Current, null);
 			else if (history.Current is History.Entry entry) entry.Next = null;
 		}
 	}
 
-	private void GoToPrevious(EntityRef window, History history, EntityRef? selected)
+	private static void GoToPrevious<T>(Entity<T> window, History history, Entity<T>? selected)
 	{
 		if (selected != null) {
 			if (history.Current?.Prev == null) return;
 			history.Current = history.Current.Prev;
 		} else if (history.Current == null) return;
 
-		var entity = EntityRef.CreateOrNull(window.World, history.Current.Entity);
+		var entity = Entity<T>.GetOrNull(window.World, history.Current.Entity);
 		SetSelected(window, null, entity);
 		// TODO: Set path if entity could not be found.
 	}
 
-	private void GoToNext(EntityRef window, History history)
+	private static void GoToNext<T>(Entity<T> window, History history)
 	{
 		if (history.Current?.Next == null) return;
 		history.Current = history.Current.Next;
 
-		var entity = EntityRef.CreateOrNull(window.World, history.Current.Entity);
+		var entity = Entity<T>.GetOrNull(window.World, history.Current.Entity);
 		SetSelected(window, null, entity);
 		// TODO: Set path if entity could not be found.
 	}
 
-	private Rule? _findDisplayTypeRule;
-	private (EntityRef? DisplayType, float Priority) FindDisplayType(EntityRef entity)
+	private static object? _findDisplayTypeRule;
+	private static (Entity<T>? DisplayType, float Priority) FindDisplayType<T>(Entity<T> entity)
 	{
 		var world = entity.World;
-		var component = world.LookupByTypeOrThrow<Component>();
+		var component = world.Entity<Core.Component>();
 
-		var rule = _findDisplayTypeRule ??= new Rule(world, new(
-			$"$Type, gaemstone.Doc.DisplayType($Type)"));
+		var rule = (Rule<T>)(_findDisplayTypeRule ??= new Rule<T>(world, new(
+			$"$Type, gaemstone.Doc.DisplayType($Type)")));
 		var typeVar = rule.Variables["Type"]!;
 
-		var curType     = (EntityRef?)null;
+		var curType     = (Entity<T>?)null;
 		var curPriority = float.MaxValue;
-		foreach (var iter in _findDisplayTypeRule.Iter().SetVar(rule.ThisVar!, entity))
+		foreach (var iter in rule.Iter().SetVar(rule.ThisVar!, entity))
 			for (var i = 0; i < iter.Count; i++) {
 				var type = iter.GetVar(typeVar);
-				if ((type == component) && (entity.GetOrNull<Component>(component)?.Size == 0))
-					type = world.LookupByTypeOrThrow<Tag>();
-				var priority = type.GetOrNull<DocPriority>()?.Value ?? float.MaxValue;
+				if ((type == component) && (entity.GetOrNull<Core.Component>(component)?.Size == 0))
+					type = world.Entity<Core.Tag>();
+				var priority = type?.GetOrNull<DocPriority>()?.Value ?? float.MaxValue;
 				if (priority <= curPriority) { curType = type; curPriority = priority; }
 			}
 
@@ -658,24 +672,24 @@ public partial class EntityInspector
 	// == Utility ImGui Functions ==
 	// =============================
 
-	private void RenderIdentifier(EntityRef window, History? history, IdRef id)
+	private static void RenderIdentifier<T>(Entity<T> window, History? history, Id<T> id)
 	{
 		ImGui.PushStyleVar(ImGuiStyleVar.ItemSpacing, new Vector2(2, ImGui.GetStyle().ItemSpacing.Y));
-		if (id.AsPair() is (EntityRef relation, EntityRef target)) {
+		if (id.AsPair() is (Entity<T> relation, Entity<T> target)) {
 			ImGui.TextUnformatted("("); ImGui.SameLine();
 			RenderEntity(window, history, relation);
 			ImGui.SameLine(); ImGui.TextUnformatted(","); ImGui.SameLine();
 			RenderEntity(window, history, target);
 			ImGui.SameLine(); ImGui.TextUnformatted(")");
-		} else if (id.AsEntity() is EntityRef entity)
+		} else if (id.AsEntity() is Entity<T> entity)
 			RenderEntity(window, history, entity);
 		else
 			ImGui.TextUnformatted(id.ToString());
 		ImGui.PopStyleVar();
 	}
 
-	private void RenderEntity(
-		EntityRef window, History? history, EntityRef entity,
+	private static void RenderEntity<T>(
+		Entity<T> window, History? history, Entity<T> entity,
 		RenderEntityFlags flags = default)
 	{
 		var spanAvailWidth = (flags & RenderEntityFlags.SpanAvailWidth) != 0;
@@ -696,7 +710,7 @@ public partial class EntityInspector
 		var docName  = entity.GetDocName(false);
 
 		var isDisabled  = entity.IsDisabled;
-		var displayName = (docName != null) ? $"\"{docName}\"" : entity.Name ?? entity.Id.ToString();
+		var displayName = (docName != null) ? $"\"{docName}\"" : entity.Name ?? entity.NumericId.ToString();
 		if (docIcon != null) displayName = $"{docIcon} {displayName}";
 
 		// Gotta push the font to calculate size properly.
@@ -715,10 +729,10 @@ public partial class EntityInspector
 			                                                 : Color.Transparent.RGBA);
 			ImGui.PushStyleColor(ImGuiCol.ButtonActive , ImGui.GetColorU32(ImGuiCol.HeaderActive));
 			ImGui.PushStyleColor(ImGuiCol.ButtonHovered, ImGui.GetColorU32(ImGuiCol.HeaderHovered));
-			ImGui.Button($"##{entity.Id}", size);
+			ImGui.Button($"##{entity.NumericId}", size);
 			ImGui.PopStyleColor(3);
 			ImGui.PopStyleVar();
-		} else ImGui.InvisibleButton($"##{entity.Id}", size);
+		} else ImGui.InvisibleButton($"##{entity.NumericId}", size);
 
 		var shift    = ImGui.IsKeyDown(ImGuiKey.ModShift);
 		var hovered  = ImGui.IsItemHovered();
@@ -746,7 +760,7 @@ public partial class EntityInspector
 		if (hovered) {
 			ImGui.BeginTooltip();
 			ImGui.PushFont(ImGui.GetIO().Fonts.Fonts[1]);
-			ImGui.TextUnformatted(entity.GetFullPath().ToString());
+			ImGui.TextUnformatted(entity.Path.ToString());
 			ImGui.PopFont();
 			if (isDisabled) {
 				ImGui.SameLine();

src/gaemstone.Client/Systems/FreeCameraController.cs

@@ -21,24 +21,25 @@ public partial class FreeCameraController
 	}
 
 	[System]
-	public static void UpdateCamera(
-		Universe universe, TimeSpan delta,
-		in Camera camera, ref GlobalTransform transform, ref CameraController controller)
+	public static void UpdateCamera<T>(
+		World<T> world, TimeSpan delta,
+		in Camera camera, ref GlobalTransform transform, in CameraController controller)
 	{
-		var input    = universe.LookupByType<Input>();
-		var mouse    = universe.LookupByType<Mouse>();
-		var keyboard = universe.LookupByType<Keyboard>();
-		if ((input == null) || (mouse == null) || (keyboard == null)) return;
+		var input    = world.Entity<Input>();
+		var mouse    = world.Entity<Mouse>();
+		var keyboard = world.Entity<Keyboard>();
+		// FIXME: Is it okay if this crashes?
+		// if ((input == null) || (mouse == null) || (keyboard == null)) return;
 
-		var module = universe.LookupByTypeOrThrow<FreeCameraController>();
+		var module = world.Entity<FreeCameraController>();
 		var capturedBy      = input.GetTargets<CursorCapturedBy>().FirstOrDefault();
 		var inputCapturedBy = input.GetTargets<MouseInputCapturedBy>().FirstOrDefault();
-		var isCaptured = (capturedBy != null);
+		var isCaptured = capturedBy.IsSome;
 		// If another system has the mouse captured, don't do anything here.
 		if (isCaptured && (capturedBy != module)) return;
 
-		var isMouseDown = ((inputCapturedBy == null) || (inputCapturedBy == module))
-			           && mouse.LookupChild("Buttons/Right")?.Has<Active>() == true;
+		var isMouseDown = (inputCapturedBy.IsNone || (inputCapturedBy == module))
+			           && mouse.LookupChildOrNull("Buttons/Right")?.Has<Active>() == true;
 		if (isMouseDown != isCaptured) {
 			if (isMouseDown)
 				input.Add<CursorCapturedBy>(module);
@@ -51,18 +52,18 @@ public partial class FreeCameraController
 
 		var mouseMovement = Vector2.Zero;
 		if (isCaptured) {
-			var raw = (Vector2?)mouse.LookupChild("Delta")?.GetOrThrow<RawValue2D>() ?? default;
+			var raw = (Vector2?)mouse.LookupChildOrNull("Delta")?.GetOrThrow<RawValue2D>() ?? default;
 			mouseMovement = raw * controller.MouseSensitivity * (float)delta.TotalSeconds;
 		}
 
 		if (camera.IsOrthographic) {
 			transform *= Matrix4x4.CreateTranslation(-mouseMovement.X, -mouseMovement.Y, 0);
 		} else {
-			var shift = keyboard.LookupChild("ShiftLeft")?.Has<Active>() == true;
-			var w = keyboard.LookupChild("W")?.Has<Active>() == true;
-			var a = keyboard.LookupChild("A")?.Has<Active>() == true;
-			var s = keyboard.LookupChild("S")?.Has<Active>() == true;
-			var d = keyboard.LookupChild("D")?.Has<Active>() == true;
+			var shift = keyboard.LookupChildOrNull("ShiftLeft")?.Has<Active>() == true;
+			var w = keyboard.LookupChildOrNull("W")?.Has<Active>() == true;
+			var a = keyboard.LookupChildOrNull("A")?.Has<Active>() == true;
+			var s = keyboard.LookupChildOrNull("S")?.Has<Active>() == true;
+			var d = keyboard.LookupChildOrNull("D")?.Has<Active>() == true;
 
 			var speed = (shift ? 12 : 4) * (float)delta.TotalSeconds;
 			var forwardMovement = ((w ? -1 : 0) + (s ? 1 : 0)) * speed;

src/gaemstone.Client/Systems/ImGuiDemoWindow.cs

@@ -9,10 +9,10 @@ namespace gaemstone.Client.Systems;
 [DependsOn<gaemstone.Client.Systems.ImGuiManager>]
 public partial class ImGuiDemoWindow
 {
-	private bool _isOpen = false;
+	private static bool _isOpen = false;
 
 	[System]
-	public void Show(ImGuiData _)
+	public static void Show(ImGuiData _)
 	{
 		if (ImGuiUtility.UIButtonToggle(2, ForkAwesome.WindowMaximize, "ImGui Demo Window", ref _isOpen))
 			ImGui.ShowDemoWindow(ref _isOpen);

src/gaemstone.Client/Systems/ImGuiInputDebug.cs

@@ -17,13 +17,12 @@ namespace gaemstone.Client.Systems;
 [DependsOn<gaemstone.Client.Systems.ImGuiManager>]
 public partial class ImGuiInputDebug
 {
-	private bool _isOpen = false;
+	private static bool _isOpen = false;
 
 	[System]
-	public void ShowInputDebugWindow(Universe universe, ImGuiData _)
+	public static void ShowInputDebugWindow<T>(World<T> world, ImGuiData _)
 	{
-		var input = universe.LookupByType<Input>();
-		if (input == null) return;
+		var input = world.Entity<Input>();
 
 		if (!ImGuiUtility.UIButtonToggle(1, ForkAwesome.Gamepad, "Input Information", ref _isOpen)) return;
 
@@ -34,19 +33,20 @@ public partial class ImGuiInputDebug
 		                ImGuiWindowFlags.NoCollapse | ImGuiWindowFlags.NoResize)) {
 			ImGui.PushFont(ImGui.GetIO().Fonts.Fonts[0]);
 
-			if (universe.LookupByType<Keyboard>() is EntityRef keyboard)
-				DrawKeyboard(keyboard);
+			var keyboard = world.Entity<Keyboard>();
+			DrawKeyboard(keyboard);
 
-			if (universe.LookupByType<Mouse>() is EntityRef mouse) {
+			var mouse = world.Entity<Mouse>();
+			{
 				ImGui.BeginChild("Mouse Info", new(160, 180), true);
 
-				ImGui.Text("Position: " + (Vector2?)mouse.LookupChild("Position")?.GetOrNull<RawValue2D>());
-				ImGui.Text("Delta: "    + (Vector2?)mouse.LookupChild("Delta"   )?.GetOrNull<RawValue2D>());
-				ImGui.Text("Wheel: "    + (float?)  mouse.LookupChild("Wheel"   )?.GetOrNull<RawValue1D>());
+				ImGui.Text("Position: " + (Vector2?)mouse.LookupChildOrNull("Position")?.GetOrNull<RawValue2D>());
+				ImGui.Text("Delta: "    + (Vector2?)mouse.LookupChildOrNull("Delta"   )?.GetOrNull<RawValue2D>());
+				ImGui.Text("Wheel: "    + (float?)  mouse.LookupChildOrNull("Wheel"   )?.GetOrNull<RawValue1D>());
 
 				ImGui.Spacing();
 
-				var buttons = mouse.LookupChild("Buttons")?.GetChildren().ToArray() ?? Array.Empty<EntityRef>();
+				var buttons = mouse.LookupChildOrNull("Buttons")?.Children.ToArray() ?? Array.Empty<Entity<T>>();
 				ImGui.Text("Buttons: " + string.Join(" ", buttons
 					.Where (button => button.Has<Active>())
 					.Select(button => $"{button.Name} ({button.GetOrThrow<Active>().Duration.TotalSeconds:f2}s)")));
@@ -60,11 +60,11 @@ public partial class ImGuiInputDebug
 				ImGui.EndChild();
 			}
 
-			for (var index = 0; input.LookupChild("Gamepad" + index) is EntityRef gamepad; index++) {
+			for (var index = 0; input.LookupChildOrNull("Gamepad" + index) is Entity<T> gamepad; index++) {
 				ImGui.SameLine();
 				ImGui.BeginChild($"{gamepad.Name} Info", new(160, 180), true);
 
-				var buttons = gamepad.LookupChild("Buttons")?.GetChildren().ToArray() ?? Array.Empty<EntityRef>();
+				var buttons = gamepad.LookupChildOrNull("Buttons")?.Children.ToArray() ?? Array.Empty<Entity<T>>();
 				ImGui.Text("Buttons: " + string.Join(" ", buttons.Where(b => b.Has<Active>())
 					.Select(b => $"{b.Name} ({b.GetOrThrow<Active>().Duration.TotalSeconds:f2}s)")));
 				ImGui.Text("  Pressed: "  + string.Join(" ", buttons.Where(b => b.Has<Activated>()).Select(b => b.Name)));
@@ -73,7 +73,7 @@ public partial class ImGuiInputDebug
 				ImGui.Spacing();
 
 				ImGui.Text("Triggers:");
-				for (var i = 0; gamepad.LookupChild("Trigger" + i) is EntityRef trigger; i++) {
+				for (var i = 0; gamepad.LookupChildOrNull("Trigger" + i) is Entity<T> trigger; i++) {
 					var text = $"  {i}: {(float?)trigger.GetOrNull<RawValue1D>() ?? default:f2}";
 					if      (trigger.Has<Activated>())   text += " pressed!";
 					else if (trigger.Has<Deactivated>()) text += " released!";
@@ -83,7 +83,7 @@ public partial class ImGuiInputDebug
 				}
 
 				ImGui.Text("Thumbsticks:");
-				for (var i = 0; gamepad.LookupChild("Thumbstick" + i) is EntityRef thumbstick; i++)
+				for (var i = 0; gamepad.LookupChildOrNull("Thumbstick" + i) is Entity<T> thumbstick; i++)
 					ImGui.Text($"  {i}: {(Vector2?)thumbstick.GetOrNull<RawValue2D>() ?? default:f2}");
 
 				ImGui.EndChild();
@@ -139,7 +139,7 @@ public partial class ImGuiInputDebug
 		[Key.NumLock] = "Num\nLck", [Key.KeypadEnter] = "=",
 	};
 
-	public static void DrawKeyboard(EntityRef keyboard)
+	public static void DrawKeyboard<T>(Entity<T> keyboard)
 	{
 		const float UnitKeySize = 32.0f;
 
@@ -150,10 +150,10 @@ public partial class ImGuiInputDebug
 		var current = Vector2.Zero;
 		foreach (var (widths, keys) in KeyboardLayout.Zip(KeyboardKeys)) {
 			foreach (var (width, key) in widths.Zip(keys)) {
-				var active = (key != null) && (keyboard.LookupChild(key.Value.ToString())?.Has<Active>() == true);
+				var active = (key != null) && (keyboard.LookupChildOrNull(key.Value.ToString())?.Has<Active>() == true);
 
 				var keySize = new Vector2(width, 1.0f);
-				if      (width ==  T) keySize = new Vector2((-keySize.X - 10), 2.0f);
+				if      (width == ImGuiInputDebug.T) keySize = new Vector2((-keySize.X - 10), 2.0f);
 				else if (width < -10) keySize = new Vector2((-keySize.X - 10), 1.0f);
 				else if (width <   0) { current += new Vector2(-keySize.X * UnitKeySize, 0); continue; }
 

src/gaemstone.Client/Systems/ImGuiManager.cs

@@ -4,8 +4,8 @@ using System.Linq;
 using System.Text;
 using gaemstone.Client.Utility;
 using gaemstone.ECS;
+using gaemstone.ECS.Utility;
 using gaemstone.Flecs;
-using gaemstone.Utility;
 using ImGuiNET;
 using Silk.NET.Input;
 using Silk.NET.OpenGL.Extensions.ImGui;
@@ -22,14 +22,17 @@ namespace gaemstone.Client.Systems;
 public partial class ImGuiManager
 {
 	[Entity, Add<Pipeline.Phase>]
-	[DependsOn<SystemPhase.OnLoad>]
+	[DependsOn<Pipeline.OnLoad>]
 	public struct ImGuiUpdatePhase {  }
 
 	[Entity, Add<Pipeline.Phase>]
-	[DependsOn<SystemPhase.OnStore>]
+	[DependsOn<Pipeline.OnStore>]
 	public struct ImGuiRenderPhase {  }
 
-	[Singleton(AutoAdd = false)]
+	// Start out disabled, because we first need to set it with a valid instance.
+	// TODO: Add a hint when a Singleton class is used without adding Disabled.
+	// TODO: Add a [Disabled] attribute as shortcut?
+	[Singleton, Add<Core.Disabled>]
 	public class ImGuiData
 	{
 		public ImGuiController Controller { get; }
@@ -84,10 +87,11 @@ public partial class ImGuiManager
 			return io.Fonts.AddFontFromMemoryTTF((IntPtr)dataPtr, size, size, cfg);
 	}
 
-	[System<SystemPhase.OnLoad>]
-	public unsafe void Initialize(Universe universe, GameWindow window, Canvas canvas,
-			[Source<Input>] InputContext inputContext, [Not] ImGuiData _)
-		=> universe.LookupByTypeOrThrow<ImGuiData>().Set(new ImGuiData(
+	[System]
+	[DependsOn<Pipeline.OnLoad>]
+	public static unsafe void Initialize<T>(World<T> world, GameWindow window, Canvas canvas,
+			[Source<Input>] InputContext inputContext, Not<ImGuiData> _)
+		=> world.Entity<ImGuiData>().Enable().Set(new ImGuiData(
 			new(canvas.GL, window.Handle, inputContext.Value, () => {
 				var io    = ImGui.GetIO();
 				var style = ImGui.GetStyle();
@@ -139,15 +143,16 @@ public partial class ImGuiManager
 				}
 			})));
 
-	[System<SystemPhase.OnLoad>]
-	public static void UpdateMouse(Universe universe,
+	[System]
+	[DependsOn<Pipeline.OnLoad>]
+	public static void UpdateMouse<T>(World<T> world,
 		[Source<Mouse>] MouseImpl impl, ImGuiData _)
 	{
 		var mouse  = impl.Value;
-		var input  = universe.LookupByTypeOrThrow<Input>();
-		var module = universe.LookupByTypeOrThrow<ImGuiManager>();
+		var input  = world.Entity<Input>();
+		var module = world.Entity<ImGuiManager>();
 		var capturedBy = input.GetTargets<MouseInputCapturedBy>().FirstOrDefault();;
-		var isCaptured = (capturedBy != null);
+		var isCaptured = capturedBy.IsSome;
 		// If another system has the mouse captured, don't do anything here.
 		if (isCaptured && (capturedBy != module)) return;
 
@@ -180,11 +185,13 @@ public partial class ImGuiManager
 		};
 	}
 
-	[System<ImGuiUpdatePhase>]
+	[System]
+	[DependsOn<ImGuiUpdatePhase>]
 	public static void Update(TimeSpan delta, ImGuiData imgui)
 		=> imgui.Controller.Update((float)delta.TotalSeconds);
 
-	[System<ImGuiRenderPhase>]
+	[System]
+	[DependsOn<ImGuiRenderPhase>]
 	public static void Render(ImGuiData imgui)
 		=> imgui.Controller.Render();
 }

src/gaemstone.Client/Systems/InputManager.cs

@@ -21,11 +21,12 @@ public partial class InputManager
 	[Component] public record class KeyboardImpl(IKeyboard Value) {  }
 	[Component] public record class GamepadImpl(IGamepad Value) {  }
 
-	[System<SystemPhase.OnLoad>]
-	public static void Initialize(Universe universe,
-		[Game] GameWindow window, [Source<Input>, Not] InputContext _)
+	[System]
+	[DependsOn<Pipeline.OnLoad>]
+	public static void Initialize<T>(World<T> world,
+		GameWindow window, [Source<Input>] Not<InputContext> _)
 	{
-		var input   = universe.LookupByTypeOrThrow<Input>();
+		var input   = world.Entity<Input>();
 		var context = window.Handle.CreateInput();
 		input.Set(new InputContext(context));
 
@@ -42,24 +43,25 @@ public partial class InputManager
 	}
 
 
-	[Observer<ObserverEvent.OnAdd>]
-	[Expression("CursorCapturedBy(Input, *)")]
-	public static void OnCursorCaptured(Universe universe)
-		=> universe.LookupByTypeOrThrow<Mouse>().GetOrThrow<MouseImpl>()
+	[Observer<Core.OnAdd>]
+	public static void OnCursorCaptured<T>(World<T> universe,
+			[Source<Input>] Has<CursorCapturedBy, Core.Wildcard> _)
+		=> universe.Entity<Mouse>().GetOrThrow<MouseImpl>()
 			.Value.Cursor.CursorMode = CursorMode.Raw;
 
-	[Observer<ObserverEvent.OnRemove>]
-	[Expression("CursorCapturedBy(Input, *)")]
-	public static void OnCursorReleased(Universe universe)
-		=> universe.LookupByTypeOrThrow<Mouse>().GetOrThrow<MouseImpl>()
+	[Observer<Core.OnRemove>]
+	public static void OnCursorReleased<T>(World<T> universe,
+			[Source<Input>] Has<CursorCapturedBy, Core.Wildcard> _)
+		=> universe.Entity<Mouse>().GetOrThrow<MouseImpl>()
 			.Value.Cursor.CursorMode = CursorMode.Normal;
 
 
-	[System<SystemPhase.OnLoad>]
-	public static void ProcessMouse(TimeSpan delta, EntityRef entity, MouseImpl impl)
+	[System]
+	[DependsOn<Pipeline.OnLoad>]
+	public static void ProcessMouse<T>(TimeSpan delta, Entity<T> entity, MouseImpl impl)
 	{
 		var mouse = impl.Value;
-		var isCaptured = entity.Parent!.Has<CursorCapturedBy, Core.Any>();
+		var isCaptured = entity.Parent?.Has<CursorCapturedBy, Core.Any>() ?? false;
 		ref var position = ref entity.NewChild("Position").Build().GetMut<RawValue2D>();
 		ref var posDelta = ref entity.NewChild("Delta"   ).Build().GetMut<RawValue2D>();
 		posDelta = mouse.Position - position;
@@ -74,8 +76,9 @@ public partial class InputManager
 			                mouse.IsButtonPressed(button) ? 1 : 0);
 	}
 
-	[System<SystemPhase.OnLoad>]
-	public static void ProcessKeyboard(TimeSpan delta, EntityRef entity, KeyboardImpl impl)
+	[System]
+	[DependsOn<Pipeline.OnLoad>]
+	public static void ProcessKeyboard<T>(TimeSpan delta, Entity<T> entity, KeyboardImpl impl)
 	{
 		var keyboard = impl.Value;
 		foreach (var key in keyboard.SupportedKeys) {
@@ -84,8 +87,9 @@ public partial class InputManager
 		}
 	}
 
-	[System<SystemPhase.OnLoad>]
-	public static void ProcessGamepad(TimeSpan delta, EntityRef entity, GamepadImpl impl)
+	[System]
+	[DependsOn<Pipeline.OnLoad>]
+	public static void ProcessGamepad<T>(TimeSpan delta, Entity<T> entity, GamepadImpl impl)
 	{
 		var gamepad = impl.Value;
 		var buttons = entity.NewChild("Buttons").Build();
@@ -101,7 +105,7 @@ public partial class InputManager
 	private const float ActivationThreshold   = 0.90f;
 	private const float DeactivationThreshold = 0.75f;
 
-	private static void Update1D(TimeSpan delta, EntityRef entity, float current)
+	private static void Update1D<T>(TimeSpan delta, Entity<T> entity, float current)
 	{
 		entity.GetMut<RawValue1D>() = current;
 		if (current >= ActivationThreshold) {
@@ -114,7 +118,7 @@ public partial class InputManager
 			entity.Remove<Active>();
 	}
 
-	private static void Update2D(TimeSpan delta, EntityRef entity, Vector2 current)
+	private static void Update2D<T>(TimeSpan delta, Entity<T> entity, Vector2 current)
 	{
 		entity.GetMut<RawValue2D>() = current;
 		var magnitude = current.Length();
@@ -134,11 +138,12 @@ public partial class InputManager
 	// public static void OnActiveAdded(EntityRef entity, Active _)
 	// 	=> entity.Add<Activated>();
 
-	[Observer<ObserverEvent.OnRemove>]
-	public static void OnActiveRemoved(EntityRef entity, Active _)
+	[Observer<Core.OnRemove>]
+	public static void OnActiveRemoved<T>(Entity<T> entity, Active _)
 		=> entity.Add<Deactivated>();
 
-	[System<SystemPhase.PostFrame>]
-	public static void ClearDeActivated(EntityRef entity, [Or] Activated _1, [Or] Deactivated _2)
+	[System]
+	[DependsOn<Pipeline.PostFrame>]
+	public static void ClearDeActivated<T>(Entity<T> entity, Has<Or<Activated, Deactivated>> _)
 		=> entity.Remove<Activated>().Remove<Deactivated>();
 }

src/gaemstone.Client/Systems/MeshManager.cs

@@ -21,12 +21,11 @@ public partial class MeshManager
 	private const uint UvAttribIndex       = 2;
 
 	[System]
-	public static void LoadMeshWhenDefined(
-		[Game] Canvas canvas, EntityRef entity,
-		Mesh _1, [Not] MeshHandle _2)
+	public static void LoadMeshWhenDefined<T>(
+		Canvas canvas, Entity<T> entity,
+		Has<Mesh> _1, Not<MeshHandle> _2)
 	{
-		var path = entity.GetFullPath();
-		using var stream = Resources.GetStream(path);
+		using var stream = Resources.GetStream(entity.Path);
 		var handle = CreateFromStream(canvas.GL, stream);
 		entity.Set(handle);
 	}

src/gaemstone.Client/Systems/Renderer.cs

@@ -3,8 +3,8 @@ using System.Diagnostics;
 using System.Numerics;
 using System.Runtime.InteropServices;
 using gaemstone.ECS;
+using gaemstone.ECS.Utility;
 using gaemstone.Flecs;
-using gaemstone.Utility;
 using Silk.NET.OpenGL;
 using Silk.NET.Windowing;
 using static gaemstone.Client.Components.CameraComponents;
@@ -21,13 +21,13 @@ namespace gaemstone.Client.Systems;
 [DependsOn<gaemstone.Components.TransformComponents>]
 public partial class Renderer
 {
-	private uint _program;
-	private int _cameraMatrixUniform;
-	private int _modelMatrixUniform;
-	private Rule? _renderEntityRule;
+	private static uint _program;
+	private static int _cameraMatrixUniform;
+	private static int _modelMatrixUniform;
+	private static object? _renderEntityRule;
 
-	[Observer<ObserverEvent.OnSet>]
-	public void OnCanvasSet(Canvas canvas)
+	[Observer<Core.OnSet>]
+	public static void OnCanvasSet(Canvas canvas)
 	{
 		var GL = canvas.GL;
 
@@ -54,8 +54,9 @@ public partial class Renderer
 		_modelMatrixUniform  = GL.GetUniformLocation(_program, "modelMatrix");
 	}
 
-	[System<SystemPhase.PreStore>]
-	public void Clear(Canvas canvas)
+	[System]
+	[DependsOn<Pipeline.PreStore>]
+	public static void Clear(Canvas canvas)
 	{
 		var GL = canvas.GL;
 		GL.UseProgram(_program);
@@ -64,8 +65,9 @@ public partial class Renderer
 		GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
 	}
 
-	[System<SystemPhase.OnStore>]
-	public void Render(Universe universe, [Game] Canvas canvas,
+	[System]
+	[DependsOn<Pipeline.OnStore>]
+	public static void Render<T>(World<T> world, Canvas canvas,
 		in GlobalTransform cameraTransform, in Camera camera, CameraViewport? viewport)
 	{
 		var color  = viewport?.ClearColor ?? Color.FromRGB(0.3f, 0.0f, 0.5f);
@@ -93,12 +95,12 @@ public partial class Renderer
 		var cameraMatrix = invertedTransform * cameraProjection;
 		GL.UniformMatrix4(_cameraMatrixUniform, 1, false, in cameraMatrix.M11);
 
-		_renderEntityRule ??= new(universe, new("""
+		var rule = (Rule<T>)(_renderEntityRule ??= new Rule<T>(world, new("""
 			[in] GlobalTransform,
 			(Mesh, $mesh), [in] MeshHandle($mesh),
 			?(Texture, $tex), [in] ?TextureHandle($tex)
-			"""));
-		foreach (var iter in _renderEntityRule.Iter()) {
+			""")));
+		foreach (var iter in rule.Iter()) {
 			var transforms = iter.Field<GlobalTransform>(1);
 			var meshes     = iter.Field<MeshHandle>(3);
 			// var texPairs   = iter.FieldOrEmpty<Identifier>(4);
@@ -125,7 +127,8 @@ public partial class Renderer
 		}
 	}
 
-	[System<SystemPhase.PostFrame>]
+	[System]
+	[DependsOn<Pipeline.PostFrame>]
 	public static void SwapBuffers(GameWindow window)
 		=> window.Handle.SwapBuffers();
 

src/gaemstone.Client/Systems/TextureManager.cs

@@ -17,7 +17,7 @@ namespace gaemstone.Client.Systems;
 [DependsOn<gaemstone.Client.Systems.Windowing>]
 public partial class TextureManager
 {
-	[Observer<ObserverEvent.OnSet>]
+	[Observer<Core.OnSet>]
 	public static void OnCanvasSet(Canvas canvas)
 	{
 		var GL = canvas.GL;
@@ -31,12 +31,11 @@ public partial class TextureManager
 	}
 
 	[System]
-	public static void LoadTextureWhenDefined(
-		[Game] Canvas canvas, EntityRef entity,
-		Texture _1, [Not] TextureHandle _2)
+	public static void LoadTextureWhenDefined<T>(
+		Canvas canvas, Entity<T> entity,
+		Has<Texture> _1, Not<TextureHandle> _2)
 	{
-		var path = entity.GetFullPath();
-		using var stream = Resources.GetStream(path);
+		using var stream = Resources.GetStream(entity.Path);
 		var handle = CreateFromStream(canvas.GL, stream);
 		entity.Set(handle);
 	}

src/gaemstone.Client/Systems/Windowing.cs

@@ -9,7 +9,9 @@ namespace gaemstone.Client.Systems;
 [Module]
 public partial class Windowing
 {
-	[Component]
+	// TODO: Canvas and GameWindow should maybe not be Singleton?
+
+	[Singleton]
 	public class Canvas
 	{
 		public GL GL { get; }
@@ -19,14 +21,15 @@ public partial class Windowing
 		public Color BackgroundColor { get; set; }
 	}
 
-	[Component]
+	[Singleton]
 	public class GameWindow
 	{
 		public IWindow Handle { get; }
 		public GameWindow(IWindow handle) => Handle = handle;
 	}
 
-	[System<SystemPhase.PreFrame>]
+	[System]
+	[DependsOn<Pipeline.PreFrame>]
 	public static void ProcessWindow(GameWindow window, Canvas canvas)
 	{
 		canvas.Size = new(window.Handle.Size.X, window.Handle.Size.Y);

src/gaemstone.Client/gaemstone.Client.csproj

@@ -6,10 +6,11 @@
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <ImplicitUsings>disable</ImplicitUsings>
     <Nullable>enable</Nullable>
+    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles> <!-- -->
   </PropertyGroup>
 
   <ItemGroup>
-    <EmbeddedResource Include="Resources/*" />
+    <EmbeddedResource Include="Resources/**" />
   </ItemGroup>
 
   <ItemGroup>

src/gaemstone.ECS

@@ -1 +1 @@
-Subproject commit 46e171940eea723f2ad8376b5056afd9b9d8a340
+Subproject commit f7d17d46ab68ae91487d5d6275c3a501e1ee0980

src/gaemstone.SourceGen/Descriptors.cs

@@ -0,0 +1,220 @@
+using Microsoft.CodeAnalysis;
+
+
+namespace gaemstone.SourceGen;
+
+public static class Descriptors
+{
+	public const string DiagnosticCategory = "gaemstone.SourceGen";
+
+	// TODO: Replace this counter with proper hardcoded IDs.
+	private static int _idCounter = 1;
+
+
+	// Diagnostics relating to where the symbol occurs.
+
+	public static readonly DiagnosticDescriptor ModuleMustNotBeNested = new(
+		$"gSG{_idCounter++:00}", "Module must not be a nested class",
+		"A [Module] must be defined as a top-level class.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor EntityMustBeInModule = new(
+		$"gSG{_idCounter++:00}", "Entity must be part of a Module",
+		"Entity type must be defined within a [Module].",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor InstanceMethodOnlyValidInSingleton = new(
+		$"gSG{_idCounter++:00}", "Non-static method is only valid in Singleton Module",
+		"Non-static [System] or [Observer] is only valid in a [Module] which is also marked as a [Singleton].",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor ParamMustBeInMethod = new(
+		$"gSG{_idCounter++:00}", "Parameter must be part of a System or Observer",
+		"This parameter must be part of a method marked as [System] or [Observer].",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+
+	// Diagnostics relating to the combined usage of attributes.
+
+	public static readonly DiagnosticDescriptor InvalidAttributeCombination = new(
+		$"gSG{_idCounter++:00}", "Invalid combination of entity attributes",
+		"The combination of entity attributes {0} is not valid.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor ValidModuleAttributesHint = new(
+		$"gSG{_idCounter++:00}", "Module may be a Singleton",
+		"A [Module] may be marked as a [Singleton].",
+		DiagnosticCategory, DiagnosticSeverity.Info, true);
+
+	public static readonly DiagnosticDescriptor ValidRelationAttributesHint = new(
+		$"gSG{_idCounter++:00}", "Relation may be a Tag or Component",
+		"A [Relation] may be marked as a [Tag] or [Component].",
+		DiagnosticCategory, DiagnosticSeverity.Info, true);
+
+	public static readonly DiagnosticDescriptor SingletonImpliesComponentHint = new(
+		$"gSG{_idCounter++:00}", "Singleton implies Component",
+		"A [Singleton] is already implied to be a [Component].",
+		DiagnosticCategory, DiagnosticSeverity.Info, true);
+
+	public static readonly DiagnosticDescriptor BuiltInModuleMustHavePath = new(
+		$"gSG{_idCounter++:00}", "BuiltIn Module must have Path",
+		"A [BuiltIn, Module] must also have a [Path] set.",
+		DiagnosticCategory, DiagnosticSeverity.Info, true);
+
+
+
+
+	// Diagnostics relating to keywords / modifiers used with the symbol.
+
+	public static readonly DiagnosticDescriptor ModuleMustBePartial = new(
+		$"gSG{_idCounter++:00}", "ModuleMustBePartial",
+		"A [Module] type must be marked as partial.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor TypeMustNotBeStatic = new(
+		$"gSG{_idCounter++:00}", "Entity type must not be static",
+		"Entity type must not be static.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor TypeMustNotBeAbstract = new(
+		$"gSG{_idCounter++:00}", "Entity type must not be abstract",
+		"Entity type must not be abstract.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+
+	public static readonly DiagnosticDescriptor MethodMustNotBeAbstract = new(
+		$"gSG{_idCounter++:00}", "System / Observer must not be abstract",
+		"A [System] or [Observer] method must not be marked as abstract.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor MethodMustNotBeAsync = new(
+		$"gSG{_idCounter++:00}", "System / Observer must not be async",
+		"A [System] or [Observer] method must not be marked as async.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	// TODO: Check for any other weird modifiers we don't want.
+
+
+	// Diagnostics relating to (generic) parameters on the attributes themselves.
+
+	// TODO: Be more specific?
+	public static readonly DiagnosticDescriptor InvalidTypeArgument = new(
+		$"gSG{_idCounter++:00}", "Invalid type argument",
+		"The specified type argument is not valid here.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+
+	// Diagnostics relating to system / observer generic parameters.
+
+	public static readonly DiagnosticDescriptor MethodMustNotBeExtension = new(
+		$"gSG{_idCounter++:00}", "System / Observer must not be extension method",
+		"A [System] or [Observer] method must not be an extension method.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor MethodMustHaveParameters = new(
+		$"gSG{_idCounter++:00}", "System / Observer must have parameters",
+		"A [System] or [Observer] must have parameters.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor MethodMustHaveExpression = new(
+		$"gSG{_idCounter++:00}", "Iterator-only System / Observer must have expression",
+		"An Iterator-only [System] or [Observer] must have an [Expression] set.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+
+	public static readonly DiagnosticDescriptor MethodGenericParamAtMostOne = new(
+		$"gSG{_idCounter++:00}", "System / Observer must have at most one generic parameter",
+		"A [System] or [Observer] must at most one generic parameter.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	// TODO: See if we can change this wording, but try to use the correct one. (Does "open generic" work?)
+	public static readonly DiagnosticDescriptor MethodGenericParamMustNotBeSubstutited = new(
+		$"gSG{_idCounter++:00}", "System / Observer generic parameter must not be substituted",
+		"The generic parameter of a [System] or [Observer] must not be substituted.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor MethodGenericParamMustNotBeConstrained = new(
+		$"gSG{_idCounter++:00}", "System / Observer generic parameter must not be substituted",
+		"The generic parameter of a [System] or [Observer] must not be constrained.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+
+	// Diagnostics relating to system / observer parameters.
+
+	public static readonly DiagnosticDescriptor ParamMustNotBeArray = new(
+		$"gSG{_idCounter++:00}", "Parameter must not be array",
+		"Parameter must not be an array type.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor ParamMustNotBePointer = new(
+		$"gSG{_idCounter++:00}", "Parameter must not be pointer",
+		"Parameter must not be a pointer type.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor ParamMustNotBeGenericType = new(
+		$"gSG{_idCounter++:00}", "Parameter must not be generic type",
+		"Parameter must not be a generic type parameter.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor ParamMustNotBePrimitive = new(
+		$"gSG{_idCounter++:00}", "Parameter must not be primitive",
+		"Parameter must not be a primitive type.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor ParamMustNotBeGeneric = new(
+		$"gSG{_idCounter++:00}", "Parameter must not be geric",
+		"Parameter must not be generic (except Iterator, World, Entity, Nullable, Has, Not and Or).",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor ParamDoesNotSupportOptional = new(
+		$"gSG{_idCounter++:00}", "Parameter does not support optional",
+		"Parameter does not support optional value syntax.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor ParamByRefMustBeValueType = new(
+		$"gSG{_idCounter++:00}", "ByRef parameter must be a value type",
+		"ByRef (in/out/ref) parameter must be a value type (struct).",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor ParamByRefMustNotBeNullable = new(
+		$"gSG{_idCounter++:00}", "ByRef parameter must not be nullable",
+		"ByRef (in/out/ref) parameter must not be nullable.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+
+	public static readonly DiagnosticDescriptor SpecialMustNotBeByRef = new(
+		$"gSG{_idCounter++:00}", "Special parameter must not be ByRef",
+		"Special parameter must not be ByRef (in/out/ref).",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor SpecialArgMustNotBeNullable = new(
+		$"gSG{_idCounter++:00}", "Special type argument must not be nullable",
+		"Special type argument must not be nullable.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor SpecialArgMustNotBeGeneric = new(
+		$"gSG{_idCounter++:00}", "Special type argument must not be generic",
+		"Special type argument must not be generic.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+
+	public static readonly DiagnosticDescriptor UniqueParamMustNotBeByRef = new(
+		$"gSG{_idCounter++:00}", "Unique parameter must not be ByRef",
+		"Unique parameter must not be ByRef (in/out/ref).",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor UniqueParamMustNotBeNullable = new(
+		$"gSG{_idCounter++:00}", "Unique parameter must not be nullable",
+		"Unique parameter must not be nullable.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor UniqueParamGenericMustMatch = new(
+		$"gSG{_idCounter++:00}", "Unique parameter generic type parameter must match method",
+		"Unique parameter's generic type parameter must match the method's type parameter.",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+	public static readonly DiagnosticDescriptor UniqueParamNotSupported = new(
+		$"gSG{_idCounter++:00}", "Unique parameter does not support this attribute",
+		"Unique parameter does not support {0}",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+}

src/gaemstone.SourceGen/Generators/AutoRegisterComponentsGenerator.cs

@@ -1,261 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using gaemstone.SourceGen.Utility;
-using Microsoft.CodeAnalysis;
-using Microsoft.CodeAnalysis.CSharp;
-using Microsoft.CodeAnalysis.CSharp.Syntax;
-
-namespace gaemstone.SourceGen.Generators;
-
-// TODO: "Jump to Definition" feature to open the file + line in which a module / component / system is defined.
-
-[Generator]
-public partial class AutoRegisterComponentsGenerator
-	: ISourceGenerator
-{
-	private static readonly DiagnosticDescriptor ComponentNotPartOfModule = new(
-		"gaem0101", "Components must be part of a module",
-		"Type {0} isn't defined as part of a [Module]",
-		nameof(AutoRegisterComponentsGenerator), DiagnosticSeverity.Error, true);
-
-	private static readonly DiagnosticDescriptor ComponentMultipleTypes = new(
-		"gaem0102", "Components may not have multiple component types",
-		"Type {0} is marked with multiple component types ({1})",
-		nameof(AutoRegisterComponentsGenerator), DiagnosticSeverity.Error, true);
-
-	private static readonly DiagnosticDescriptor ComponentRelationInvalidType = new(
-		"gaem0103", "Relations may only be marked with [Component] or [Tag]",
-		"Type {0} marked as [Relation] may not be marked with [{1}], only [Component] or [Tag] are valid",
-		nameof(AutoRegisterComponentsGenerator), DiagnosticSeverity.Error, true);
-
-
-	public void Initialize(GeneratorInitializationContext context)
-		=> context.RegisterForSyntaxNotifications(() => new SyntaxReceiver());
-
-	private class SyntaxReceiver
-		: ISyntaxContextReceiver
-	{
-		public Dictionary<INamedTypeSymbol, HashSet<INamedTypeSymbol>> Modules { get; }
-			= new(SymbolEqualityComparer.Default);
-		public HashSet<INamedTypeSymbol> ComponentsNotInModule { get; }
-			= new(SymbolEqualityComparer.Default);
-
-		public void OnVisitSyntaxNode(GeneratorSyntaxContext context)
-		{
-			if (context.Node is not AttributeSyntax attrNode) return;
-			var model = context.SemanticModel;
-
-			var attrType = model.GetTypeInfo(attrNode).Type!;
-			if ((attrType.GetNamespace() != "gaemstone.ECS") || !attrType.Name.EndsWith("Attribute")) return;
-			var attrName = attrType.Name.Substring(0, attrType.Name.Length - "Attribute".Length);
-			if (!Enum.TryParse<RegisterType>(attrName, out _)) return;
-
-			var symbol = (model.GetDeclaredSymbol(attrNode.Parent?.Parent!) as INamedTypeSymbol)!;
-			if ((symbol.ContainingSymbol is INamedTypeSymbol module)
-				&& module.HasAttribute("gaemstone.ECS.ModuleAttribute"))
-			{
-				if (!Modules.TryGetValue(module, out var components))
-					Modules.Add(module, components = new(SymbolEqualityComparer.Default));
-				components.Add(symbol);
-			}
-			else ComponentsNotInModule.Add(symbol);
-		}
-	}
-
-	public void Execute(GeneratorExecutionContext context)
-	{
-		if (context.SyntaxContextReceiver is not SyntaxReceiver receiver) return;
-
-		foreach (var symbol in receiver.ComponentsNotInModule)
-			context.ReportDiagnostic(Diagnostic.Create(ComponentNotPartOfModule,
-				symbol.Locations.FirstOrDefault(), symbol.GetFullName()));
-
-		var modules = new Dictionary<INamedTypeSymbol, ModuleInfo>(
-			SymbolEqualityComparer.Default);
-		foreach (var pair in receiver.Modules) {
-			var moduleSymbol = pair.Key;
-			if (!modules.TryGetValue(moduleSymbol, out var module))
-				modules.Add(moduleSymbol, module = new(moduleSymbol));
-
-			foreach (var symbol in pair.Value) {
-				var componentTypes = symbol.GetAttributes()
-					.Where (attr => (attr.AttributeClass?.GetNamespace() == "gaemstone.ECS"))
-					.Select(attr => attr.AttributeClass!.Name)
-					.Where (name => name.EndsWith("Attribute"))
-					.Select(name => name.Substring(0, name.Length - "Attribute".Length))
-					.SelectMany(name => Enum.TryParse<RegisterType>(name, out var type)
-						? new[] { type } : Enumerable.Empty<RegisterType>())
-					.ToList();
-				if (componentTypes.Count == 0) continue;
-
-				var isRelation = componentTypes.Contains(RegisterType.Relation);
-				if (isRelation && (componentTypes.Count == 2)) {
-					var other = componentTypes.Where(type => (type != RegisterType.Relation)).Single();
-					if (other is RegisterType.Component or RegisterType.Tag)
-						componentTypes.Remove(RegisterType.Relation);
-					else context.ReportDiagnostic(Diagnostic.Create(ComponentRelationInvalidType,
-						symbol.Locations.FirstOrDefault(), symbol.GetFullName(), other.ToString()));
-				}
-
-				if (componentTypes.Count >= 2)
-					context.ReportDiagnostic(Diagnostic.Create(ComponentMultipleTypes,
-						symbol.Locations.FirstOrDefault(), symbol.GetFullName(),
-						string.Join(", ", componentTypes.Select(s => $"[{s}]"))));
-				var componentType = componentTypes[0];
-
-				var addedEntities  = new List<ITypeSymbol>();
-				var addedRelations = new List<(ITypeSymbol, ITypeSymbol)>();
-				foreach (var attr in symbol.GetAttributes())
-					for (var type = attr.AttributeClass; type != null; type = type.BaseType)
-						switch (type.GetFullName(true)) {
-							case "gaemstone.ECS.AddAttribute`1": addedEntities.Add(type.TypeArguments[0]); break;
-							case "gaemstone.ECS.AddAttribute`2": addedRelations.Add((type.TypeArguments[0], type.TypeArguments[1])); break;
-						}
-
-				module.Components.Add(new(symbol, module, componentType,
-				                          addedEntities, addedRelations));
-			}
-		}
-
-		foreach (var module in modules.Values) {
-			var sb = new StringBuilder();
-			sb.AppendLine($$"""
-				// <auto-generated/>
-				using gaemstone.ECS;
-
-				namespace {{ module.Namespace }};
-
-				""");
-
-			if (module.IsStatic) {
-				sb.AppendLine($$"""
-					public static partial class {{ module.Name }}
-					{
-						public static void RegisterComponents(World world)
-						{
-							var module = world.LookupByPathOrThrow({{ module.Path.ToStringLiteral() }});
-					""");
-
-				foreach (var c in module.Components) {
-					var @var = $"entity{c.Name}";
-					var path = (c.Path ?? c.Name).ToStringLiteral();
-					                  sb.AppendLine($$"""		var {{ @var }} = world.LookupByPathOrThrow(module, {{ path }})""");
-					if (c.IsRelation) sb.AppendLine($$"""			.Add<gaemstone.Doc.Relation>()""");
-					                  sb.AppendLine($$"""			.CreateLookup<{{ c.Name }}>()""");
-					sb.Insert(sb.Length - 1, ";");
-				}
-			} else {
-				sb.AppendLine($$"""
-					public partial class {{ module.Name }}
-						: IModuleAutoRegisterComponents
-					{
-						public void RegisterComponents(EntityRef module)
-						{
-							var world = module.World;
-					""");
-
-				foreach (var c in module.Components) {
-					var @var = $"entity{c.Name}";
-					var path = (c.Path ?? c.Name).ToStringLiteral();
-					                        sb.AppendLine($$"""		var {{ @var }} = world.New(module, {{ path }})""");
-					if (c.IsPublic)         sb.AppendLine($$"""			.Symbol("{{ c.Name }}")""");
-					if (c.IsRelation)       sb.AppendLine($$"""			.Add<gaemstone.Doc.Relation>()""");
-					if (c.IsTag)            sb.AppendLine($$"""			.Add<gaemstone.Flecs.Core.Tag>()""");
-					if (c.IsComponent)      sb.AppendLine($$"""			.Build().InitComponent<{{ c.Name }}>()""");
-					else                    sb.AppendLine($$"""			.Build().CreateLookup<{{ c.Name }}>()""");
-					if (c.SingletonAddSelf) sb.AppendLine($$"""			.Add<{{ c.Name }}>()""");
-					sb.Insert(sb.Length - 1, ";");
-				}
-
-				foreach (var c in module.Components) {
-					var @var = $"entity{c.Name}";
-					foreach (var e in c.EntitiesToAdd)
-						sb.AppendLine($$"""		{{ @var }}.Add<{{ e.GetFullName() }}>();""");
-					foreach (var (r, t) in c.RelationsToAdd)
-						sb.AppendLine($$"""		{{ @var }}.Add<{{ r.GetFullName() }}, {{ t.GetFullName() }}>();""");
-				}
-			}
-
-			sb.AppendLine($$"""
-					}
-				}
-				""");
-
-			context.AddSource($"{module.FullName}_Components.g.cs", sb.ToString());
-		}
-	}
-
-	private class ModuleInfo
-	{
-		public INamedTypeSymbol Symbol { get; }
-		public string Name      => Symbol.Name;
-		public string FullName  => Symbol.GetFullName();
-		public string Namespace => Symbol.GetNamespace()!;
-
-		public List<ComponentInfo> Components { get; } = new();
-		public string? Path { get; }
-		public bool IsPublic { get; }
-		public bool IsStatic => Symbol.IsStatic;
-
-		public ModuleInfo(INamedTypeSymbol symbol)
-		{
-			Symbol = symbol;
-			Path   = symbol.GetAttribute("gaemstone.ECS.PathAttribute")?
-			               .ConstructorArguments.FirstOrDefault().Value as string;
-			IsPublic = symbol.HasAttribute("gaemstone.ECS.PublicAttribute");
-		}
-	}
-
-	private class ComponentInfo
-	{
-		public INamedTypeSymbol Symbol { get; }
-		public string Name => Symbol.Name;
-
-		public ModuleInfo Module { get; }
-		public RegisterType Type { get; }
-		public List<ITypeSymbol> EntitiesToAdd { get; }
-		public List<(ITypeSymbol, ITypeSymbol)> RelationsToAdd { get; }
-
-		public string? Path { get; }
-		public bool IsPublic { get; }
-		public bool IsRelation { get; }
-		public bool IsTag       => (Type is RegisterType.Tag);
-		public bool IsComponent => (Type is RegisterType.Component
-		                                 or RegisterType.Singleton);
-		public bool IsSingleton => (Type is RegisterType.Singleton);
-		public bool SingletonAddSelf { get; }
-
-		public ComponentInfo(INamedTypeSymbol symbol, ModuleInfo module, RegisterType type,
-			List<ITypeSymbol> entitiesToAdd, List<(ITypeSymbol, ITypeSymbol)> relationsToAdd)
-		{
-			Symbol = symbol;
-			Module = module;
-			Type   = type;
-
-			EntitiesToAdd  = entitiesToAdd;
-			RelationsToAdd = relationsToAdd;
-
-			Path = symbol.GetAttribute("gaemstone.ECS.PathAttribute")?
-			             .ConstructorArguments.FirstOrDefault().Value as string;
-			IsPublic = symbol.HasAttribute("gaemstone.ECS.PublicAttribute")
-				|| (Module.IsPublic && !symbol.HasAttribute("gaemstone.ECS.PrivateAttribute"));
-			IsRelation = symbol.HasAttribute("gaemstone.ECS.RelationAttribute");
-
-			SingletonAddSelf = IsSingleton
-				&& (symbol.GetAttribute("gaemstone.ECS.SingletonAttribute")!
-				         .NamedArguments.FirstOrDefault() is not ("AutoAdd", TypedConstant typedConst)
-					|| (bool)typedConst.Value!);
-		}
-	}
-
-	private enum RegisterType
-	{
-		Entity,
-		Singleton,
-		Relation,
-		Component,
-		Tag,
-	}
-}

src/gaemstone.SourceGen/Generators/ModuleGenerator.cs

@@ -1,141 +0,0 @@
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using gaemstone.SourceGen.Utility;
-using Microsoft.CodeAnalysis;
-using Microsoft.CodeAnalysis.CSharp;
-using Microsoft.CodeAnalysis.CSharp.Syntax;
-
-namespace gaemstone.SourceGen.Generators;
-
-[Generator]
-public class ModuleGenerator
-	: ISourceGenerator
-{
-	private static readonly DiagnosticDescriptor ModuleMayNotBeNested = new(
-		"gaem0001", "Module may not be nested",
-		"Type {0} marked with [Module] may not be a nested type",
-		nameof(ModuleGenerator), DiagnosticSeverity.Error, true);
-
-	private static readonly DiagnosticDescriptor ModuleMustBePartial = new(
-		"gaem0002", "Module must be partial",
-		"Type {0} marked with [Module] must be a partial type",
-		nameof(ModuleGenerator), DiagnosticSeverity.Error, true);
-
-	private static readonly DiagnosticDescriptor ModuleBuiltInMustHavePath = new(
-		"gaem0003", "Built-in module must have [Path]",
-		"Type {0} marked with [Module] is a built-in module (static), and therefore must have [Path] set",
-		nameof(ModuleGenerator), DiagnosticSeverity.Error, true);
-
-
-	public void Initialize(GeneratorInitializationContext context)
-		=> context.RegisterForSyntaxNotifications(() => new SyntaxReceiver());
-
-	private class SyntaxReceiver
-		: ISyntaxContextReceiver
-	{
-		public HashSet<INamedTypeSymbol> Symbols { get; }
-			= new(SymbolEqualityComparer.Default);
-
-		public void OnVisitSyntaxNode(GeneratorSyntaxContext context)
-		{
-			if (context.Node is not AttributeSyntax attrNode) return;
-			var model = context.SemanticModel;
-
-			var attrType = model.GetTypeInfo(attrNode).Type!;
-			if (attrType.GetFullName(true) != "gaemstone.ECS.ModuleAttribute") return;
-
-			var memberNode   = attrNode.Parent?.Parent!;
-			var memberSymbol = model.GetDeclaredSymbol(memberNode) as INamedTypeSymbol;
-			Symbols.Add(memberSymbol!);
-		}
-	}
-
-	public void Execute(GeneratorExecutionContext context)
-	{
-		if (context.SyntaxContextReceiver is not SyntaxReceiver receiver) return;
-
-		foreach (var symbol in receiver.Symbols) {
-			var isNested = (symbol.ContainingType != null);
-			if (isNested)
-				context.ReportDiagnostic(Diagnostic.Create(ModuleMayNotBeNested,
-					symbol.Locations.FirstOrDefault(), symbol.GetFullName()));
-
-			var isPartial = symbol.DeclaringSyntaxReferences
-				.Any(r => (r.GetSyntax() as ClassDeclarationSyntax)?.Modifiers
-					.Any(t => t.IsKind(SyntaxKind.PartialKeyword)) ?? false);
-			if (!isPartial)
-				context.ReportDiagnostic(Diagnostic.Create(ModuleMustBePartial,
-					symbol.Locations.FirstOrDefault(), symbol.GetFullName()));
-
-			if (symbol.IsStatic && (symbol.GetAttribute("gaemstone.ECS.PathAttribute")?
-			                              .ConstructorArguments.FirstOrDefault().Value == null))
-				context.ReportDiagnostic(Diagnostic.Create(ModuleBuiltInMustHavePath,
-					symbol.Locations.FirstOrDefault(), symbol.GetFullName()));
-
-			// Static classes can't implement interfaces.
-			if (symbol.IsStatic) continue;
-
-			var modulePath = GetModulePath(symbol).ToStringLiteral();
-			var dependencies = new List<string>();
-			foreach (var attr in symbol.GetAttributes())
-				for (var type = attr.AttributeClass; type != null; type = type.BaseType)
-					if ((type.GetFullName(true) == "gaemstone.ECS.AddAttribute`2")
-							&& type.TypeArguments[0].GetFullName() == "gaemstone.Flecs.Core.DependsOn")
-						dependencies.Add(GetModulePath(type.TypeArguments[1]));
-
-			var sb = new StringBuilder();
-			sb.AppendLine($$"""
-				// <auto-generated/>
-				using System.Collections.Generic;
-				using System.Linq;
-				using gaemstone.ECS;
-
-				namespace {{ symbol.GetNamespace() }};
-
-				public partial class {{ symbol.Name }}
-					: IModule
-				{
-					public static string ModulePath { get; }
-						= {{ modulePath }};
-
-				""");
-
-			if (dependencies.Count > 0) {
-				sb.AppendLine($$"""
-						public static IEnumerable<string> Dependencies { get; } = new[] {
-					""");
-				foreach (var dependency in dependencies)
-					sb.AppendLine($$"""		{{ dependency.ToStringLiteral() }},""");
-				sb.AppendLine($$"""
-						};
-					""");
-			} else sb.AppendLine($$"""
-					public static IEnumerable<string> Dependencies { get; } = Enumerable.Empty<string>();
-				""");
-
-
-			sb.AppendLine($$"""
-				}
-
-				""");
-
-			context.AddSource($"{symbol.GetFullName()}_Module.g.cs", sb.ToString());
-		}
-	}
-
-	private static string GetModulePath(ISymbol module)
-	{
-		var path = module.GetAttribute("gaemstone.ECS.PathAttribute")?
-							.ConstructorArguments.FirstOrDefault().Value as string;
-		var isAbsolute = (path?.FirstOrDefault() == '/');
-		if (isAbsolute) return path!;
-
-		var fullPath = module.GetFullName().Replace('.', '/');
-		if (path != null) {
-			var index = fullPath.LastIndexOf('/');
-			fullPath = $"{fullPath.Substring(0, index)}/{path}";
-		}
-		return $"/{fullPath}";
-	}
-}

src/gaemstone.SourceGen/ModuleGenerator.cs

@@ -0,0 +1,369 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using gaemstone.SourceGen.Structure;
+using gaemstone.SourceGen.Utility;
+using Microsoft.CodeAnalysis;
+
+namespace gaemstone.SourceGen;
+
+[Generator]
+public class ModuleGenerator
+	: ISourceGenerator
+{
+	public void Initialize(GeneratorInitializationContext context)
+	{
+#if DEBUG
+		// while (!System.Diagnostics.Debugger.IsAttached)
+		// 	System.Threading.Thread.Sleep(500);
+#endif
+		context.RegisterForSyntaxNotifications(
+			() => new RelevantSymbolReceiver());
+	}
+
+	private Dictionary<ISymbol, BaseInfo>? _symbolToInfoLookup;
+
+	public BaseInfo? Lookup(ISymbol symbol)
+		=> (_symbolToInfoLookup ?? throw new InvalidOperationException())
+			.TryGetValue(symbol, out var info) ? info : null;
+
+	public void Execute(GeneratorExecutionContext context)
+	{
+		if (context.SyntaxContextReceiver is not RelevantSymbolReceiver receiver) return;
+		_symbolToInfoLookup = receiver.Symbols;
+		var infos = receiver.Symbols.Values;
+
+		// Go through all entity infos (types and methods), populate their
+		// Parent property and add them to their parent's Children property.
+		foreach (var info in infos.OfType<BaseEntityInfo>()) {
+			if ((info.Symbol.ContainingType is INamedTypeSymbol parentSymbol)
+				&& (Lookup(parentSymbol) is TypeEntityInfo parentInfo))
+			{
+				info.Parent = parentInfo;
+				parentInfo.Children.Add(info);
+			}
+		}
+
+		// Go through all the method infos and lookup / construct
+		// their parameter infos from the method's parameters.
+		foreach (var info in infos.OfType<MethodEntityInfo>()) {
+			foreach (var paramSymbol in info.Symbol.Parameters) {
+				if (Lookup(paramSymbol) is not ParameterInfo paramInfo)
+					paramInfo = new ParameterInfo(paramSymbol);
+				info.Parameters.Add(paramInfo);
+				paramInfo.Parent = info;
+			}
+		}
+
+		// Validate all instances of base info without a Parent / Method set.
+		// Nested infos are validated by their containing info.
+		foreach (var info in infos.Where(info => info.Parent == null))
+			info.Validate();
+
+		// Report all the diagnostics we found.
+		foreach (var info in infos)
+			foreach (var diag in info.Diagnostics)
+				context.ReportDiagnostic(diag);
+
+		foreach (var module in infos.OfType<ModuleEntityInfo>()) {
+			if (module.IsErrored) continue;
+			var sb = new StringBuilder();
+			AppendHeader(sb, module.Namespace);
+			AppendModuleType(sb, module);
+			context.AddSource($"{module.FullName}.g.cs", sb.ToString());
+		}
+
+		_symbolToInfoLookup = null;
+	}
+
+	private void AppendHeader(StringBuilder sb, string @namespace)
+		=> sb.AppendLine($$"""
+			// <auto-generated/>
+			using System.Collections.Generic;
+			using System.Collections.Immutable;
+			using gaemstone.ECS;
+			using gaemstone.ECS.Utility;
+
+			namespace {{ @namespace }};
+
+			""");
+
+	private void AppendModuleType(StringBuilder sb, ModuleEntityInfo module)
+	{
+		var type = module.Symbol.IsValueType ? "struct" : "class";
+		sb.AppendLine($$"""
+			public partial {{ type }} {{ module.Name }}
+				: IModule
+			""");
+
+		var modulePath = module.GetModulePath().ToStringLiteral();
+		sb.AppendLine($$"""
+			{
+				static string IModule.Path { get; } = {{ modulePath }};
+
+				static bool IModule.IsBuiltIn { get; } = {{( module.IsBuiltIn ? "true" : "false" )}};
+
+			""");
+
+		var dependencies = module.GetDependencies().ToList();
+		sb.Append("\tstatic IReadOnlyList<string> IModule.Dependencies { get; } = ");
+		if (dependencies.Count > 0) {
+			sb.AppendLine("ImmutableList.Create(");
+			foreach (var dependency in dependencies)
+				sb.AppendLine($"\t\t{dependency.ToStringLiteral()},");
+			sb.Length -= 2; sb.AppendLine();
+			sb.AppendLine("\t);");
+		} else sb.AppendLine("ImmutableList.Create<string>();");
+		sb.AppendLine();
+
+		sb.AppendLine($$"""
+				static void IModule.Initialize<T>(Entity<T> module)
+				{
+					var world = module.World;
+			""");
+
+		// TODO: Might want to add things related to the module entity.
+
+		AppendEntityRegistration(sb, module);
+		AppendMethodRegistration(sb, module);
+		AppendEntityToAdd(sb, module);
+		AppendShimMethods(sb, module);
+
+		// TODO: Can BuiltIn modules have systems and such?
+
+		sb.AppendLine("\t}");
+
+		sb.AppendLine("}");
+	}
+
+	private void AppendEntityRegistration(
+		StringBuilder sb, ModuleEntityInfo module)
+	{
+		var entities = module.Children
+			.OfType<TypeEntityInfo>()
+			.Where(e => !e.IsErrored)
+			.ToList();
+		if (entities.Count == 0) return;
+
+		sb.AppendLine();
+		sb.AppendLine("\t\t// Register entities.");
+		foreach (var e in entities) {
+			var @var = $"_{e.Name}_Entity";
+			var path = (e.Path ?? e.Name).ToStringLiteral();
+			sb.AppendLine($"\t\tvar {@var} = world.New({path}, module)");
+
+			// Since this is a custom gaemstone tag, we want to add it even for [BuiltIn] modules.
+			if (e.IsRelation) sb.AppendLine("\t\t\t.Add<gaemstone.Doc.Relation>()");
+
+			if (module.IsBuiltIn)
+			{
+				sb.AppendLine($"\t\t\t.Build().CreateLookup<{e.FullName}>()");
+			}
+			else
+			{
+				// TODO: if (e.IsPublic) sb.AppendLine($"\t\t\t.Symbol(\"{e.Name}\")");
+
+				// Tags and relations in Flecs are marked as empty components.
+				if (e.IsTag || e.IsRelation) sb.AppendLine("\t\t\t.Add<gaemstone.Flecs.Core.Component>()");
+				if (e.IsTag && e.IsRelation) sb.AppendLine("\t\t\t.Add<gaemstone.Flecs.Core.Tag>()");
+
+				                   sb.Append( "\t\t\t.Build()");
+				if (e.IsComponent) sb.Append($".InitComponent<{e.FullName}>()");
+				else               sb.Append($".CreateLookup<{e.FullName}>()");
+				                   sb.AppendLine();
+
+				if (e.IsSingleton) sb.AppendLine($"\t\t\t.Add<{e.FullName}>()");
+			}
+
+			sb.Insert(sb.Length - 1, ";");
+		}
+	}
+
+	private void AppendMethodRegistration(
+		StringBuilder sb, ModuleEntityInfo module)
+	{
+		var methods = module.Children
+			.OfType<MethodEntityInfo>()
+			.Where(e => !e.IsErrored)
+			.ToList();
+		if (methods.Count == 0) return;
+
+		sb.AppendLine();
+		sb.AppendLine("\t\t// Register systems / observers.");
+		foreach (var m in methods) {
+			var @var = $"_{m.Name}_Entity";
+			var path = (m.Path ?? m.Name).ToStringLiteral();
+			sb.AppendLine($"\t\tvar {@var} = world.New({path}, module)");
+
+			sb.Append("\t\t\t.Build()");
+			if (m.IsSystem)   sb.AppendLine(".InitSystem(");
+			if (m.IsObserver) sb.AppendLine(".InitObserver(");
+
+			if (m.IsIteratorOnly) {
+				var expression = m.Expression.ToStringLiteral();
+				sb.AppendLine($"\t\t\t\tnew({expression}), {m.Name},");
+			} else {
+				sb.AppendLine("\t\t\t\tnew(");
+				foreach (var p in m.Parameters)
+					if (p.HasTerm) {
+						// TODO: Throw error if multiple Or terms appear next to each other.
+						foreach (var term in p.TermTypes) {
+							sb.Append($"\t\t\t\t\tnew Term(");
+							switch (term) {
+								case ITypeSymbol type:
+									AppendTypeEntity(sb, module, type);
+									break;
+								case ParameterInfo.Pair pair:
+									AppendTypeEntity(sb, module, pair.Relation);
+									sb.Append(',');
+									AppendTypeEntity(sb, module, pair.Target);
+									break;
+								default: throw new InvalidOperationException(
+									$"Unexpected term type {term.GetType()}");
+							}
+							sb.Append(')');
+							if (p.Source != null) {
+								sb.Append("{ Source = ");
+								AppendTypeEntity(sb, module, p.Source);
+								sb.Append(" }");
+							}
+							sb.Append(p.Kind switch {
+								ParameterKind.Or    => ".Or",
+								ParameterKind.HasOr => ".Or.None",
+								ParameterKind.Has => ".None",
+								ParameterKind.Not => ".None.Not",
+								ParameterKind.Ref => ".InOut",
+								ParameterKind.Out => ".Out",
+								_ when !p.IsValueType => ".InOut", // Reference types always imply writability.
+								_ => ".In",
+							});
+							if (p.IsNullable) sb.Append(".Optional");
+							sb.AppendLine(",");
+						}
+					}
+				if (m.Parameters.Any(p => p.HasTerm))
+					{ sb.Length -= 2; sb.AppendLine(); }
+				sb.AppendLine( "\t\t\t\t),");
+				sb.AppendLine($"\t\t\t\t_{m.Name}_Shim,");
+			}
+
+			if (m.IsObserver)
+				foreach (var ev in m.ObserverEvents!) {
+					sb.Append("\t\t\t\t");
+					AppendTypeEntity(sb, module, ev);
+					sb.AppendLine(",");
+				}
+
+			sb.Length -= 2;
+			sb.AppendLine(");");
+		}
+	}
+
+	private void AppendEntityToAdd(
+		StringBuilder sb, ModuleEntityInfo module)
+	{
+		var entities = module.Children
+			.Where(e => !e.IsErrored)
+			.Where(e => e.HasEntitiesToAdd)
+			.ToList();
+		if (entities.Count == 0) return;
+
+		sb.AppendLine();
+		sb.AppendLine("\t\t// Add things to entities.");
+		foreach (var e in entities) {
+			var @var = $"_{e.Name}_Entity";
+			foreach (var a in e.EntitiesToAdd)
+				sb.AppendLine($"\t\t{@var}.Add<{a.GetFullName()}>();");
+			foreach (var (r, t) in e.RelationsToAdd)
+				sb.AppendLine($"\t\t{@var}.Add<{r.GetFullName()}, {t.GetFullName()}>();");
+		}
+	}
+
+	private void AppendShimMethods(
+		StringBuilder sb, ModuleEntityInfo module)
+	{
+		var methods = module.Children
+			.OfType<MethodEntityInfo>()
+			.Where(m => !m.IsErrored)
+			.Where(m => !m.IsIteratorOnly)
+			.ToList();
+
+		foreach (var method in methods)
+			AppendShimMethod(sb, module, method);
+	}
+
+	private void AppendShimMethod(
+		StringBuilder sb, ModuleEntityInfo module,
+		MethodEntityInfo method)
+	{
+		sb.AppendLine();
+		sb.AppendLine($$"""
+				void _{{ method.Name }}_Shim(Iterator<T> iter)
+				{
+		""");
+
+		foreach (var param in method.Parameters)
+			if (param.HasField)
+				sb.Append($"\t\t\tvar _{param.Name}_Field = ")
+					.Append(param.IsNullable ? "iter.FieldOrEmpty" : "iter.Field")
+					.AppendLine($"<{param.FieldType!.GetFullName()}>({param.TermIndex});");
+
+		sb.AppendLine("\t\t\tfor (var i = 0; i < iter.Count; i++) {");
+
+		sb.Append($"\t\t\t\t{method.Name}");
+		if (method.IsGeneric) sb.Append("<T>");
+		sb.Append($"(");
+		foreach (var param in method.Parameters) {
+			switch (param.Kind) {
+				case ParameterKind.Unique:
+					sb.Append(param.UniqueReplacement);
+					break;
+
+				case ParameterKind.In:
+				case ParameterKind.Out:
+				case ParameterKind.Ref:
+					var modifier = param.Kind.ToString().ToLowerInvariant();
+					sb.Append(modifier).Append(' ');
+					goto case ParameterKind.Normal;
+
+				case ParameterKind.Normal:
+				case ParameterKind.Nullable:
+					// FIXME: Handle pairs.
+					sb.Append($"_{param.Name}_Field")
+						.Append(param.IsNullable ? ".GetOrNull(i)" : "[i]");
+					break;
+
+				case ParameterKind.Has:
+				case ParameterKind.Not:
+				case ParameterKind.HasOr:
+					sb.Append("default");
+					break;
+
+				case ParameterKind.Or:
+					throw new NotSupportedException(
+						$"ParameterKind {param.Kind} not supported");
+			}
+			sb.Append(", ");
+		}
+		if (method.Parameters.Any())
+			sb.Length -= 2;
+		sb.AppendLine(");");
+
+		sb.AppendLine($$"""
+						}
+					}
+			""");
+	}
+
+
+	private void AppendTypeEntity(
+		StringBuilder sb, ModuleEntityInfo module,
+		ITypeSymbol type)
+	{
+		var found = module.Children.Where(c => !c.IsErrored)
+			.Any(c => SymbolEqualityComparer.Default.Equals(c.Symbol, type));
+		sb.Append(found ? $"_{type.Name}_Entity"
+			: $"world.Entity<{type.GetFullName()}>()");
+	}
+}

src/gaemstone.SourceGen/RelevantSymbolReceiver.cs

@@ -0,0 +1,82 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using gaemstone.SourceGen.Structure;
+using gaemstone.SourceGen.Utility;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+namespace gaemstone.SourceGen;
+
+public class RelevantSymbolReceiver
+	: ISyntaxContextReceiver
+{
+	private static readonly HashSet<string> RelevantAttributeNames = new(){
+		// Base entity attributes
+		"Module",     // Can also be [Singleton]
+		"Entity",
+		"Relation",   // Can also be [Tag] or [Component]
+		"Tag",
+		"Component",
+		"Singleton",  // Implies [Component]
+		"System",
+		"Observer",
+
+		// Entity properties that specify additional info / behavior
+		"Public",
+		"Private",
+		"Path",
+		"Add",
+		"BuiltIn",    // Valid on [Module]
+		"Expression", // Valid on [System] and [Observer]
+
+		// Term properties (on [System] and [Observer] parameters)
+		"Source",
+		"Pair",
+		"Has",
+		"Not",
+		"Or",
+	};
+
+	public Dictionary<ISymbol, BaseInfo> Symbols { get; } = new(SymbolEqualityComparer.Default);
+
+	public void OnVisitSyntaxNode(GeneratorSyntaxContext context)
+	{
+		var model = context.SemanticModel;
+		if (context.Node is not AttributeSyntax node) return;
+		if (node.Parent?.Parent is not SyntaxNode parent) return;
+		if (model.GetDeclaredSymbol(parent) is not ISymbol symbol) return;
+		if (model.GetTypeInfo(node).Type is not INamedTypeSymbol type) return;
+
+		// Go through the attribute's type hierarchy to see if it matches any
+		// of the attributes we care about. This is to make sure attributes
+		// based on [Add<...>] are picked up correctly, including custom ones.
+		for (var baseType = type; baseType != null; baseType = baseType.BaseType) {
+			if ((ToRelevantAttributeName(baseType) is string name)
+				&& RelevantAttributeNames.Contains(name) // Check if we found a relevant attribute.
+				&& !Symbols.ContainsKey(symbol))         // Check if this is already a known symbol.
+			{
+				Symbols.Add(symbol, symbol switch {
+					INamedTypeSymbol typeSymbol =>
+						typeSymbol.GetAttributes().Any(attr => attr.AttributeClass!
+								.GetFullName() == "gaemstone.ECS.ModuleAttribute")
+							? new ModuleEntityInfo(typeSymbol)
+							: new TypeEntityInfo(typeSymbol),
+					IMethodSymbol methodSymbol => new MethodEntityInfo(methodSymbol),
+					IParameterSymbol paramSymbol => new ParameterInfo(paramSymbol),
+					_ => throw new InvalidOperationException(
+						$"Unhandled symbol type {symbol.GetType()}"),
+				});
+				break;
+			}
+		}
+	}
+
+	public static string? ToRelevantAttributeName(INamedTypeSymbol symbol)
+	{
+		if (symbol.GetNamespace() != "gaemstone.ECS") return null;
+		var name = symbol.MetadataName.Split('`')[0];
+		return name.EndsWith("Attribute") ? name[..^"Attribute".Length] : null;
+	}
+}

src/gaemstone.SourceGen/Structure/BaseEntityInfo.cs

@@ -0,0 +1,78 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.CodeAnalysis;
+
+namespace gaemstone.SourceGen.Structure;
+
+public abstract class BaseEntityInfo : BaseInfo
+{
+	public new TypeEntityInfo? Parent { get => (TypeEntityInfo?)base.Parent; set => base.Parent = value; }
+
+	public string? Path { get; }
+	// TODO: Rename this to something sensible, like [Symbol].
+	// public bool IsPublic { get; private set; }
+	// private bool IsPrivate { get; }
+
+	public List<INamedTypeSymbol> EntitiesToAdd { get; } = new();
+	public List<(INamedTypeSymbol, INamedTypeSymbol)> RelationsToAdd { get; } = new();
+	public bool HasEntitiesToAdd => (EntitiesToAdd.Count > 0) || (RelationsToAdd.Count > 0);
+
+	public BaseEntityInfo(ISymbol symbol)
+		: base(symbol)
+	{
+		Path = Get("Path")?.ConstructorArguments.FirstOrDefault().Value as string;
+
+		// IsPublic  = Symbol.HasAttribute("gaemstone.ECS.PublicAttribute");
+		// IsPrivate = Symbol.HasAttribute("gaemstone.ECS.PrivateAttribute");
+	}
+
+	protected override IEnumerable<Diagnostic> ValidateSelf()
+	{
+		if (this is ModuleEntityInfo) {
+			// If this entity is a module, it must not be nested.
+			if (Symbol.ContainingType != null) yield return Diagnostic.Create(
+				Descriptors.ModuleMustNotBeNested, Location);
+		} else {
+			// Otherwise, it must occur within a module
+			if (Parent is not ModuleEntityInfo) yield return Diagnostic.Create(
+				Descriptors.EntityMustBeInModule, Location);
+		}
+
+		// var moduleIsPublic = (Parent?.IsPublic == true);
+		// var inheritsPublic = (this is MethodEntityInfo); // Observers and systems don't inherit [Public] from their module.
+		// IsPublic = IsPublic || (moduleIsPublic && inheritsPublic && !IsPrivate);
+
+		// Add entities and relationships specified using [Add<...>] attributes.
+		foreach (var attr in Symbol.GetAttributes()) {
+			for (var attrType = attr.AttributeClass; attrType != null; attrType = attrType.BaseType) {
+				if (RelevantSymbolReceiver.ToRelevantAttributeName(attrType) != "Add") continue;
+
+				var allTypeArgumentsValid = true;
+				for (var i = 0; i < attrType.TypeArguments.Length; i++) {
+					var arg   = attrType.TypeArguments[i];
+					var param = attrType.TypeParameters[i];
+					if (arg is not INamedTypeSymbol) {
+						yield return Diagnostic.Create(
+							Descriptors.InvalidTypeArgument, param.Locations.Single());
+						allTypeArgumentsValid = false;
+					}
+					// TODO: Make sure entities being added have appropriate attributes as well.
+				}
+
+				if (allTypeArgumentsValid) {
+					switch (attrType.TypeArguments) {
+						case [ INamedTypeSymbol entity ]:
+							EntitiesToAdd.Add(entity);
+							break;
+						case [ INamedTypeSymbol relation, INamedTypeSymbol target ]:
+							RelationsToAdd.Add((relation, target));
+							break;
+						default: throw new InvalidOperationException(
+							"Type argument pattern matching failed");
+					}
+				}
+			}
+		}
+	}
+}

src/gaemstone.SourceGen/Structure/BaseInfo.cs

@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+using System.Linq;
+using gaemstone.SourceGen.Utility;
+using Microsoft.CodeAnalysis;
+
+namespace gaemstone.SourceGen.Structure;
+
+public abstract class BaseInfo
+{
+	public ISymbol Symbol { get; }
+
+	public BaseEntityInfo? Parent { get; set; }
+
+	public string Name      => Symbol.Name;
+	public string FullName  => Symbol.GetFullName();
+	public string Namespace => Symbol.GetNamespace()!;
+
+	public Location Location => Symbol.Locations.First();
+	public List<Diagnostic> Diagnostics { get; } = new();
+	public bool IsErrored => Diagnostics.Any(d => d.Severity == DiagnosticSeverity.Error);
+
+	public BaseInfo(ISymbol symbol) => Symbol = symbol;
+
+	public void Validate()
+	{
+		// All of the children are validated before the parent is,
+		// in case their state affects the parent in some way.
+		foreach (var child in GetChildren())
+			child.Validate();
+		foreach (var diag in ValidateSelf())
+			Diagnostics.Add(diag);
+	}
+
+	protected virtual IEnumerable<BaseInfo> GetChildren() => Enumerable.Empty<BaseInfo>();
+	protected abstract IEnumerable<Diagnostic> ValidateSelf();
+
+	protected bool Has(string name)
+		=> Get(name) != null;
+
+	protected AttributeData? Get(string name)
+		=> Symbol.GetAttributes().FirstOrDefault(attr =>
+			RelevantSymbolReceiver.ToRelevantAttributeName(attr.AttributeClass!) == name);
+}

src/gaemstone.SourceGen/Structure/MethodEntityInfo.cs

@@ -0,0 +1,85 @@
+using System.Collections.Generic;
+using System.Linq;
+using gaemstone.SourceGen.Utility;
+using Microsoft.CodeAnalysis;
+
+namespace gaemstone.SourceGen.Structure;
+
+public class MethodEntityInfo : BaseEntityInfo
+{
+	public new IMethodSymbol Symbol => (IMethodSymbol)base.Symbol;
+
+	public List<ParameterInfo> Parameters { get; } = new();
+	protected override IEnumerable<BaseInfo> GetChildren() => Parameters;
+
+	public bool IsSystem { get; }
+	public bool IsObserver { get; }
+
+	public bool IsIteratorOnly { get; }
+	public string? Expression { get; }
+	public ITypeSymbol[]? ObserverEvents { get; }
+
+	public bool IsStatic  => Symbol.IsStatic;
+	public bool IsGeneric => Symbol.IsGenericMethod;
+
+	public MethodEntityInfo(ISymbol symbol)
+		: base(symbol)
+	{
+		IsSystem   = Has("System");
+		IsObserver = Has("Observer");
+
+		IsIteratorOnly = (Parameters.Count == 1) && (Parameters[0].Symbol.GetFullName(FullNameStyle.NoGeneric) == "gaemstone.ECS.Iterator");
+		Expression     = Get("Expression")?.ConstructorArguments.FirstOrDefault().Value as string;
+		ObserverEvents = Get("Observer")?.AttributeClass!.TypeArguments.ToArray();
+	}
+
+	protected override IEnumerable<Diagnostic> ValidateSelf()
+	{
+		foreach (var diag in base.ValidateSelf()) yield return diag;
+
+		if (IsSystem && IsObserver) yield return Diagnostic.Create(
+			Descriptors.InvalidAttributeCombination, Location, "[System, Observer]");
+
+		if (Symbol.IsAbstract) yield return Diagnostic.Create(
+			Descriptors.MethodMustNotBeAbstract, Location);
+		if (Symbol.IsAsync) yield return Diagnostic.Create(
+			Descriptors.MethodMustNotBeAsync, Location);
+
+		if (Symbol.Parameters.Length == 0) yield return Diagnostic.Create(
+			Descriptors.MethodMustHaveParameters, Location);
+		if (Symbol.IsExtensionMethod) yield return Diagnostic.Create(
+			Descriptors.MethodMustNotBeExtension, Location);
+
+		if (IsIteratorOnly && (Expression == null)) yield return Diagnostic.Create(
+			Descriptors.MethodMustHaveExpression, Location);
+
+		if (!IsStatic && Parent is ModuleEntityInfo { IsSingleton: false })
+			yield return Diagnostic.Create(
+				Descriptors.InstanceMethodOnlyValidInSingleton, Location);
+
+		if (IsGeneric) {
+			if (Symbol.TypeParameters.Length > 1) yield return Diagnostic.Create(
+				Descriptors.MethodGenericParamAtMostOne, Location);
+			else {
+				var param = Symbol.TypeParameters[0];
+				if (!SymbolEqualityComparer.Default.Equals(param, Symbol.TypeArguments[0]))
+					yield return Diagnostic.Create(
+						Descriptors.MethodGenericParamMustNotBeSubstutited, param.Locations.Single());
+				if (param.HasReferenceTypeConstraint || param.HasValueTypeConstraint
+					|| param.HasUnmanagedTypeConstraint || param.HasNotNullConstraint
+					|| param.HasConstructorConstraint || (param.ConstraintTypes.Length > 0))
+					yield return Diagnostic.Create(
+						Descriptors.MethodGenericParamMustNotBeConstrained, param.Locations.Single());
+			}
+		}
+
+		// Set TermIndex of parameters.
+		var termIndex = 1;
+		foreach (var param in Parameters)
+			if (param.HasTerm)
+				param.TermIndex = termIndex++;
+
+		// TODO: Handle systems with [Source].
+		// TODO: Validate ObserverEvents.
+	}
+}

src/gaemstone.SourceGen/Structure/ModuleEntityInfo.cs

@@ -0,0 +1,60 @@
+using System.Collections.Generic;
+using System.Linq;
+using gaemstone.SourceGen.Utility;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+namespace gaemstone.SourceGen.Structure;
+
+public class ModuleEntityInfo : TypeEntityInfo
+{
+	public bool IsPartial { get; }
+	public bool IsBuiltIn { get; }
+
+	// TODO: Support [Source].
+
+	public ModuleEntityInfo(ISymbol symbol)
+		: base(symbol)
+	{
+		var classDecl = (TypeDeclarationSyntax)Symbol.DeclaringSyntaxReferences.First().GetSyntax();
+		IsPartial = classDecl.Modifiers.Any(t => t.IsKind(SyntaxKind.PartialKeyword));
+		IsBuiltIn = Has("BuiltIn");
+	}
+
+	protected override IEnumerable<Diagnostic> ValidateSelf()
+	{
+		foreach (var diag in base.ValidateSelf()) yield return diag;
+
+		if (!IsPartial) yield return Diagnostic.Create(
+			Descriptors.ModuleMustBePartial, Location);
+
+		if (IsBuiltIn && (Path == null)) yield return Diagnostic.Create(
+			Descriptors.BuiltInModuleMustHavePath, Location);
+	}
+
+	public IEnumerable<string> GetDependencies()
+	{
+		foreach (var (relation, target) in RelationsToAdd)
+			if (relation.GetFullName(FullNameStyle.NoGeneric) == "gaemstone.Flecs.Core.DependsOn")
+				yield return GetModulePath(target);
+	}
+
+	public string GetModulePath()
+		=> GetModulePath(Symbol);
+
+	private static string GetModulePath(ISymbol module)
+	{
+		var pathAttr = module.GetAttributes().FirstOrDefault(attr =>
+			attr.AttributeClass!.GetFullName() == "gaemstone.ECS.PathAttribute");
+		var path = pathAttr?.ConstructorArguments.FirstOrDefault().Value as string;
+
+		var isAbsolute = (path?.FirstOrDefault() == '/');
+		if (isAbsolute) return path!;
+
+		var fullPath = module.GetFullName().Replace('.', '/');
+		return (path != null)
+			? $"/{fullPath[..(fullPath.LastIndexOf('/'))]}/{path}"
+			: $"/{fullPath}";
+	}
+}

src/gaemstone.SourceGen/Structure/ParameterInfo.cs

@@ -0,0 +1,216 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Linq;
+using gaemstone.SourceGen.Utility;
+using Microsoft.CodeAnalysis;
+
+namespace gaemstone.SourceGen.Structure;
+
+public class ParameterInfo : BaseInfo
+{
+	public record Pair(ITypeSymbol Relation, ITypeSymbol Target);
+
+	private static readonly Dictionary<string, string> UniqueParameters = new() {
+		["gaemstone.ECS.Iterator`1"] = "iter",
+		["gaemstone.ECS.World"]      = "iter.World",
+		["gaemstone.ECS.World`1"]    = "iter.World",
+		["gaemstone.ECS.Entity"]     = "iter.Entity(i)",
+		["gaemstone.ECS.Entity`1"]   = "iter.Entity(i)",
+		["System.TimeSpan"]          = "iter.DeltaTime",
+	};
+
+	public new IParameterSymbol Symbol => (IParameterSymbol)base.Symbol;
+	public new MethodEntityInfo? Parent { get => (MethodEntityInfo?)base.Parent; set => base.Parent = value; }
+
+	public bool IsGeneric   => Symbol.Type is INamedTypeSymbol { IsGenericType: true };
+	public bool IsValueType => Symbol.Type.IsValueType;
+	public bool IsNullable  => Symbol.Type.IsNullable();
+	public bool IsByRef     => (Symbol.RefKind != RefKind.None);
+
+	public ITypeSymbol? Source { get; } // TODO: Support [Source] for each term?
+	public int? TermIndex { get; set; } // Assigned by MethodEntityInfo
+
+	public string? UniqueReplacement { get; }
+	public IReadOnlyList<object> TermTypes { get; } // Either ITypeSymbol or Pair for relationships.
+	public ITypeSymbol? FieldType { get; }
+	public ParameterKind Kind { get; }
+
+	public bool HasTerm  => (Kind != ParameterKind.Unique);
+	public bool HasField => HasTerm && !(Kind is ParameterKind.Has or ParameterKind.Not or ParameterKind.HasOr);
+
+	public ParameterInfo(ISymbol symbol)
+		: base(symbol)
+	{
+		Source = Get("Source")?.AttributeClass!.TypeArguments[0]
+			// If the type of the parameter has the [Singleton] attribute, use it as the default Source.
+			?? (Symbol.Type.HasAttribute("gaemstone.ECS.SingletonAttribute") ? Symbol.Type : null);
+
+		var typeFullName = Symbol.Type.GetFullName(FullNameStyle.Metadata);
+		if (UniqueParameters.TryGetValue(typeFullName, out var replacement))
+		{
+			Kind = ParameterKind.Unique;
+			TermTypes = Array.Empty<ITypeSymbol>();
+			UniqueReplacement = replacement;
+		}
+		else
+		{
+			var isHas = typeFullName.StartsWith("gaemstone.ECS.Has");
+			var isNot = typeFullName.StartsWith("gaemstone.ECS.Not");
+			var isOr  = typeFullName.StartsWith("gaemstone.ECS.Or");
+
+			if (IsGeneric)
+			{
+				var type = (INamedTypeSymbol)Symbol.Type; // Checked by IsGeneric.
+				var args = type.TypeArguments;
+
+				// Has<...> usually doesn't support a generic type as its own type parameter.
+				// However, Has<Or<...>> is an exception to this rule, so we check for this here.
+				if (isHas && (args is [ INamedTypeSymbol { IsGenericType: true } argType ])
+				          && argType.GetFullName(FullNameStyle.NoGeneric) == "gaemstone.ECS.Or")
+				{
+					TermTypes = argType.TypeArguments.ToImmutableList();
+					FieldType = null;
+					isOr = true;
+				}
+				else if ((isHas || isNot) && (args is [ INamedTypeSymbol relation, INamedTypeSymbol target ]))
+				{
+					TermTypes = ImmutableList.Create(new Pair(relation, target));
+					FieldType = null;
+				}
+				else
+				{
+					TermTypes = args.ToImmutableList();
+					FieldType = IsNullable ? args[0] : type;
+				}
+			}
+			else
+			{
+				TermTypes = ImmutableList.Create(Symbol.Type);
+				FieldType = Symbol.Type;
+			}
+
+			Kind = (isHas && isOr) ? ParameterKind.HasOr
+				:   isHas          ? ParameterKind.Has
+				:   isOr           ? ParameterKind.Or
+				:   isNot          ? ParameterKind.Not
+				:   IsNullable     ? ParameterKind.Nullable
+				: Symbol.RefKind switch {
+					RefKind.In    => ParameterKind.In,
+					RefKind.Out   => ParameterKind.Out,
+					RefKind.Ref   => ParameterKind.Ref,
+					_             => ParameterKind.Normal,
+				};
+		}
+	}
+
+	protected override IEnumerable<Diagnostic> ValidateSelf()
+	{
+		if (Parent == null) { yield return Diagnostic.Create(
+			Descriptors.ParamMustBeInMethod, Location); yield break; }
+
+		// TODO: Support optionals.
+		if (Symbol.IsOptional) yield return Diagnostic.Create(
+			Descriptors.ParamDoesNotSupportOptional, Location);
+
+		if (IsByRef && !IsValueType) { yield return Diagnostic.Create(
+			Descriptors.ParamByRefMustBeValueType, Location); yield break; }
+		if (IsByRef && IsNullable) { yield return Diagnostic.Create(
+			Descriptors.ParamByRefMustNotBeNullable, Location); yield break; }
+
+		if (UniqueReplacement != null)
+		{
+			if (Source != null) yield return Diagnostic.Create(
+				Descriptors.UniqueParamNotSupported, Location, "[Source]");
+
+			if (IsByRef) { yield return Diagnostic.Create(
+				Descriptors.UniqueParamMustNotBeByRef, Location); yield break; }
+			if (IsNullable) { yield return Diagnostic.Create(
+				Descriptors.UniqueParamMustNotBeNullable, Location); yield break; }
+
+			if (IsGeneric) {
+				if (!Parent.IsGeneric) { yield return Diagnostic.Create(
+					Descriptors.UniqueParamGenericMustMatch, Location); yield break; }
+				var paramParam  = ((INamedTypeSymbol)Symbol.Type).TypeArguments[0];
+				var methodParam = Parent.Symbol.TypeParameters[0];
+				if (!SymbolEqualityComparer.Default.Equals(paramParam, methodParam)) {
+					yield return Diagnostic.Create(Descriptors.UniqueParamGenericMustMatch, Location);
+					yield break;
+				}
+			}
+		}
+		else
+		{
+			var isSpecial = Kind is ParameterKind.Has or ParameterKind.HasOr
+			                     or ParameterKind.Or  or ParameterKind.Not;
+			if (isSpecial && IsByRef) yield return Diagnostic.Create(
+				Descriptors.SpecialMustNotBeByRef, Location);
+			if (isSpecial && IsNullable) yield return Diagnostic.Create(
+				Descriptors.SpecialArgMustNotBeNullable, Location);
+
+			foreach (var termTypeBase in TermTypes) {
+				var termTypes = termTypeBase switch {
+					ITypeSymbol type => new[]{ type },
+					Pair pair => new[]{ pair.Relation, pair.Target },
+					_ => throw new InvalidOperationException(
+						$"Unexpected term type {termTypeBase.GetType()}")
+				};
+
+				foreach (var termType in termTypes) {
+					var location = termType.Locations.Single();
+
+					switch (termType.TypeKind) {
+						case TypeKind.Array: yield return Diagnostic.Create(
+							Descriptors.ParamMustNotBeArray, location); continue;
+						case TypeKind.Pointer: yield return Diagnostic.Create(
+							Descriptors.ParamMustNotBePointer, location); continue;
+						case TypeKind.TypeParameter: yield return Diagnostic.Create(
+							Descriptors.ParamMustNotBeGenericType, location); continue;
+					}
+
+					if (termType.IsPrimitiveType()) yield return Diagnostic.Create(
+						Descriptors.ParamMustNotBePrimitive, location);
+
+					if (isSpecial) {
+						if (termType is INamedTypeSymbol { IsGenericType: true })
+							yield return Diagnostic.Create(
+								Descriptors.SpecialArgMustNotBeGeneric, location);
+						if (termType.IsNullable())
+							yield return Diagnostic.Create(
+								Descriptors.SpecialArgMustNotBeNullable, location);
+					}
+				}
+			}
+
+			if (IsGeneric && !isSpecial && !(IsValueType && IsNullable)) {
+				yield return Diagnostic.Create(
+					Descriptors.ParamMustNotBeGeneric, Location);
+				yield return Diagnostic.Create(
+					Descriptors.ParamMustNotBeGeneric, Location);
+			}
+		}
+	}
+}
+
+public enum ParameterKind
+{
+	Unique,   // for example "Iterator<T>", "Entity<T>", "Entity"
+	Normal,   // Pass by value (copy)
+	Nullable, // Pass by value, but may be missing (null)
+	In,  // Pass by reference (read-only)
+	Out, // Pass by reference (write-only)
+	Ref, // Pass by reference (read/write)
+	Has, // Required present (no read/write)
+	Not, // Required missing (no read/write)
+	Or,  // Only one of multiple terms is required
+	HasOr, // Both Has and Or at the same time
+}
+
+// public enum TermKind
+// {
+// 	In,  // Pass by reference (read-only)
+// 	Out, // Pass by reference (write-only)
+// 	Ref, // Pass by reference (read/write)
+// 	Has, // Required present (no read/write)
+// 	Not, // Required missing (no read/write)
+// }

src/gaemstone.SourceGen/Structure/TypeEntityInfo.cs

@@ -0,0 +1,76 @@
+using System.Collections.Generic;
+using Microsoft.CodeAnalysis;
+
+namespace gaemstone.SourceGen.Structure;
+
+public class TypeEntityInfo : BaseEntityInfo
+{
+	public new INamedTypeSymbol Symbol => (INamedTypeSymbol)base.Symbol;
+
+	public List<BaseEntityInfo> Children { get; } = new();
+	protected override IEnumerable<BaseInfo> GetChildren() => Children;
+
+	public bool IsModule { get; }
+	private bool IsEntity { get; }
+	public bool IsTag { get; }
+	public bool IsComponent { get; private set; }
+	public bool IsSingleton { get; }
+	public bool IsRelation { get; }
+
+	public bool IsValueType     => Symbol.IsValueType;
+	public bool IsReferenceType => Symbol.IsReferenceType;
+	// TODO: Make sure that component has (instance) fields, non-component entity does not have fields.
+
+	public TypeEntityInfo(ISymbol symbol)
+		: base(symbol)
+	{
+		IsModule    = Has("Module");
+		IsEntity    = Has("Entity");
+		IsTag       = Has("Tag");
+		IsComponent = Has("Component");
+		IsSingleton = Has("Singleton");
+		IsRelation  = Has("Relation");
+	}
+
+	protected override IEnumerable<Diagnostic> ValidateSelf()
+	{
+		foreach (var diag in base.ValidateSelf()) yield return diag;
+
+		if (Symbol.IsStatic) yield return Diagnostic.Create(
+			Descriptors.TypeMustNotBeStatic, Location);
+		if (Symbol.IsAbstract) yield return Diagnostic.Create(
+			Descriptors.TypeMustNotBeAbstract, Location);
+
+		var attributeList = new List<string>();
+		if (IsModule)    attributeList.Add("Module");
+		if (IsEntity)    attributeList.Add("Entity");
+		if (IsRelation)  attributeList.Add("Relation");
+		if (IsTag)       attributeList.Add("Tag");
+		if (IsComponent) attributeList.Add("Component");
+		if (IsSingleton) attributeList.Add("Singleton");
+
+		switch (attributeList) {
+			// A single attribute is valid.
+			case [ _ ]:
+			// The following are valid attribute combinations.
+			case [ "Module", "Singleton" ]:
+			case [ "Relation", "Tag" ]:
+			case [ "Relation", "Component" ]:
+				break;
+			default:
+				yield return Diagnostic.Create(
+					Descriptors.InvalidAttributeCombination, Location,
+					$"[{string.Join(", ", attributeList)}]");
+				if (IsModule) yield return Diagnostic.Create(
+					Descriptors.ValidModuleAttributesHint, Location);
+				if (IsRelation) yield return Diagnostic.Create(
+					Descriptors.ValidRelationAttributesHint, Location);
+				if (IsSingleton && IsComponent) yield return Diagnostic.Create(
+					Descriptors.SingletonImpliesComponentHint, Location);
+				break;
+		}
+
+		// Singletons are special kinds of components.
+		if (IsSingleton) IsComponent = true;
+	}
+}

src/gaemstone.SourceGen/Utility/IsExternalInit.cs

@@ -0,0 +1,8 @@
+// Unfortunately, this is necessary for record types or init
+// properties to work under netstandard2.0 target framework.
+namespace System.Runtime.CompilerServices
+{
+	public static class IsExternalInit
+	{
+	}
+}

src/gaemstone.SourceGen/Utility/SymbolExtensions.cs

@@ -7,45 +7,112 @@ namespace gaemstone.SourceGen.Utility;
 
 public static class SymbolExtensions
 {
-	public static string GetFullName(this ISymbol symbol, bool metadata = false)
+	public static bool IsNullable(this ITypeSymbol type) => type.IsValueType
+		? (type.OriginalDefinition?.SpecialType == SpecialType.System_Nullable_T)
+		: (type.NullableAnnotation == NullableAnnotation.Annotated);
+
+	public static bool IsPrimitiveType(this ITypeSymbol symbol)
+		=> symbol.SpecialType switch {
+			SpecialType.System_Boolean or
+			SpecialType.System_SByte or
+			SpecialType.System_Int16 or
+			SpecialType.System_Int32 or
+			SpecialType.System_Int64 or
+			SpecialType.System_Byte or
+			SpecialType.System_UInt16 or
+			SpecialType.System_UInt32 or
+			SpecialType.System_UInt64 or
+			SpecialType.System_Single or
+			SpecialType.System_Double or
+			SpecialType.System_Char or
+			SpecialType.System_String or
+			SpecialType.System_Object => true,
+			_ => false,
+		};
+
+	public static string GetFullName(
+		this ISymbol symbol, FullNameStyle style = FullNameStyle.Full)
 	{
 		var builder = new StringBuilder();
-		AppendFullName(symbol, builder, metadata);
+		AppendFullName(symbol, builder, style);
 		return builder.ToString();
 	}
 
-	public static void AppendFullName(this ISymbol symbol, StringBuilder builder, bool metadata = false)
+	public static void AppendFullName(
+		this ISymbol symbol, StringBuilder builder,
+		FullNameStyle style = FullNameStyle.Full)
 	{
-		if ((symbol.ContainingSymbol is ISymbol parent)
-			&& ((parent as INamespaceSymbol)?.IsGlobalNamespace != true))
+		var withGeneric  = (style != FullNameStyle.NoGeneric);
+		var withMetadata = (style == FullNameStyle.Metadata);
+
+		if ((symbol.Kind != SymbolKind.TypeParameter)
+			&& (symbol.ContainingSymbol is ISymbol parent)
+			&& (parent is not INamespaceSymbol { IsGlobalNamespace: true }))
 		{
-			AppendFullName(parent, builder, metadata);
-			builder.Append(((parent is ITypeSymbol) && metadata) ? '+' : '.');
+			AppendFullName(parent, builder, style);
+			builder.Append((withMetadata && (parent is ITypeSymbol)) ? '+' : '.');
 		}
 
-		if (!metadata && (symbol is INamedTypeSymbol typeSymbol) && typeSymbol.IsGenericType) {
+		if ((symbol is INamedTypeSymbol { IsGenericType: true } typeSymbol)
+			&& !(withGeneric && withMetadata))
+		{
 			var length = symbol.MetadataName.IndexOf('`');
 			builder.Append(symbol.MetadataName, 0, length);
-			builder.Append('<');
-			foreach (var genericType in typeSymbol.TypeArguments) {
-				AppendFullName(genericType, builder, true);
-				builder.Append(',');
+
+			if (withGeneric) {
+				builder.Append('<');
+				foreach (var arg in typeSymbol.TypeArguments) {
+					AppendFullName(arg, builder, style);
+					builder.Append(',');
+				}
+				builder.Length--; // Remove the last ',' character.
+				builder.Append('>');
 			}
-			builder.Length--; // Remove the last ',' character.
-			builder.Append('>');
-		} else builder.Append(symbol.MetadataName);
+		}
+		else builder.Append(symbol.MetadataName);
 	}
 
 	public static string? GetNamespace(this ISymbol symbol)
 		=> symbol.ContainingNamespace?.GetFullName();
 
-	public static bool HasAttribute(this ISymbol symbol, string attrMetadataName)
-		=> symbol.GetAttributes().Any(attr => attr.AttributeClass!.GetFullName(true) == attrMetadataName);
+	public static bool HasAttribute(this ISymbol symbol, string name,
+			FullNameStyle matchStyle = FullNameStyle.Metadata)
+		=> symbol.GetAttributes().Any(attr =>
+			attr.AttributeClass!.GetFullName(matchStyle) == name);
 
-	public static AttributeData? GetAttribute(this ISymbol symbol, string attrMetadataName)
-		=> symbol.GetAttributes().FirstOrDefault(attr => attr.AttributeClass!.GetFullName(true) == attrMetadataName);
+	public static AttributeData? FindAttribute(this ISymbol symbol, string name,
+			FullNameStyle matchStyle = FullNameStyle.Metadata)
+		=> symbol.GetAttributes().FirstOrDefault(attr =>
+			attr.AttributeClass!.GetFullName(matchStyle) == name);
 
 	public static string ToStringLiteral(this string? input)
 		=> (input != null) ? SyntaxFactory.LiteralExpression(SyntaxKind.StringLiteralExpression,
 			SyntaxFactory.Literal(input)).ToFullString() : "null";
 }
+
+public enum FullNameStyle
+{
+	Full,      // Namespace.Foo.Bar<Baz<Quux>>
+	Metadata,  // Namespace.Foo+Bar`1
+	NoGeneric, // Namespace.Foo.Bar
+}
+
+public struct StringifyOptions
+{
+	public static readonly StringifyOptions Default = new();
+	public static readonly StringifyOptions StripGeneric = new(){ Generic = GenericDisplayMode.None };
+	public static readonly StringifyOptions MetadataGeneric = new(){ Generic = GenericDisplayMode.Metadata };
+
+	public bool Namespace { get; set; } = true;
+	// TODO: public bool FriendlyNames { get; set; } = true;
+	public GenericDisplayMode Generic { get; set; } = GenericDisplayMode.Full;
+
+	public StringifyOptions() {  }
+
+	public enum GenericDisplayMode
+	{
+		None,     // Foo
+		Metadata, // Foo`1
+		Full,     // Foo<Bar<Baz>>
+	}
+}

src/gaemstone.SourceGen/gaemstone.SourceGen.csproj

@@ -9,6 +9,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="IndexRange" Version="1.0.2" />
     <PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="3.3.3">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>

src/gaemstone/ECS/FilterExtensions.cs

@@ -1,16 +0,0 @@
-using System;
-using System.Linq;
-using gaemstone.Utility.IL;
-
-namespace gaemstone.ECS;
-
-public static class FilterExtensions
-{
-	public static void RunOnce(World world, Delegate action)
-	{
-		var gen  = IterActionGenerator.GetOrBuild(world, action.Method);
-		var desc = new FilterDesc(gen.Terms.ToArray());
-		using var filter = new Filter(world, desc);
-		foreach (var iter in filter.Iter()) gen.RunWithTryCatch(action.Target, iter);
-	}
-}

src/gaemstone/ECS/Game.cs

@@ -1,13 +0,0 @@
-using System;
-
-namespace gaemstone.ECS;
-
-/// <summary>
-/// Entity for storing global game state and configuration.
-/// Parameters can use <see cref="GameAttribute"/> to source this entity.
-/// </summary>
-public struct Game {  }
-
-/// <summary> Equivalent to <see cref="SourceAttribute{Game}"/>. </summary>
-[AttributeUsage(AttributeTargets.Parameter)]
-public class GameAttribute : SourceAttribute<Game> {  }

src/gaemstone/ECS/Module+Attributes.cs

@@ -3,16 +3,6 @@ using static gaemstone.Flecs.Core;
 
 namespace gaemstone.ECS;
 
-/// <summary>
-/// Use a custom name or path for this entity instead of the type's name.
-/// </summary>
-[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
-public class PathAttribute : Attribute
-{
-	public string Value { get; }
-	public PathAttribute(string value) => Value = value;
-}
-
 /// <summary>
 /// <p>
 /// Components marked with this attribute are automatically registered with
@@ -24,40 +14,44 @@ public class PathAttribute : Attribute
 /// components, except ones marked with <see cref="PrivateAttribute"/>.
 /// </p>
 /// </summary>
-[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct
+              | AttributeTargets.Enum  | AttributeTargets.Method)]
 public class PublicAttribute : Attribute {  }
 
 /// <seealso cref="PublicAttribute"/>
-[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct
+              | AttributeTargets.Enum  | AttributeTargets.Method)]
 public class PrivateAttribute : Attribute {  }
 
 
+// TODO: Should this be renamed from [Add<...>] to something else?
 /// <summary>
 /// Marked entity automatically has the specified entity added to it when
-/// automatically registered. Equivalent to <see cref="EntityBase.Add{T}"/>.
+/// automatically registered. Equivalent to <see cref="EntityBuilder.Add{T}"/>.
 /// </summary>
-[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum, AllowMultiple = true)]
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct
+              | AttributeTargets.Enum  | AttributeTargets.Method,
+                AllowMultiple = true)]
 public class AddAttribute<TEntity> : Attribute {  }
 
 /// <summary>
 /// Marked entity automatically has the specified relationship pair added to it when
-/// automatically registered. Equivalent to <see cref="EntityBase.Add{TRelation, TTarget}"/>.
+/// automatically registered. Equivalent to <see cref="EntityBuilder.Add{TRelation, TTarget}"/>.
 /// </summary>
-[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum, AllowMultiple = true)]
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct
+              | AttributeTargets.Enum  | AttributeTargets.Method,
+                AllowMultiple = true)]
 public class AddAttribute<TRelation, TTarget> : Attribute {  }
 
 
 /// <seealso cref="IsA"/>
 public class IsAAttribute<TTarget> : AddAttribute<IsA, TTarget> {  }
-
 /// <seealso cref="ChildOf"/>
 public class ChildOfAttribute<TTarget> : AddAttribute<ChildOf, TTarget> {  }
-
 /// <seealso cref="DependsOn"/>
 public class DependsOnAttribute<TTarget> : AddAttribute<DependsOn, TTarget> {  }
 
 /// <seealso cref="Exclusive"/>
 public class ExclusiveAttribute : AddAttribute<Exclusive> {  }
-
 /// <seealso cref="With"/>
 public class WithAttribute<TTarget> : AddAttribute<With, TTarget> {  }

src/gaemstone/ECS/Module+Components.cs

@@ -1,27 +1,33 @@
 using System;
-using static gaemstone.Flecs.Core;
 
 namespace gaemstone.ECS;
 
+/// <summary> Use a custom name or path for this entity instead of the type's name. </summary>
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct
+              | AttributeTargets.Enum  | AttributeTargets.Method)]
+public class PathAttribute : Attribute
+{
+	public string Value { get; }
+	public PathAttribute(string value) => Value = value;
+}
+
 [AttributeUsage(AttributeTargets.Struct)]
 public class EntityAttribute : Attribute {  }
 
-/// <seealso cref="Tag"/>
+// On types marked as [Relation], this has an effect on the relation's behavior.
+// Otherwise this has the same behavior as [Entity], but informs people
 [AttributeUsage(AttributeTargets.Struct)]
 public class TagAttribute : Attribute {  }
 
-/// <seealso cref="Component"/>
 [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
 public class ComponentAttribute : Attribute {  }
 
 /// <summary>
-/// A singleton is a single instance of a tag or component that can be retrieved
+/// A singleton is a single instance of a [Component] that can be retrieved
 /// without explicitly specifying an entity in a query, where it is equivalent
 /// to <see cref="SourceAttribute{}"/> with itself as the generic type parameter.
 /// </summary>
-[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
-public class SingletonAttribute : Attribute
-	{ public bool AutoAdd { get; init; } = true; }
+public class SingletonAttribute : ComponentAttribute {  }
 
 /// <summary>
 /// Marked entity represents a relationship type.

src/gaemstone/ECS/Module.cs

@@ -3,25 +3,29 @@ using System.Collections.Generic;
 
 namespace gaemstone.ECS;
 
-[AttributeUsage(AttributeTargets.Class)]
-public class ModuleAttribute : Attribute {  }
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
+public class ModuleAttribute : SingletonAttribute {  }
 
-public interface IModuleInitializer
-{
-	void Initialize(EntityRef module);
-}
+[AttributeUsage(AttributeTargets.Class)]
+public class BuiltInAttribute : Attribute {  }
 
-// The following will be implemented on [Module]
-// types automatically for you by source generators.
 
+/// <summary>
+/// A concrete implementation of this interface is generated by a source
+/// generator for each type marked as <see cref="ModuleAttribute"/>.
+/// </summary>
 public interface IModule
 {
-	static abstract string ModulePath { get; }
+	static abstract string Path { get; }
+
+	static abstract bool IsBuiltIn { get; }
 
-	static abstract IEnumerable<string> Dependencies { get; }
+	static abstract IReadOnlyList<string> Dependencies { get; }
+
+	static abstract void Initialize<TContext>(Entity<TContext> module);
 }
 
-public interface IModuleAutoRegisterComponents
+public interface IModuleInitializer
 {
-	void RegisterComponents(EntityRef module);
+	static abstract void Initialize<TContext>(Entity<TContext> module);
 }

src/gaemstone/ECS/Observer.cs

@@ -1,45 +0,0 @@
-using System;
-using System.Linq;
-using System.Reflection;
-using gaemstone.Utility;
-using gaemstone.Utility.IL;
-
-namespace gaemstone.ECS;
-
-[AttributeUsage(AttributeTargets.Method)]
-public class ObserverAttribute : Attribute
-{
-	public Type Event { get; }
-	internal ObserverAttribute(Type @event) => Event = @event; // Use generic type instead.
-}
-public class ObserverAttribute<TEvent> : ObserverAttribute
-	{ public ObserverAttribute() : base(typeof(TEvent)) {  } }
-
-public static class ObserverExtensions
-{
-	public static EntityRef InitObserver(this World world,
-		object? instance, MethodInfo method)
-	{
-		var attr = method.Get<ObserverAttribute>() ?? throw new ArgumentException(
-			"Observer must specify ObserverAttribute", nameof(method));
-		var expr = method.Get<ExpressionAttribute>()?.Value;
-		FilterDesc filter;
-		Action<Iterator> iterAction;
-
-		var param = method.GetParameters();
-		if ((param.Length == 1) && (param[0].ParameterType == typeof(Iterator))) {
-			filter = new(expr ?? throw new Exception(
-				"Observer must specify ExpressionAttribute"));
-			if (method.IsStatic) instance = null;
-			iterAction = (Action<Iterator>)Delegate.CreateDelegate(
-				typeof(Action<Iterator>), instance, method);
-		} else {
-			var gen = IterActionGenerator.GetOrBuild(world, method);
-			filter  = (expr != null) ? new(expr) : new(gen.Terms.ToArray());
-			iterAction = iter => gen.RunWithTryCatch(instance, iter);
-		}
-
-		var @event = world.LookupByTypeOrThrow(attr.Event);
-		return world.New(method.Name).Build().InitObserver(@event, filter, iterAction);
-	}
-}

src/gaemstone/ECS/System+Terms.cs

@@ -0,0 +1,88 @@
+using System;
+using gaemstone.Utility;
+
+namespace gaemstone.ECS;
+
+[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Method)]
+public class SourceAttribute<T> : Attribute {  }
+
+
+// TODO: Implement [Pair] somehow.
+[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.GenericParameter)]
+public class PairAttribute<TRelation, TTarget> : Attribute {  }
+
+public static class Pair<TRelation, TTarget>
+{
+	[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.GenericParameter)]
+	public class RelationAttribute : Attribute {  }
+	[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.GenericParameter)]
+	public class TargetAttribute : Attribute {  }
+}
+
+
+public readonly ref struct Has<T> {  }
+public readonly ref struct Has<TRelation, TTarget> {  }
+
+public readonly ref struct Not<T> {  }
+public readonly ref struct Not<TRelation, TTarget> {  }
+
+
+public readonly struct Or<T1, T2>
+{
+	private readonly Union<T1, T2> _union;
+	public int Case { get; }
+
+	public Or(T1 value) { Case = 1; _union = new() { Value1 = value }; }
+	public Or(T2 value) { Case = 2; _union = new() { Value2 = value }; }
+
+	public T1 Value1 => (Case == 1) ? _union.Value1 : throw new InvalidOperationException();
+	public T2 Value2 => (Case == 2) ? _union.Value2 : throw new InvalidOperationException();
+}
+
+public readonly struct Or<T1, T2, T3>
+{
+	private readonly Union<T1, T2, T3> _union;
+	public int Case { get; }
+
+	public Or(T1 value) { Case = 1; _union = new() { Value1 = value }; }
+	public Or(T2 value) { Case = 2; _union = new() { Value2 = value }; }
+	public Or(T3 value) { Case = 3; _union = new() { Value3 = value }; }
+
+	public T1 Value1 => (Case == 1) ? _union.Value1 : throw new InvalidOperationException();
+	public T2 Value2 => (Case == 2) ? _union.Value2 : throw new InvalidOperationException();
+	public T3 Value3 => (Case == 3) ? _union.Value3 : throw new InvalidOperationException();
+}
+
+public readonly struct Or<T1, T2, T3, T4>
+{
+	private readonly Union<T1, T2, T3, T4> _union;
+	public int Case { get; }
+
+	public Or(T1 value) { Case = 1; _union = new() { Value1 = value }; }
+	public Or(T2 value) { Case = 2; _union = new() { Value2 = value }; }
+	public Or(T3 value) { Case = 3; _union = new() { Value3 = value }; }
+	public Or(T4 value) { Case = 4; _union = new() { Value4 = value }; }
+
+	public T1 Value1 => (Case == 1) ? _union.Value1 : throw new InvalidOperationException();
+	public T2 Value2 => (Case == 2) ? _union.Value2 : throw new InvalidOperationException();
+	public T3 Value3 => (Case == 3) ? _union.Value3 : throw new InvalidOperationException();
+	public T4 Value4 => (Case == 4) ? _union.Value4 : throw new InvalidOperationException();
+}
+
+public readonly struct Or<T1, T2, T3, T4, T5>
+{
+	private readonly Union<T1, T2, T3, T4, T5> _union;
+	public int Case { get; }
+
+	public Or(T1 value) { Case = 1; _union = new() { Value1 = value }; }
+	public Or(T2 value) { Case = 2; _union = new() { Value2 = value }; }
+	public Or(T3 value) { Case = 3; _union = new() { Value3 = value }; }
+	public Or(T4 value) { Case = 4; _union = new() { Value4 = value }; }
+	public Or(T5 value) { Case = 5; _union = new() { Value5 = value }; }
+
+	public T1 Value1 => (Case == 1) ? _union.Value1 : throw new InvalidOperationException();
+	public T2 Value2 => (Case == 2) ? _union.Value2 : throw new InvalidOperationException();
+	public T3 Value3 => (Case == 3) ? _union.Value3 : throw new InvalidOperationException();
+	public T4 Value4 => (Case == 4) ? _union.Value4 : throw new InvalidOperationException();
+	public T5 Value5 => (Case == 5) ? _union.Value5 : throw new InvalidOperationException();
+}

src/gaemstone/ECS/System.cs

@@ -1,21 +1,20 @@
 using System;
-using System.Linq;
-using System.Reflection;
-using gaemstone.Flecs;
-using gaemstone.Utility;
-using gaemstone.Utility.IL;
 
 namespace gaemstone.ECS;
 
 [AttributeUsage(AttributeTargets.Method)]
-public class SystemAttribute : Attribute
-{
-	public Type Phase { get; }
-	public SystemAttribute() : this(typeof(SystemPhase.OnUpdate)) {  }
-	internal SystemAttribute(Type phase) => Phase = phase; // Use generic type instead.
-}
-public class SystemAttribute<TPhase> : SystemAttribute
-	{ public SystemAttribute() : base(typeof(TPhase)) {  } }
+public class SystemAttribute : Attribute {  }
+
+[AttributeUsage(AttributeTargets.Method)]
+public class ObserverAttribute<T> : Attribute {  }
+[AttributeUsage(AttributeTargets.Method)]
+public class ObserverAttribute<T1, T2> : Attribute {  }
+[AttributeUsage(AttributeTargets.Method)]
+public class ObserverAttribute<T1, T2, T3> : Attribute {  }
+[AttributeUsage(AttributeTargets.Method)]
+public class ObserverAttribute<T1, T2, T3, T4> : Attribute {  }
+[AttributeUsage(AttributeTargets.Method)]
+public class ObserverAttribute<T1, T2, T3, T4, T5> : Attribute {  }
 
 [AttributeUsage(AttributeTargets.Method)]
 public class ExpressionAttribute : Attribute
@@ -23,48 +22,3 @@ public class ExpressionAttribute : Attribute
 	public string Value { get; }
 	public ExpressionAttribute(string value) => Value = value;
 }
-
-public static class SystemExtensions
-{
-	public static EntityRef InitSystem(this World world, Delegate action)
-	{
-		var attr = action.Method.Get<SystemAttribute>();
-		var expr = action.Method.Get<ExpressionAttribute>()?.Value;
-
-		QueryDesc query;
-		if (action is Action<Iterator> callback) {
-			query = new(expr ?? throw new ArgumentException(
-				"System must specify ExpressionAttribute", nameof(action)));
-		} else {
-			var gen  = IterActionGenerator.GetOrBuild(world, action.Method);
-			query    = (expr != null) ? new(expr) : new(gen.Terms.ToArray());
-			callback = iter => gen.RunWithTryCatch(action.Target, iter);
-		}
-
-		var phase = world.LookupByTypeOrThrow(attr?.Phase ?? typeof(SystemPhase.OnUpdate));
-		return world.New(action.Method.Name).Build().InitSystem(phase, query, callback);
-	}
-
-	public static EntityRef InitSystem(this World world,
-		object? instance, MethodInfo method)
-	{
-		var attr = method.Get<SystemAttribute>();
-		var expr = method.Get<ExpressionAttribute>()?.Value;
-
-		QueryDesc query;
-		Action<Iterator> callback;
-		var param = method.GetParameters();
-		if ((param.Length == 1) && (param[0].ParameterType == typeof(Iterator))) {
-			query = new(expr ?? throw new ArgumentException(
-				"System must specify ExpressionAttribute", nameof(method)));
-			callback = (Action<Iterator>)Delegate.CreateDelegate(typeof(Action<Iterator>), instance, method);
-		} else {
-			var gen  = IterActionGenerator.GetOrBuild(world, method);
-			query    = (expr != null) ? new(expr) : new(gen.Terms.ToArray());
-			callback = iter => gen.RunWithTryCatch(instance, iter);
-		}
-
-		var phase = world.LookupByTypeOrThrow(attr?.Phase ?? typeof(SystemPhase.OnUpdate));
-		return world.New(method.Name).Build().InitSystem(phase, query, callback);
-	}
-}

src/gaemstone/ECS/TermAttributes.cs

@@ -1,32 +0,0 @@
-using System;
-
-namespace gaemstone.ECS;
-
-[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Method)]
-public class SourceAttribute : Attribute
-{
-	public Type Type { get; }
-	// TODO: Support path as source too.
-	internal SourceAttribute(Type type) => Type = type;
-}
-public class SourceAttribute<TEntity> : SourceAttribute
-	{ public SourceAttribute() : base(typeof(TEntity)) {  } }
-
-
-// Parameters types marked with [Tag] are equivalent to [Has].
-[AttributeUsage(AttributeTargets.Parameter)]
-public class HasAttribute : Attribute {  }
-
-// Parameters with "in" modifier are equivalent to [In].
-[AttributeUsage(AttributeTargets.Parameter)]
-public class InAttribute : Attribute {  }
-
-// Parameters with "out" modifier are equivalent to [Out].
-[AttributeUsage(AttributeTargets.Parameter)]
-public class OutAttribute : Attribute {  }
-
-[AttributeUsage(AttributeTargets.Parameter)]
-public class OrAttribute : Attribute {  }
-
-[AttributeUsage(AttributeTargets.Parameter)]
-public class NotAttribute : Attribute {  }

src/gaemstone/Flecs/Core.cs

@@ -2,8 +2,8 @@ using gaemstone.ECS;
 
 namespace gaemstone.Flecs;
 
-[Module, Path("/flecs/core")]
-public static partial class Core
+[BuiltIn, Module, Path("/flecs/core")]
+public partial class Core
 {
 	// Entity Tags
 
@@ -17,27 +17,23 @@ public static partial class Core
 	[Tag] public struct Disabled {  }
 	[Tag] public struct Empty {  }
 
-	// Can't be in a module class with the same name.
-	// [Path("/flecs/system/System")]
-	// [Tag] public struct System {  }
 
 	// Entities
 
-	// Conflicts with World class, and not required?
-	// [Entity]    public struct World {  }
-
 	[Entity, Path("*")] public struct Wildcard {  }
 	[Entity, Path("_")] public struct Any {  }
 	[Entity]            public struct This {  }
 	[Entity, Path("$")] public struct Variable {  }
 	[Entity]            public struct Flag {  }
 
+
 	// Entity Relationships
 
 	[Relation, Tag] public struct IsA {  }
 	[Relation, Tag] public struct ChildOf {  }
 	[Relation, Tag] public struct DependsOn {  }
 
+
 	// Component / Relationship Properties
 
 	[Tag] public struct Transitive {  }
@@ -52,6 +48,27 @@ public static partial class Core
 	[Relation, Tag] public struct With {  }
 	[Tag] public struct OneOf {  }
 
+
+	// Observer Events
+
+	[Entity] public struct OnAdd {  }
+	[Entity] public struct OnRemove {  }
+	[Entity] public struct OnSet {  }
+	[Entity] public struct UnSet {  }
+	[Entity] public struct OnTableEmpty {  }
+	[Entity] public struct OnTableFilled {  }
+
+
+	// Related to Deletion Events
+
+	[Relation, Tag] public struct OnDelete {  }
+	[Relation, Tag] public struct OnDeleteTarget {  }
+
+	[Tag] public struct Remove {  }
+	[Tag] public struct Delete {  }
+	[Tag] public struct Panic {  }
+
+
 	// Components
 
 	[Component]
@@ -75,6 +92,7 @@ public static partial class Core
 #pragma warning restore
 
 		public override string? ToString() { unsafe {
+			// TODO: What does Flecs do if you give it "" as an identifier?
 			if ((_value == null) || (_length == 0)) return null;
 			else return new UTF8View(new(_value, (int)_length)).ToString();
 		} }

src/gaemstone/Flecs/DeletionEvent.cs

@@ -1,18 +0,0 @@
-using gaemstone.ECS;
-
-namespace gaemstone.Flecs;
-
-[Module, Path("/flecs/core")]
-public static partial class DeletionEvent
-{
-	[Relation, Tag] public struct OnDelete {  }
-	[Relation, Tag] public struct OnDeleteTarget {  }
-}
-
-[Module, Path("/flecs/core")]
-public static partial class DeletionBehavior
-{
-	[Tag] public struct Remove {  }
-	[Tag] public struct Delete {  }
-	[Tag] public struct Panic {  }
-}

src/gaemstone/Flecs/Doc.cs

@@ -1,13 +1,12 @@
-using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using gaemstone.ECS;
-using gaemstone.Utility;
+using gaemstone.ECS.Utility;
 using static flecs_hub.flecs;
 
 namespace gaemstone.Flecs;
 
-[Module, Path("/flecs/doc")]
-public static partial class Doc
+[BuiltIn, Module, Path("/flecs/doc")]
+public partial class Doc
 {
 	[Tag] public struct Brief {  }
 	[Tag] public struct Detail {  }
@@ -28,52 +27,44 @@ public static partial class Doc
 
 public static unsafe class DocExtensions
 {
-	private static EntityRef Set<T>(EntityRef entity, string? value)
+	private static Entity<TContext> Set<TContext, T>(Entity<TContext> entity, string? value)
 	{
-		var world = entity.World;
-		var id = IdRef.Pair<Doc.Description, T>(world);
+		var id = entity.World.Pair<Doc.Description, T>();
 
-		var hadId = entity.Has(id);
-		var alloc = GlobalHeapAllocator.Instance;
 		if (value != null) {
-			var ptr = ecs_get_mut_id(world, entity, id);
-			ref var desc = ref Unsafe.AsRef<Doc.Description>(ptr);
-			// FIXME: Why does freeing these cause crashes?
-			// if (has) alloc.Free((nint)desc.Value); // Free previous value.
-			desc.Value = (void*)(nint)alloc.AllocateCString(value);
-		} else if (hadId) {
-			// var @ref = ecs_ref_init_id(world, entity, id);
-			// var ptr  = ecs_ref_get_id(world, &@ref, id);
-			// var desc = Unsafe.AsRef<Doc.Description>(ptr);
-			// alloc.Free((nint)desc.Value); // Free previous value.
+			var str  = GlobalHeapAllocator.Instance.AllocateCString(value);
+			var desc = new Doc.Description { Value = (void*)(nint)str };
+			entity.Set(id, desc);
+		} else {
 			entity.Remove(id);
 		}
+
 		return entity;
 	}
 
-	public static string? GetDocName(this EntityRef entity, bool fallbackToEntityName = true)
+	public static string? GetDocName<TContext>(this Entity<TContext> entity, bool fallbackToEntityName = true)
 		=> fallbackToEntityName || entity.Has<Doc.Description, Core.Name>()
 			? ecs_doc_get_name(entity.World, entity).FlecsToString() : null;
-	public static EntityRef SetDocName(this EntityRef entity, string? value)
-		=> Set<Core.Name>(entity, value);
+	public static Entity<TContext> SetDocName<TContext>(this Entity<TContext> entity, string? value)
+		=> Set<TContext, Core.Name>(entity, value);
 
-	public static string? GetDocBrief(this EntityRef entity)
+	public static string? GetDocBrief<TContext>(this Entity<TContext> entity)
 		=> ecs_doc_get_brief(entity.World, entity).FlecsToString()!;
-	public static EntityRef SetDocBrief(this EntityRef entity, string? value)
-		=> Set<Doc.Brief>(entity, value);
+	public static Entity<TContext> SetDocBrief<TContext>(this Entity<TContext> entity, string? value)
+		=> Set<TContext, Doc.Brief>(entity, value);
 
-	public static string? GetDocDetail(this EntityRef entity)
+	public static string? GetDocDetail<TContext>(this Entity<TContext> entity)
 		=> ecs_doc_get_detail(entity.World, entity).FlecsToString()!;
-	public static EntityRef SetDocDetail(this EntityRef entity, string? value)
-		=> Set<Doc.Detail>(entity, value);
+	public static Entity<TContext> SetDocDetail<TContext>(this Entity<TContext> entity, string? value)
+		=> Set<TContext, Doc.Detail>(entity, value);
 
-	public static string? GetDocLink(this EntityRef entity)
+	public static string? GetDocLink<TContext>(this Entity<TContext> entity)
 		=> ecs_doc_get_link(entity.World, entity).FlecsToString()!;
-	public static EntityRef SetDocLink(this EntityRef entity, string? value)
-		=> Set<Doc.Link>(entity, value);
+	public static Entity<TContext> SetDocLink<TContext>(this Entity<TContext> entity, string? value)
+		=> Set<TContext, Doc.Link>(entity, value);
 
-	public static string? GetDocColor(this EntityRef entity)
+	public static string? GetDocColor<TContext>(this Entity<TContext> entity)
 		=> ecs_doc_get_color(entity.World, entity).FlecsToString()!;
-	public static EntityRef SetDocColor(this EntityRef entity, string? value)
-		=> Set<Doc.Color>(entity, value);
+	public static Entity<TContext> SetDocColor<TContext>(this Entity<TContext> entity, string? value)
+		=> Set<TContext, Doc.Color>(entity, value);
 }

src/gaemstone/Flecs/ObserverEvent.cs

@@ -1,14 +0,0 @@
-using gaemstone.ECS;
-
-namespace gaemstone.Flecs;
-
-[Module, Path("/flecs/core")]
-public static partial class ObserverEvent
-{
-	[Entity] public struct OnAdd {  }
-	[Entity] public struct OnRemove {  }
-	[Entity] public struct OnSet {  }
-	[Entity] public struct UnSet {  }
-	[Entity] public struct OnTableEmpty {  }
-	[Entity] public struct OnTableFilled {  }
-}

src/gaemstone/Flecs/Pipeline.cs

@@ -2,8 +2,92 @@ using gaemstone.ECS;
 
 namespace gaemstone.Flecs;
 
-[Module, Path("/flecs/pipeline")]
-public static partial class Pipeline
+[BuiltIn, Module, Path("/flecs/pipeline")]
+public partial class Pipeline
 {
 	[Entity] public struct Phase {  }
+
+
+	[Entity, Add<Phase>]
+	public struct PreFrame {  }
+
+	/// <summary>
+	/// This phase contains all the systems that load data into your ECS.
+	/// This would be a good place to load keyboard and mouse inputs.
+	/// </summary>
+	[Entity, Add<Phase>]
+	[DependsOn<PreFrame>]
+	public struct OnLoad {  }
+
+	/// <summary>
+	/// Often the imported data needs to be processed. Maybe you want to associate
+	/// your keypresses with high level actions rather than comparing explicitly
+	/// in your game code if the user pressed the 'K' key.
+	/// </summary>
+	[Entity, Add<Phase>]
+	[DependsOn<OnLoad>]
+	public struct PostLoad {  }
+
+	/// <summary>
+	/// Now that the input is loaded and processed, it's time to get ready to
+	/// start processing our game logic. Anything that needs to happen after
+	/// input processing but before processing the game logic can happen here.
+	/// This can be a good place to prepare the frame, maybe clean up some
+	/// things from the previous frame, etcetera.
+	/// </summary>
+	[Entity, Add<Phase>]
+	[DependsOn<PostLoad>]
+	public struct PreUpdate {  }
+
+	/// <summary>
+	/// This is usually where the magic happens! This is where you put all of
+	/// your gameplay systems. By default systems are added to this phase.
+	/// </summary>
+	[Entity, Add<Phase>]
+	[DependsOn<PreUpdate>]
+	public struct OnUpdate {  }
+
+	/// <summary>
+	/// This phase was introduced to deal with validating the state of the game
+	/// after processing the gameplay systems. Sometimes you entities too close
+	/// to each other, or the speed of an entity is increased too much.
+	/// This phase is for righting that wrong. A typical feature to implement
+	/// in this phase would be collision detection.
+	/// </summary>
+	[Entity, Add<Phase>]
+	[DependsOn<OnUpdate>]
+	public struct OnValidate {  }
+
+	/// <summary>
+	/// When your game logic has been updated, and your validation pass has ran,
+	/// you may want to apply some corrections. For example, if your collision
+	/// detection system detected collisions in the <c>OnValidate</c> phase,
+	/// you may want to move the entities so that they no longer overlap.
+	/// </summary>
+	[Entity, Add<Phase>]
+	[DependsOn<OnValidate>]
+	public struct PostUpdate {  }
+
+	/// <summary>
+	/// Now that all of the frame data is computed, validated and corrected for,
+	/// it is time to prepare the frame for rendering. Any systems that need to
+	/// run before rendering, but after processing the game logic should go here.
+	/// A good example would be a system that calculates transform matrices from
+	/// a scene graph.
+	/// </summary>
+	[Entity, Add<Phase>]
+	[DependsOn<PostUpdate>]
+	public struct PreStore {  }
+
+	/// <summary>
+	/// This is where it all comes together. Your frame is ready to be
+	/// rendered, and that is exactly what you would do in this phase.
+	/// </summary>
+	[Entity, Add<Phase>]
+	[DependsOn<PreStore>]
+	public struct OnStore {  }
+
+	[Entity, Add<Phase>]
+	[DependsOn<OnStore>]
+	public struct PostFrame {  }
 }

src/gaemstone/Flecs/SystemPhase.cs

@@ -1,91 +0,0 @@
-using gaemstone.ECS;
-using static gaemstone.Flecs.Pipeline;
-
-namespace gaemstone.Flecs;
-
-[Module, Path("/flecs/pipeline")]
-public static partial class SystemPhase
-{
-	[Entity, Add<Phase>]
-	public struct PreFrame {  }
-
-	/// <summary>
-	/// This phase contains all the systems that load data into your ECS.
-	/// This would be a good place to load keyboard and mouse inputs.
-	/// </summary>
-	[Entity, Add<Phase>]
-	[DependsOn<PreFrame>]
-	public struct OnLoad {  }
-
-	/// <summary>
-	/// Often the imported data needs to be processed. Maybe you want to associate
-	/// your keypresses with high level actions rather than comparing explicitly
-	/// in your game code if the user pressed the 'K' key.
-	/// </summary>
-	[Entity, Add<Phase>]
-	[DependsOn<OnLoad>]
-	public struct PostLoad {  }
-
-	/// <summary>
-	/// Now that the input is loaded and processed, it's time to get ready to
-	/// start processing our game logic. Anything that needs to happen after
-	/// input processing but before processing the game logic can happen here.
-	/// This can be a good place to prepare the frame, maybe clean up some
-	/// things from the previous frame, etcetera.
-	/// </summary>
-	[Entity, Add<Phase>]
-	[DependsOn<PostLoad>]
-	public struct PreUpdate {  }
-
-	/// <summary>
-	/// This is usually where the magic happens! This is where you put all of
-	/// your gameplay systems. By default systems are added to this phase.
-	/// </summary>
-	[Entity, Add<Phase>]
-	[DependsOn<PreUpdate>]
-	public struct OnUpdate {  }
-
-	/// <summary>
-	/// This phase was introduced to deal with validating the state of the game
-	/// after processing the gameplay systems. Sometimes you entities too close
-	/// to each other, or the speed of an entity is increased too much.
-	/// This phase is for righting that wrong. A typical feature to implement
-	/// in this phase would be collision detection.
-	/// </summary>
-	[Entity, Add<Phase>]
-	[DependsOn<OnUpdate>]
-	public struct OnValidate {  }
-
-	/// <summary>
-	/// When your game logic has been updated, and your validation pass has ran,
-	/// you may want to apply some corrections. For example, if your collision
-	/// detection system detected collisions in the <c>OnValidate</c> phase,
-	/// you may want to move the entities so that they no longer overlap.
-	/// </summary>
-	[Entity, Add<Phase>]
-	[DependsOn<OnValidate>]
-	public struct PostUpdate {  }
-
-	/// <summary>
-	/// Now that all of the frame data is computed, validated and corrected for,
-	/// it is time to prepare the frame for rendering. Any systems that need to
-	/// run before rendering, but after processing the game logic should go here.
-	/// A good example would be a system that calculates transform matrices from
-	/// a scene graph.
-	/// </summary>
-	[Entity, Add<Phase>]
-	[DependsOn<PostUpdate>]
-	public struct PreStore {  }
-
-	/// <summary>
-	/// This is where it all comes together. Your frame is ready to be
-	/// rendered, and that is exactly what you would do in this phase.
-	/// </summary>
-	[Entity, Add<Phase>]
-	[DependsOn<PreStore>]
-	public struct OnStore {  }
-
-	[Entity, Add<Phase>]
-	[DependsOn<OnStore>]
-	public struct PostFrame {  }
-}

src/gaemstone/Flecs/Systems/Monitor.cs

@@ -1,19 +1,20 @@
 using System.Runtime.InteropServices;
 using gaemstone.ECS;
-using gaemstone.Utility;
+using gaemstone.ECS.Utility;
 using static flecs_hub.flecs;
 
 namespace gaemstone.Flecs.Systems;
 
-[Module, Path("/flecs/monitor")]
+[BuiltIn, Module, Path("/flecs/monitor")]
 public unsafe partial class Monitor
 	: IModuleInitializer
 {
-	public void Initialize(EntityRef module)
+	public static void Initialize<T>(Entity<T> module)
 	{
 		using var alloc = TempAllocator.Use();
-		ecs_import_c(module.World, new() { Data = new() {
-			Pointer = &MonitorImport } }, alloc.AllocateCString("FlecsMonitor"));
+		ecs_import_c(module.World,
+			new() { Data = new() { Pointer = &MonitorImport } },
+			alloc.AllocateCString("FlecsMonitor"));
 	}
 
 	[UnmanagedCallersOnly]

src/gaemstone/Flecs/Systems/Rest.cs

@@ -1,19 +1,20 @@
 using System.Runtime.InteropServices;
 using gaemstone.ECS;
-using gaemstone.Utility;
+using gaemstone.ECS.Utility;
 using static flecs_hub.flecs;
 
 namespace gaemstone.Flecs.Systems;
 
-[Module, Path("/flecs/rest")]
+[BuiltIn, Module, Path("/flecs/rest")]
 public unsafe partial class Rest
 	: IModuleInitializer
 {
-	public void Initialize(EntityRef module)
+	public static void Initialize<T>(Entity<T> module)
 	{
 		using (var alloc = TempAllocator.Use())
-			ecs_import_c(module.World, new() { Data = new() {
-				Pointer = &RestImport } }, alloc.AllocateCString("FlecsRest"));
+			ecs_import_c(module.World,
+				new() { Data = new() { Pointer = &RestImport } },
+				alloc.AllocateCString("FlecsRest"));
 
 		module.NewChild("Rest").Build()
 			.CreateLookup<EcsRest>()

src/gaemstone/Universe+Modules.cs

@@ -1,30 +1,31 @@
 using System;
+using System.Collections;
 using System.Collections.Generic;
-using System.Reflection;
+using System.Collections.Immutable;
+using System.Linq;
 using gaemstone.ECS;
-using gaemstone.Utility;
 using static gaemstone.Flecs.Core;
 using Module = gaemstone.Flecs.Core.Module;
 
 namespace gaemstone;
 
-public class ModuleManager
+public class ModuleManager<TContext>
+	: IEnumerable<ModuleManager<TContext>.IModuleInfo>
 {
-	private readonly Dictionary<Entity, ModuleInfo> _modules = new();
+	private readonly Dictionary<Entity<TContext>, IModuleInfo> _modules = new();
 
-	public Universe Universe { get; }
-	public ModuleManager(Universe universe)
+	public Universe<TContext> Universe { get; }
+	public ModuleManager(Universe<TContext> universe)
 		=> Universe = universe;
 
-	internal ModuleInfo? Lookup(Entity entity)
+	internal IModuleInfo? Lookup(Entity<TContext> entity)
 		=> _modules.GetValueOrDefault(entity);
 
-	public EntityRef Register<T>()
-		where T : class, IModule, new()
+	public Entity<TContext> Register<T>()
+		where T : IModule
 	{
-		var moduleType = typeof(T);
-		if (moduleType.IsGenericType) throw new Exception(
-			$"Module {moduleType} must be a non-generic class");
+		// if (!typeof(T).IsAssignableTo(typeof(IModule))) throw new ArgumentException(
+		// 	$"The specified type {typeof(T)} does not implement IModule", nameof(T));
 
 		var module = new ModuleInfo<T>(Universe);
 		_modules.Add(module.Entity, module);
@@ -32,87 +33,97 @@ public class ModuleManager
 		return module.Entity;
 	}
 
-	private void TryEnableModule(ModuleInfo module)
+	private void TryEnableModule(IModuleInfo module)
 	{
-		if (module.UnmetDependencies.Count > 0) return;
+		if (!module.Dependencies.All(dep => dep.IsDependencyMet)) return;
+
+		Console.WriteLine($"Enabling module {module.Entity.Path}");
 
-		Console.WriteLine($"Enabling module {module.Entity.GetFullPath()} ...");
 		module.Enable();
 
 		// Find other modules that might be missing this module as a dependency.
 		foreach (var other in _modules.Values) {
-			if (other.IsActive) continue;
-			if (!other.UnmetDependencies.Contains(module.Entity)) continue;
+			if (other.IsInitialized) continue;
+			var dependency = other.Dependencies.FirstOrDefault(dep => dep.Entity == module.Entity);
+			if (dependency == null) continue;
 
-			// Move the just enabled module from unmet to met depedencies.
-			other.UnmetDependencies.Remove(module.Entity);
-			other.MetDependencies.Add(module);
+			dependency.Info = module;
+			dependency.IsDependencyMet = true;
 
 			TryEnableModule(other);
 		}
 	}
 
-	internal abstract class ModuleInfo
+	public interface IModuleInfo
 	{
-		public abstract EntityRef Entity { get; }
-		public abstract bool IsActive { get; }
+		Entity<TContext> Entity { get; }
+		IReadOnlyCollection<ModuleDependency> Dependencies { get; }
+		bool IsInitialized { get; }
+		void Enable();
+	}
 
-		public HashSet<ModuleInfo> MetDependencies { get; } = new();
-		public HashSet<Entity> UnmetDependencies { get; } = new();
+	// IEnumerable implementation
+	public IEnumerator<IModuleInfo> GetEnumerator() => _modules.Values.GetEnumerator();
+	IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
-		public abstract void Enable();
-	}
 
-	internal class ModuleInfo<T> : ModuleInfo
-		where T : IModule, new()
+	public class ModuleDependency
 	{
-		public override EntityRef Entity { get; }
-		public override bool IsActive => (Instance != null);
-		public T? Instance { get; internal set; }
+		public Entity<TContext> Entity { get; }
+		public IModuleInfo? Info { get; internal set; }
+		public bool IsDependencyMet { get; internal set; }
 
-		public ModuleInfo(Universe universe)
+		public ModuleDependency(Entity<TContext> entity,
+			IModuleInfo? info = null, bool isDependencyMet = false)
 		{
-			var builder = universe.New(T.ModulePath).Add<Module>();
-
-			foreach (var dependsPath in T.Dependencies) {
-				var dependency = universe.LookupByPath(dependsPath) ??
-					universe.New(dependsPath).Add<Module>().Disable().Build();
+			Entity = entity;
+			Info = info;
+			IsDependencyMet = isDependencyMet;
+		}
+	}
 
-				var depModule = universe.Modules.Lookup(dependency);
-				if (depModule?.IsActive == true) MetDependencies.Add(depModule);
-				else { UnmetDependencies.Add(dependency); builder.Disable(); }
+	internal class ModuleInfo<T> : IModuleInfo
+		 where T : IModule
+	{
+		public Entity<TContext> Entity { get; }
+		public IReadOnlyCollection<ModuleDependency> Dependencies { get; }
+		public bool IsInitialized { get; private set; }
 
-				builder.Add<DependsOn>(dependency);
+		public ModuleInfo(Universe<TContext> universe)
+		{
+			var world   = universe.World;
+			var builder = world.New(T.Path);
+			var deps    = new List<ModuleDependency>();
+
+			if (!T.IsBuiltIn) {
+				builder.Add<Module>();
+				foreach (var dependsPath in T.Dependencies) {
+					var dependency = world.LookupPathOrNull(dependsPath) ??
+						world.New(dependsPath).Add<Module>().Add<Disabled>().Build();
+
+					var depModule = universe.Modules.Lookup(dependency);
+					var isDepInit = (depModule?.IsInitialized == true);
+
+					deps.Add(new(dependency, depModule, isDepInit));
+					if (!isDepInit) builder.Add<Disabled>();
+					builder.Add<DependsOn>(dependency);
+				}
 			}
 
 			Entity = builder.Build().CreateLookup<T>();
+			Dependencies = deps.AsReadOnly();
 
-			// Ensure all parent entities have Module set.
-			for (var p = Entity.Parent; p != null; p = p.Parent)
-				p.Add<Module>();
+			if (!T.IsBuiltIn)
+				// Ensure all parent entities have the Module tag set.
+				for (var p = Entity.Parent; p is Entity<TContext> parent; p = parent.Parent)
+					parent.Add<Module>();
 		}
 
-		public override void Enable()
+		public void Enable()
 		{
 			Entity.Enable();
-			Instance = new T();
-			(Instance as IModuleAutoRegisterComponents)?.RegisterComponents(Entity);
-			(Instance as IModuleInitializer)?.Initialize(Entity);
-			RegisterMethods(Instance);
-		}
-
-		private void RegisterMethods(object? instance)
-		{
-			var world = Entity.World;
-			foreach (var method in typeof(T).GetMethods(
-				BindingFlags.Public | BindingFlags.NonPublic |
-				BindingFlags.Static | BindingFlags.Instance
-			)) {
-				if (method.Has<SystemAttribute>())
-					world.InitSystem(instance, method).ChildOf(Entity);
-				if (method.Has<ObserverAttribute>())
-					world.InitObserver(instance, method).ChildOf(Entity);
-			}
+			T.Initialize(Entity);
+			IsInitialized = true;
 		}
 	}
 }

src/gaemstone/Universe.cs

@@ -2,32 +2,25 @@ using gaemstone.ECS;
 
 namespace gaemstone;
 
-public class Universe : World
+public class Universe<TContext>
 {
-	public ModuleManager Modules { get; }
+	public World<TContext> World { get; }
+	public ModuleManager<TContext> Modules { get; }
 
 	public Universe(params string[] args)
-		: base(args)
 	{
+		World   = new(args);
 		Modules = new(this);
 
 		// Bootstrap [Relation] tag, since it will be added to some Flecs types.
-		New("/gaemstone/Doc/Relation").Build().CreateLookup<Doc.Relation>();
+		World.New("/gaemstone/Doc/Relation").Build()
+			.CreateLookup<Doc.Relation>();
 
 		// Bootstrap built-in (static) modules from Flecs.
-		// These methods are generated by gaemstone.SourceGen.
-		Flecs.Core            .RegisterComponents(this);
-		Flecs.DeletionEvent   .RegisterComponents(this);
-		Flecs.DeletionBehavior.RegisterComponents(this);
-		Flecs.Doc             .RegisterComponents(this);
-		Flecs.ObserverEvent   .RegisterComponents(this);
-		Flecs.Pipeline        .RegisterComponents(this);
-		Flecs.SystemPhase     .RegisterComponents(this);
+		Modules.Register<Flecs.Core>();
+		Modules.Register<Flecs.Doc>();
+		Modules.Register<Flecs.Pipeline>();
 
 		Modules.Register<Doc>();
-
-		// Create "Game" entity to store global state.
-		New("/Game").Symbol("Game").Build()
-			.CreateLookup<Game>().Add<Game>();
 	}
 }

src/gaemstone/Utility/IL/ILGeneratorWrapper.cs

@@ -1,299 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Reflection;
-using System.Reflection.Emit;
-using System.Text;
-
-namespace gaemstone.Utility.IL;
-
-public class ILGeneratorWrapper
-{
-	private readonly DynamicMethod _method;
-	private readonly ILGenerator _il;
-	private readonly List<ILocal> _locals = new();
-	private readonly List<(int Offset, int Indent, OpCode Code, object? Arg)> _instructions = new();
-	private readonly Dictionary<Label, int> _labelToOffset = new();
-	private readonly Stack<BlockImpl> _indents = new();
-
-	public ILGeneratorWrapper(DynamicMethod method)
-	{
-		_method = method;
-		_il     = method.GetILGenerator();
-	}
-
-	public string ToReadableString()
-	{
-		var sb = new StringBuilder();
-		sb.AppendLine("Parameters:");
-		foreach (var (param, index) in _method.GetParameters().Select((p, i) => (p, i)))
-			sb.AppendLine($"  Argument({index}, {param.ParameterType.GetFriendlyName()})");
-		sb.AppendLine("Return:");
-			sb.AppendLine($"  {_method.ReturnType.GetFriendlyName()}");
-		sb.AppendLine();
-
-		sb.AppendLine("Locals:");
-		foreach (var local in _locals)
-			sb.AppendLine($"  {local}");
-		sb.AppendLine();
-
-		sb.AppendLine("Instructions:");
-		foreach (var (offset, indent, code, arg) in _instructions) {
-			sb.Append("  ");
-
-			// Append instruction offset.
-			if (offset < 0) sb.Append("        ");
-			else sb.Append($"0x{offset:X4}  ");
-
-			// Append instruction opcode.
-			if (code == OpCodes.Nop) sb.Append("            ");
-			else sb.Append($"{code.Name,-12}");
-
-			// Append indents.
-			for (var i = 0; i < indent; i++)
-				sb.Append("| ");
-
-			// Append instruction argument.
-			if (code == OpCodes.Nop) sb.Append("// ");
-			switch (arg) {
-				case Label label: sb.Append($"Label(0x{_labelToOffset.GetValueOrDefault(label, -1):X4})"); break;
-				case not null: sb.Append(arg); break;
-			}
-
-			sb.AppendLine();
-		}
-		return sb.ToString();
-	}
-
-
-	public IArgument Argument(int index)
-	{
-		var type = _method.GetParameters()[index].ParameterType;
-		if (type.IsByRefLike) return new ArgumentImpl(index, type);
-		return (IArgument)Activator.CreateInstance(typeof(ArgumentImpl<>).MakeGenericType(type), index)!;
-	}
-	public IArgument<T> Argument<T>(int index) => (IArgument<T>)Argument(index);
-
-	public ILocal Local(Type type, string? name = null)
-	{
-		var builder = _il.DeclareLocal(type);
-		var local = type.IsByRefLike ? new LocalImpl(builder, name)
-			: (ILocal)Activator.CreateInstance(typeof(LocalImpl<>).MakeGenericType(type), builder, name)!;
-		_locals.Add(local);
-		return local;
-	}
-	public ILocal<T>     Local<T>(string? name = null)              => (ILocal<T>)Local(typeof(T), name);
-	public ILocal<Array> LocalArray(Type type, string? name = null) => (ILocal<Array>)Local(type.MakeArrayType(), name);
-	public ILocal<T[]>   LocalArray<T>(string? name = null)         => (ILocal<T[]>)Local(typeof(T).MakeArrayType(), name);
-
-	public Label DefineLabel() => _il.DefineLabel();
-	public void MarkLabel(Label label)
-	{
-		_instructions.Add((-1, _indents.Count, OpCodes.Nop, label));
-		_labelToOffset.Add(label, _il.ILOffset);
-		_il.MarkLabel(label);
-	}
-
-
-	private void AddInstr(OpCode code, object? arg = null) => _instructions.Add((_il.ILOffset, _indents.Count, code, arg));
-	public void Comment(string comment) => _instructions.Add((-1, _indents.Count, OpCodes.Nop, comment));
-
-	private static readonly MethodInfo _writeLine = typeof(Console).GetMethod(nameof(Console.WriteLine), new[] { typeof(string) })!;
-	public void Log(string str) { Load(str); Call(_writeLine); }
-
-	internal void Emit(OpCode code)                { AddInstr(code, null);  _il.Emit(code); }
-	internal void Emit(OpCode code, int arg)       { AddInstr(code, arg);   _il.Emit(code, arg); }
-	internal void Emit(OpCode code, string arg)    { AddInstr(code, arg);   _il.Emit(code, arg); }
-	internal void Emit(OpCode code, Type type)     { AddInstr(code, type);  _il.Emit(code, type); }
-	internal void Emit(OpCode code, Label label)   { AddInstr(code, label); _il.Emit(code, label); }
-	internal void Emit(OpCode code, ILocal local)  { AddInstr(code, local); _il.Emit(code, local.Builder); }
-	internal void Emit(OpCode code, IArgument arg) { AddInstr(code, arg);   _il.Emit(code, arg.Index); }
-	internal void Emit(OpCode code, MethodInfo method)      { AddInstr(code, method); _il.Emit(code, method); }
-	internal void Emit(OpCode code, ConstructorInfo constr) { AddInstr(code, constr); _il.Emit(code, constr); }
-
-	public void Dup() => Emit(OpCodes.Dup);
-	public void Pop() => Emit(OpCodes.Pop);
-
-	public void LoadNull() => Emit(OpCodes.Ldnull);
-	public void LoadConst(int value) => Emit(OpCodes.Ldc_I4, value);
-	public void Load(string value) => Emit(OpCodes.Ldstr, value);
-
-	private static readonly MethodInfo _typeFromHandleMethod = typeof(Type).GetMethod(nameof(Type.GetTypeFromHandle))!;
-	public void Typeof(Type value) { Emit(OpCodes.Ldtoken, value); Call(_typeFromHandleMethod); }
-
-	public void Load(IArgument arg) => Emit(OpCodes.Ldarg, arg);
-	public void LoadAddr(IArgument arg) => Emit(OpCodes.Ldarga, arg);
-
-	public void Load(ILocal local) => Emit(OpCodes.Ldloc, local);
-	public void LoadAddr(ILocal local) => Emit(OpCodes.Ldloca, local);
-	public void Store(ILocal local) => Emit(OpCodes.Stloc, local);
-	public void Set(ILocal<int> local, int value) { LoadConst(value); Store(local); }
-
-	public void LoadLength() { Emit(OpCodes.Ldlen); Emit(OpCodes.Conv_I4); }
-	public void LoadLength(IArgument<Array> array) { Load(array); LoadLength(); }
-	public void LoadLength(ILocal<Array> array)    { Load(array); LoadLength(); }
-
-	public void LoadObj(Type type) => Emit(OpCodes.Ldobj, type);
-	public void LoadObj<T>() where T : struct => LoadObj(typeof(T));
-	public void LoadObj(ILocal local) { LoadAddr(local); LoadObj(local.LocalType); }
-	public void LoadObj(IArgument arg) { LoadAddr(arg); LoadObj(arg.ArgumentType); }
-
-	public void LoadElem(Type type) => Emit(OpCodes.Ldelem, type);
-	public void LoadElem(Type type, int index)         { LoadConst(index); LoadElem(type); }
-	public void LoadElem(Type type, ILocal<int> index) { Load(index); LoadElem(type); }
-	public void LoadElem(Type type, IArgument<Array> array, int index)         { Load(array); LoadElem(type, index); }
-	public void LoadElem(Type type, IArgument<Array> array, ILocal<int> index) { Load(array); LoadElem(type, index); }
-	public void LoadElem(Type type, ILocal<Array> array, int index)            { Load(array); LoadElem(type, index); }
-	public void LoadElem(Type type, ILocal<Array> array, ILocal<int> index)    { Load(array); LoadElem(type, index); }
-
-	public void LoadElemRef() => Emit(OpCodes.Ldelem_Ref);
-	public void LoadElemRef(int index)         { LoadConst(index); LoadElemRef(); }
-	public void LoadElemRef(ILocal<int> index) { Load(index); LoadElemRef(); }
-	public void LoadElemRef(IArgument<Array> array, int index)         { Load(array); LoadElemRef(index); }
-	public void LoadElemRef(IArgument<Array> array, ILocal<int> index) { Load(array); LoadElemRef(index); }
-	public void LoadElemRef(ILocal<Array> array, int index)            { Load(array); LoadElemRef(index); }
-	public void LoadElemRef(ILocal<Array> array, ILocal<int> index)    { Load(array); LoadElemRef(index); }
-
-	public void LoadElemEither(Type type)                                            { if (type.IsValueType) LoadElem(type); else LoadElemRef(); }
-	public void LoadElemEither(Type type, int index)                                 { if (type.IsValueType) LoadElem(type, index); else LoadElemRef(index); }
-	public void LoadElemEither(Type type, ILocal<int> index)                         { if (type.IsValueType) LoadElem(type, index); else LoadElemRef(index); }
-	public void LoadElemEither(Type type, IArgument<Array> array, int index)         { if (type.IsValueType) LoadElem(type, array, index); else LoadElemRef(array, index); }
-	public void LoadElemEither(Type type, IArgument<Array> array, ILocal<int> index) { if (type.IsValueType) LoadElem(type, array, index); else LoadElemRef(array, index); }
-	public void LoadElemEither(Type type, ILocal<Array> array, int index)            { if (type.IsValueType) LoadElem(type, array, index); else LoadElemRef(array, index); }
-	public void LoadElemEither(Type type, ILocal<Array> array, ILocal<int> index)    { if (type.IsValueType) LoadElem(type, array, index); else LoadElemRef(array, index); }
-
-	public void LoadElemAddr(Type type) => Emit(OpCodes.Ldelema, type);
-	public void LoadElemAddr(Type type, int index)         { LoadConst(index); LoadElemAddr(type); }
-	public void LoadElemAddr(Type type, ILocal<int> index) { Load(index); LoadElemAddr(type); }
-	public void LoadElemAddr(Type type, IArgument<Array> array, int index)         { Load(array); LoadElemAddr(type, index); }
-	public void LoadElemAddr(Type type, IArgument<Array> array, ILocal<int> index) { Load(array); LoadElemAddr(type, index); }
-	public void LoadElemAddr(Type type, ILocal<Array> array, int index)            { Load(array); LoadElemAddr(type, index); }
-	public void LoadElemAddr(Type type, ILocal<Array> array, ILocal<int> index)    { Load(array); LoadElemAddr(type, index); }
-
-	public void Load(PropertyInfo info) => CallVirt(info.GetMethod!);
-	public void Load(ILocal obj, PropertyInfo info) { Load(obj); Load(info); }
-	public void Load(IArgument obj, PropertyInfo info) { Load(obj); Load(info); }
-
-	public void Add() => Emit(OpCodes.Add);
-	public void Increment(ILocal<int> local) { Load(local); LoadConst(1); Add(); Store(local); }
-
-	public void Init(Type type) => Emit(OpCodes.Initobj, type);
-	public void Init<T>() where T : struct => Init(typeof(T));
-
-	public void New(ConstructorInfo constructor) => Emit(OpCodes.Newobj, constructor);
-	public void New(Type type) => New(type.GetConstructors().Single());
-	public void New(Type type, params Type[] paramTypes) => New(type.GetConstructor(paramTypes)!);
-
-	public void Cast(Type type) => Emit(OpCodes.Castclass, type);
-	public void Cast<T>() => Cast(typeof(T));
-	public void Box(Type type) => Emit(OpCodes.Box, type);
-	public void Box<T>() => Box(typeof(T));
-
-	public void Goto(Label label) => Emit(OpCodes.Br, label);
-	public void GotoIfTrue(Label label) => Emit(OpCodes.Brtrue, label);
-	public void GotoIfFalse(Label label) => Emit(OpCodes.Brfalse, label);
-
-	public void GotoIf(Label label, ILocal<int> a, Comparison op, ILocal<int> b)
-		{ Load(a); Load(b); Emit(op.Code, label); }
-	public void GotoIfNull(Label label, ILocal<object> local)
-		{ Load(local); Emit(OpCodes.Brfalse, label); }
-	public void GotoIfNotNull(Label label, ILocal<object> local)
-		{ Load(local); Emit(OpCodes.Brtrue, label); }
-
-	public void Call(MethodInfo method) => Emit(OpCodes.Call, method);
-	public void CallVirt(MethodInfo method) => Emit(OpCodes.Callvirt, method);
-
-	private static readonly MethodInfo _consoleWriteLineMethod = typeof(Console).GetMethod(nameof(Console.WriteLine), new[] { typeof(string) })!;
-	public void Print(string str) { Load(str); Call(_consoleWriteLineMethod); }
-
-	public void Return() => Emit(OpCodes.Ret);
-
-
-	public delegate void WhileBodyAction(Label continueLabel, Label breakLabel);
-	public delegate void WhileTestAction(Label continueLabel);
-
-	public void While(string name, WhileTestAction testAction, WhileBodyAction bodyAction) {
-		var bodyLabel  = DefineLabel();
-		var testLabel  = DefineLabel();
-		var breakLabel = DefineLabel();
-
-		Goto(testLabel);
-		Comment("BEGIN LOOP " + name);
-		MarkLabel(bodyLabel);
-		using (Indent()) bodyAction(testLabel, breakLabel);
-		Comment("TEST LOOP " + name);
-		MarkLabel(testLabel);
-		using (Indent()) testAction(bodyLabel);
-		Comment("END LOOP " + name);
-		MarkLabel(breakLabel);
-	}
-
-
-	public IDisposable Block(Action onClose)
-		=> new BlockImpl(onClose);
-	public IDisposable Indent()
-	{
-		BlockImpl indent = null!;
-		indent = new(() => { if (_indents.Pop() != indent) throw new InvalidOperationException(); });
-		_indents.Push(indent);
-		return indent;
-	}
-
-	internal class BlockImpl : IDisposable
-	{
-		public Action OnClose { get; }
-		public BlockImpl(Action onClose) => OnClose = onClose;
-		public void Dispose() => OnClose();
-	}
-
-
-	internal class ArgumentImpl : IArgument
-	{
-		public int Index { get; }
-		public Type ArgumentType { get; }
-		public ArgumentImpl(int index, Type type) {  Index = index; ArgumentType = type; }
-		public override string ToString() => $"Argument({Index}, {ArgumentType.GetFriendlyName()})";
-	}
-	internal class ArgumentImpl<T> : ArgumentImpl, IArgument<T>
-		{ public ArgumentImpl(int index) : base(index, typeof(T)) {  } }
-
-	internal class LocalImpl : ILocal
-	{
-		public LocalBuilder Builder { get; }
-		public string? Name { get; }
-		public Type LocalType => Builder.LocalType;
-		public LocalImpl(LocalBuilder builder, string? name) { Builder = builder; Name = name; }
-		public override string ToString() => $"Local({Builder.LocalIndex}, {LocalType.GetFriendlyName()}){(Name != null ? $" // {Name}" : "")}";
-	}
-	internal class LocalImpl<T> : LocalImpl, ILocal<T>
-		{ public LocalImpl(LocalBuilder builder, string? name) : base(builder, name) {  } }
-}
-
-public class Comparison
-{
-	public static Comparison NotEqual    { get; } = new(OpCodes.Bne_Un);
-	public static Comparison LessThan    { get; } = new(OpCodes.Blt);
-	public static Comparison LessOrEq    { get; } = new(OpCodes.Ble);
-	public static Comparison Equal       { get; } = new(OpCodes.Beq);
-	public static Comparison GreaterOrEq { get; } = new(OpCodes.Bge);
-	public static Comparison GreaterThan { get; } = new(OpCodes.Bgt);
-
-	public OpCode Code { get; }
-	private Comparison(OpCode code) => Code = code;
-}
-
-public interface IArgument
-{
-	int Index { get; }
-	Type ArgumentType { get; }
-}
-public interface IArgument<out T>
-	: IArgument {  }
-
-public interface ILocal
-{
-	LocalBuilder Builder { get; }
-	Type LocalType { get; }
-}
-public interface ILocal<out T>
-	: ILocal {  }

src/gaemstone/Utility/IL/IterActionGenerator.cs

@@ -1,404 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Collections.Immutable;
-using System.Linq;
-using System.Reflection;
-using System.Reflection.Emit;
-using System.Runtime.CompilerServices;
-using gaemstone.ECS;
-
-namespace gaemstone.Utility.IL;
-
-// TODO: Support tuple syntax to match relationship pairs.
-public unsafe class IterActionGenerator
-{
-	private static readonly ConstructorInfo _entityRefCtor = typeof(EntityRef).GetConstructors().Single();
-
-	private static readonly PropertyInfo _iterWorldProp     = typeof(Iterator).GetProperty(nameof(Iterator.World))!;
-	private static readonly PropertyInfo _iterDeltaTimeProp = typeof(Iterator).GetProperty(nameof(Iterator.DeltaTime))!;
-	private static readonly PropertyInfo _iterCountProp     = typeof(Iterator).GetProperty(nameof(Iterator.Count))!;
-	private static readonly MethodInfo _iterEntityMethod = typeof(Iterator).GetMethod(nameof(Iterator.Entity))!;
-	private static readonly MethodInfo _iterFieldMethod        = typeof(Iterator).GetMethod(nameof(Iterator.Field       ), 1, new[] { typeof(int) })!;
-	private static readonly MethodInfo _iterFieldOrEmptyMethod = typeof(Iterator).GetMethod(nameof(Iterator.FieldOrEmpty), 1, new[] { typeof(int) })!;
-	private static readonly MethodInfo _iterFieldRefMethod        = typeof(Iterator).GetMethod(nameof(Iterator.Field       ), 1, new[] { typeof(int), Type.MakeGenericMethodParameter(0) })!;
-	private static readonly MethodInfo _iterFieldOrEmptyRefMethod = typeof(Iterator).GetMethod(nameof(Iterator.FieldOrEmpty), 1, new[] { typeof(int), Type.MakeGenericMethodParameter(0) })!;
-
-	private static readonly ConditionalWeakTable<MethodInfo, IterActionGenerator> _cache = new();
-	private static readonly Dictionary<Type, Action<ILGeneratorWrapper, IArgument<Iterator>>> _globalUniqueParameters = new() {
-		[typeof(World)]    = (IL, iter) => { IL.Load(iter, _iterWorldProp); },
-		[typeof(Universe)] = (IL, iter) => { IL.Load(iter, _iterWorldProp); IL.Cast(typeof(Universe)); },
-		[typeof(TimeSpan)] = (IL, iter) => { IL.Load(iter, _iterDeltaTimeProp); },
-	};
-	private static readonly Dictionary<Type, Action<ILGeneratorWrapper, IArgument<Iterator>, ILocal<int>>> _uniqueParameters = new() {
-		[typeof(Iterator)]  = (IL, iter, i) => { IL.Load(iter); },
-		[typeof(EntityRef)] = (IL, iter, i) => { IL.Load(iter); IL.Load(i); IL.Call(_iterEntityMethod); },
-	};
-
-	public World World { get; }
-	public MethodInfo Method { get; }
-	public IReadOnlyList<ParamInfo> Parameters { get; }
-
-	public IReadOnlyList<Term> Terms { get; }
-	public Action<object?, Iterator> GeneratedAction { get; }
-	public string ReadableString { get; }
-
-	public void RunWithTryCatch(object? instance, Iterator iter)
-	{
-		try { GeneratedAction(instance, iter); }
-		catch { Console.Error.WriteLine(ReadableString); throw; }
-	}
-
-	public IterActionGenerator(World world, MethodInfo method)
-	{
-		World      = world;
-		Method     = method;
-		Parameters = method.GetParameters().Select(ParamInfo.Build).ToImmutableArray();
-
-		var name = "<>Query_" + string.Join("_", method.Name);
-		var genMethod = new DynamicMethod(name, null, new[] { typeof(object), typeof(Iterator) });
-		var IL = new ILGeneratorWrapper(genMethod);
-
-		var instanceArg = IL.Argument<object?>(0);
-		var iteratorArg = IL.Argument<Iterator>(1);
-
-		var fieldIndex = 0;
-		var paramData  = new List<(ParamInfo Info, Term? Term, ILocal? FieldLocal, ILocal? TempLocal)>();
-		foreach (var p in Parameters) {
-			// If the parameter is unique, we don't create a term for it.
-			if (p.Kind <= ParamKind.Unique)
-				{ paramData.Add((p, null, null, null)); continue; }
-
-			// Create a term to add to the query.
-			var term = new Term(world.LookupByTypeOrThrow(p.UnderlyingType)) {
-				Source = (p.Source != null) ? (TermId)World.LookupByTypeOrThrow(p.Source) : null,
-				InOut = p.Kind switch {
-					ParamKind.In  => TermInOutKind.In,
-					ParamKind.Out => TermInOutKind.Out,
-					ParamKind.Not or ParamKind.Not => TermInOutKind.None,
-					_ => default,
-				},
-				Oper = p.Kind switch {
-					ParamKind.Not => TermOperKind.Not,
-					ParamKind.Or  => TermOperKind.Or,
-					_ when !p.IsRequired => TermOperKind.Optional,
-					_ => default,
-				},
-			};
-
-			// If this and the previous parameter are marked with [Or], do not advance the field index.
-			if ((fieldIndex == 0) || (p.Kind != ParamKind.Or) || (paramData[^1].Info.Kind != ParamKind.Or))
-				fieldIndex++;
-
-			var spanType   = typeof(Span<>).MakeGenericType(p.FieldType);
-			var fieldLocal = (ILocal?)null;
-			var tempLocal  = (ILocal?)null;
-
-			switch (p.Kind) {
-				// FIXME: Currently would not work with [Or]'d components.
-				case ParamKind.Has or ParamKind.Not or ParamKind.Or:
-					if (!p.ParameterType.IsValueType) break;
-					// If parameter is a struct, we require a temporary local that we can
-					// later load onto the stack when loading the arguments for the action.
-					IL.Comment($"{p.Info.Name}Temp = default({p.ParameterType});");
-					tempLocal = IL.Local(p.ParameterType);
-					IL.LoadAddr(tempLocal);
-					IL.Init(tempLocal.LocalType);
-					break;
-
-				case ParamKind.Nullable or ParamKind.Or:
-					IL.Comment($"{p.Info.Name}Field = iterator.FieldOrEmpty<{p.FieldType.Name}>({fieldIndex})");
-					fieldLocal = IL.Local(spanType, $"{p.Info.Name}Field");
-					IL.Load(iteratorArg);
-					IL.LoadConst(fieldIndex);
-					IL.Call(_iterFieldOrEmptyMethod.MakeGenericMethod(p.FieldType));
-					IL.Store(fieldLocal);
-
-					if (p.UnderlyingType.IsValueType) {
-						IL.Comment($"{p.Info.Name}Temp = default({p.ParameterType});");
-						tempLocal = IL.Local(p.ParameterType);
-						IL.LoadAddr(tempLocal);
-						IL.Init(tempLocal.LocalType);
-					}
-					break;
-
-				default:
-					IL.Comment($"{p.Info.Name}Field = iterator.Field<{p.FieldType.Name}>({fieldIndex})");
-					fieldLocal = IL.Local(spanType, $"{p.Info.Name}Field");
-					IL.Load(iteratorArg);
-					IL.LoadConst(fieldIndex);
-					IL.Call(_iterFieldMethod.MakeGenericMethod(p.FieldType));
-					IL.Store(fieldLocal);
-					break;
-			}
-
-			paramData.Add((p, term, fieldLocal, tempLocal));
-		}
-
-		var indexLocal = IL.Local<int>("iter_index");
-		var countLocal = IL.Local<int>("iter_count");
-
-		IL.Set(indexLocal, 0);
-		IL.Load(iteratorArg, _iterCountProp);
-		IL.Store(countLocal);
-
-		// If all parameters are fixed, iterator count will be 0, but since
-		// the query matched, we want to run the callback at least once.
-		IL.Comment("if (iter_count == 0) iter_count = 1;");
-		var dontIncrementLabel = IL.DefineLabel();
-		IL.Load(countLocal);
-		IL.GotoIfTrue(dontIncrementLabel);
-		IL.LoadConst(1);
-		IL.Store(countLocal);
-		IL.MarkLabel(dontIncrementLabel);
-
-		IL.While("IteratorLoop", (@continue) => {
-			IL.GotoIf(@continue, indexLocal, Comparison.LessThan, countLocal);
-		}, (_, _) => {
-			if (!Method.IsStatic)
-				IL.Load(instanceArg);
-
-			foreach (var (info, term, fieldLocal, tempLocal) in paramData) {
-				var isValueType = info.UnderlyingType.IsValueType;
-				var paramName   = info.ParameterType.GetFriendlyName();
-				switch (info.Kind) {
-
-					case ParamKind.GlobalUnique:
-						IL.Comment($"Global unique parameter {paramName}");
-						_globalUniqueParameters[info.ParameterType](IL, iteratorArg);
-						break;
-
-					case ParamKind.Unique:
-						IL.Comment($"Unique parameter {paramName}");
-						_uniqueParameters[info.ParameterType](IL, iteratorArg, indexLocal!);
-						break;
-
-					// FIXME: Currently would not work with [Or]'d components.
-					case ParamKind.Has or ParamKind.Not or ParamKind.Or:
-						IL.Comment($"Has parameter {paramName}");
-						if (isValueType) IL.LoadObj(tempLocal!);
-						else             IL.LoadNull();
-						break;
-
-					default:
-						var spanType = isValueType ? typeof(Span<>) : typeof(Iterator.SpanToRef<>);
-						var concreteSpanType = spanType.MakeGenericType(info.UnderlyingType);
-						var spanItemMethod   = concreteSpanType.GetProperty("Item")!.GetMethod!;
-						var spanLengthMethod = concreteSpanType.GetProperty("Length")!.GetMethod!;
-
-						IL.Comment($"Parameter {paramName}");
-						if (info.IsByRef) {
-							IL.LoadAddr(fieldLocal!);
-							if (info.IsFixed) IL.LoadConst(0);
-							else IL.Load(indexLocal!);
-							IL.Call(spanItemMethod);
-						} else if (info.IsRequired) {
-							IL.LoadAddr(fieldLocal!);
-							if (info.IsFixed) IL.LoadConst(0);
-							else IL.Load(indexLocal!);
-							IL.Call(spanItemMethod);
-							if (isValueType) IL.LoadObj(info.FieldType);
-						} else {
-							var elseLabel = IL.DefineLabel();
-							var doneLabel = IL.DefineLabel();
-							IL.LoadAddr(fieldLocal!);
-							IL.Call(spanLengthMethod);
-							IL.GotoIfFalse(elseLabel);
-								IL.LoadAddr(fieldLocal!);
-								if (info.IsFixed) IL.LoadConst(0);
-								else IL.Load(indexLocal!);
-								IL.Call(spanItemMethod);
-								if (isValueType) {
-									IL.LoadObj(info.FieldType);
-									IL.New(info.ParameterType);
-								}
-							IL.Goto(doneLabel);
-							IL.MarkLabel(elseLabel);
-								if (isValueType) IL.LoadObj(tempLocal!);
-								else IL.LoadNull();
-							IL.MarkLabel(doneLabel);
-						}
-						break;
-				}
-			}
-			IL.Call(Method);
-
-			IL.Increment(indexLocal);
-		});
-
-		IL.Return();
-
-		Terms = paramData.Where(p => p.Term != null).Select(p => p.Term!).ToImmutableList();
-		GeneratedAction = genMethod.CreateDelegate<Action<object?, Iterator>>();
-		ReadableString  = IL.ToReadableString();
-	}
-
-	public static IterActionGenerator GetOrBuild(World world, MethodInfo method)
-		=>_cache.GetValue(method, m => new IterActionGenerator(world, m));
-
-	public class ParamInfo
-	{
-		public ParameterInfo Info { get; }
-
-		public ParamKind Kind { get; }
-		public Type ParameterType { get; }
-		public Type UnderlyingType { get; }
-		public Type FieldType { get; }
-
-		public Type? Source { get; }
-
-		public bool IsRequired => (Kind <  ParamKind.Nullable);
-		public bool IsByRef    => (Kind is ParamKind.In or ParamKind.Ref);
-		public bool IsFixed    => (Kind == ParamKind.GlobalUnique) || (Source != null);
-
-		private ParamInfo(ParameterInfo info, ParamKind kind,
-		                  Type paramType, Type underlyingType)
-		{
-			Info = info;
-			Kind = kind;
-			ParameterType  = paramType;
-			UnderlyingType = underlyingType;
-			// Reference types have a backing type of ReferenceHandle.
-			FieldType = underlyingType.IsValueType ? underlyingType : typeof(ReferenceHandle);
-
-			if (UnderlyingType.Has<SingletonAttribute>()) Source = UnderlyingType;
-			if (Info.Get<SourceAttribute>()?.Type is Type type) Source = type;
-		}
-
-		public static ParamInfo Build(ParameterInfo info)
-		{
-			if (info.IsOptional) throw new ArgumentException($"Optional parameters are not supported\nParameter: {info}");
-			if (info.ParameterType.IsArray)     throw new ArgumentException($"Arrays are not supported\nParameter: {info}");
-			if (info.ParameterType.IsPointer)   throw new ArgumentException($"Pointers are not supported\nParameter: {info}");
-			if (info.ParameterType.IsPrimitive) throw new ArgumentException($"Primitives are not supported\nParameter: {info}");
-
-			// Find out initial parameter kind from provided attribute.
-			var fromAttributes = new List<ParamKind>();
-			if (info.Has< InAttribute>()) fromAttributes.Add(ParamKind.In);
-			if (info.Has<OutAttribute>()) fromAttributes.Add(ParamKind.Out);
-			if (info.Has<HasAttribute>()) fromAttributes.Add(ParamKind.Has);
-			if (info.Has< OrAttribute>()) fromAttributes.Add(ParamKind.Or);
-			if (info.Has<NotAttribute>()) fromAttributes.Add(ParamKind.Not);
-			// Throw an error if multiple incompatible attributes were found.
-			if (fromAttributes.Count > 1) throw new ArgumentException(
-				"Parameter must not be marked with multiple attributes: "
-					+ string.Join(", ", fromAttributes.Select(a => $"[{a}]"))
-					+ $"\nParameter: {info}");
-			var kind = fromAttributes.FirstOrNull() ?? ParamKind.Normal;
-
-			// Handle unique parameters such as Universe, EntityRef, ...
-			var isGlobalUnique = _globalUniqueParameters.ContainsKey(info.ParameterType);
-			var isUnique       = _uniqueParameters.ContainsKey(info.ParameterType);
-			if (isGlobalUnique || isUnique) {
-				if (kind != ParamKind.Normal) throw new ArgumentException(
-					$"Unique parameter {info.ParameterType.Name} does not support [{kind}]\nParameter: {info}");
-				kind = isGlobalUnique ? ParamKind.GlobalUnique : ParamKind.Unique;
-				return new(info, kind, info.ParameterType, info.ParameterType);
-			}
-
-			var isNullable = info.IsNullable();
-			var isByRef    = info.ParameterType.IsByRef;
-
-			if (info.ParameterType.Has<TagAttribute>() && (kind is not (ParamKind.Has or ParamKind.Not or ParamKind.Or))) {
-				if (kind is not ParamKind.Normal) throw new ArgumentException($"Parameter does not support [{kind}]\nParameter: {info}");
-				kind = ParamKind.Has;
-			}
-
-			if (kind is ParamKind.Not or ParamKind.Has) {
-				if (isNullable) throw new ArgumentException($"Parameter does not support Nullable\nParameter: {info}");
-				if (isByRef) throw new ArgumentException($"Parameter does not support ByRef\nParameter: {info}");
-				return new(info, kind, info.ParameterType, info.ParameterType);
-			}
-
-			var underlyingType = info.ParameterType;
-
-			if (isNullable) {
-				if (isByRef) throw new ArgumentException($"Parameter does not support ByRef\nParameter: {info}");
-				if (info.ParameterType.IsValueType)
-					underlyingType = Nullable.GetUnderlyingType(info.ParameterType)!;
-				kind = ParamKind.Nullable;
-			}
-
-			if (info.ParameterType.IsByRef) {
-				if (kind != ParamKind.Normal) throw new ArgumentException(
-					$"Parameter does not support [{kind}]\nParameter: {info}");
-				underlyingType = info.ParameterType.GetElementType()!;
-				if (!underlyingType.IsValueType) throw new ArgumentException(
-					$"Reference types can't also be ByRef\nParameter: {info}");
-				kind = info.IsIn  ? ParamKind.In
-				     : info.IsOut ? ParamKind.Out
-				                  : ParamKind.Ref;
-			}
-
-			if (underlyingType.IsPrimitive) throw new ArgumentException(
-				$"Primitives are not supported\nParameter: {info}");
-
-			return new(info, kind, info.ParameterType, underlyingType);
-		}
-	}
-
-	public enum ParamKind
-	{
-		/// <summary>
-		/// Not part of the resulting query's terms.
-		/// Same value across a single invocation of a callback.
-		/// For example <see cref="ECS.World"/> or <see cref="TimeSpan"/>.
-		/// </summary>
-		GlobalUnique,
-
-		/// <summary>
-		/// Not part of the resulting query's terms.
-		/// Unique value for each iterated entity.
-		/// For example <see cref="EntityRef"/>.
-		/// </summary>
-		Unique,
-
-		/// <summary> Passed by value. </summary>
-		Normal,
-
-		/// <summary>
-		/// Struct passed with the "in" modifier, allowing direct pointer access.
-		/// Manually applied with <see cref="InAttribute"/>.
-		/// Marks a component as being read from.
-		/// </summary>
-		In,
-
-		/// <summary>
-		/// Struct passed with the "out" modifier, allowing direct pointer access.
-		/// Manually applied with <see cref="HasAttribute"/>.
-		/// Marks a component as being written to.
-		/// </summary>
-		Out,
-
-		/// <summary>
-		/// Struct passed with the "ref" modifier, allowing direct pointer access.
-		/// Marks a component as being read from and written to.
-		/// </summary>
-		Ref,
-
-		/// <summary>
-		/// Only checks for presence.
-		/// Manually applied with <see cref="HasAttribute"/>.
-		/// Automatically applied for types with <see cref="TagAttribute"/>.
-		/// Marks a component as not being accessed.
-		/// </summary>
-		Has,
-
-		/// <summary>
-		/// Struct or class passed as <see cref="T?"/>.
-		/// </summary>
-		Nullable,
-
-		/// <summary>
-		/// Only checks for absence.
-		/// Applied with <see cref="NotAttribute"/>.
-		/// </summary>
-		Not,
-
-		/// <summary>
-		/// Matches any terms in a chain of "or" terms.
-		/// Applied with <see cref="OrAttribute"/>.
-		/// Implies <see cref="Nullable"/>.
-		/// </summary>
-		Or,
-	}
-}

src/gaemstone/Utility/Union.cs

@@ -0,0 +1,37 @@
+using System.Runtime.InteropServices;
+
+namespace gaemstone.Utility;
+
+[StructLayout(LayoutKind.Explicit)]
+internal struct Union<T1, T2>
+{
+	[FieldOffset(0)] public T1 Value1;
+	[FieldOffset(0)] public T2 Value2;
+}
+
+[StructLayout(LayoutKind.Explicit)]
+internal struct Union<T1, T2, T3>
+{
+	[FieldOffset(0)] public T1 Value1;
+	[FieldOffset(0)] public T2 Value2;
+	[FieldOffset(0)] public T3 Value3;
+}
+
+[StructLayout(LayoutKind.Explicit)]
+internal struct Union<T1, T2, T3, T4>
+{
+	[FieldOffset(0)] public T1 Value1;
+	[FieldOffset(0)] public T2 Value2;
+	[FieldOffset(0)] public T3 Value3;
+	[FieldOffset(0)] public T4 Value4;
+}
+
+[StructLayout(LayoutKind.Explicit)]
+internal struct Union<T1, T2, T3, T4, T5>
+{
+	[FieldOffset(0)] public T1 Value1;
+	[FieldOffset(0)] public T2 Value2;
+	[FieldOffset(0)] public T3 Value3;
+	[FieldOffset(0)] public T4 Value4;
+	[FieldOffset(0)] public T5 Value5;
+}

src/gaemstone/gaemstone.csproj

@@ -6,6 +6,7 @@
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <ImplicitUsings>disable</ImplicitUsings>
     <Nullable>enable</Nullable>
+    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles> <!-- -->
   </PropertyGroup>
 
   <ItemGroup>