Add support for iterator variables

- Add Iterator.Variable
- Add Iterator.GetVar and .SetVar methods
- Add ThisVar to Filter, Query and Rule
- Add Rule.Variables, shows all its variables
- Remove Iterator.SetThis

src/gaemstone.ECS/Filter.cs

@@ -12,6 +12,14 @@ public unsafe class Filter
 	public World World { get; }
 	public ecs_filter_t* Handle { get; }
 
+	public Iterator.Variable? ThisVar { get {
+		var index = ecs_filter_find_this_var(this);
+		return (index >= 0) ? new(index, "This") : null;
+	} }
+
+	internal Filter(World world, ecs_filter_t* handle)
+		{ World = world; Handle = handle; }
+
 	public Filter(World world, FilterDesc desc)
 	{
 		using var alloc = TempAllocator.Use();

src/gaemstone.ECS/Iterator.cs

@@ -31,13 +31,6 @@ public unsafe class Iterator
 		return new(world, IteratorType.Term, flecsIter);
 	}
 
-	public Iterator SetThis(Entity entity)
-	{
-		fixed (ecs_iter_t* ptr = &Value)
-			ecs_iter_set_var(ptr, 0, entity);
-		return this;
-	}
-
 	public void Dispose()
 	{
 		// When an iterator is iterated until completion,
@@ -47,6 +40,19 @@ public unsafe class Iterator
 				ecs_iter_fini(ptr);
 	}
 
+	public EntityRef GetVar(Variable var)
+	{
+		fixed (ecs_iter_t* ptr = &Value)
+			return new(World, new(ecs_iter_get_var(ptr, var.Index)));
+	}
+
+	public Iterator SetVar(Variable var, Entity entity)
+	{
+		fixed (ecs_iter_t* ptr = &Value)
+			ecs_iter_set_var(ptr, var.Index, entity);
+		return this;
+	}
+
 	public bool Next()
 	{
 		fixed (ecs_iter_t* ptr = &Value) {
@@ -113,6 +119,15 @@ public unsafe class Iterator
 	// IEnumerable implementation
 	public IEnumerator<Iterator> GetEnumerator() { while (Next()) yield return this; }
 	IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+
+	public class Variable
+	{
+		public int Index { get; }
+		public string Name { get; }
+		public Variable(int index, string name)
+			{ Index = index; Name = name; }
+	}
 }
 
 public enum IteratorType

src/gaemstone.ECS/Query.cs

@@ -9,6 +9,9 @@ public unsafe class Query
 {
 	public World World { get; }
 	public ecs_query_t* Handle { get; }
+	public Filter Filter { get; }
+
+	public Iterator.Variable? ThisVar => Filter.ThisVar;
 
 	public Query(World world, QueryDesc desc)
 	{
@@ -16,6 +19,7 @@ public unsafe class Query
 		var flecsDesc = desc.ToFlecs(alloc);
 		World  = world;
 		Handle = ecs_query_init(world, &flecsDesc);
+		Filter = new(world, ecs_query_get_filter(Handle));
 	}
 
 	public void Dispose()

src/gaemstone.ECS/Rule.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections;
+using System.Collections.Generic;
 using gaemstone.Utility;
 using static flecs_hub.flecs;
 
@@ -9,6 +11,11 @@ public unsafe class Rule
 {
 	public World World { get; }
 	public ecs_rule_t* Handle { get; }
+	public Filter Filter { get; }
+
+	private VariableCollection? _variables;
+	public VariableCollection Variables => _variables ??= new(this);
+	public Iterator.Variable? ThisVar => Filter.ThisVar;
 
 	public Rule(World world, FilterDesc desc)
 	{
@@ -16,6 +23,7 @@ public unsafe class Rule
 		var flecsDesc = desc.ToFlecs(alloc);
 		World  = world;
 		Handle = ecs_rule_init(world, &flecsDesc);
+		Filter = new(World, ecs_rule_get_filter(Handle));
 	}
 
 	public void Dispose()
@@ -28,4 +36,41 @@ public unsafe class Rule
 		=> ecs_rule_str(Handle).FlecsToStringAndFree()!;
 
 	public static implicit operator ecs_rule_t*(Rule q) => q.Handle;
+
+
+	public unsafe class VariableCollection
+		: IReadOnlyCollection<Iterator.Variable>
+	{
+		private readonly List<Iterator.Variable> _variables = new();
+
+		public Rule Rule { get; }
+
+		public int Count => _variables.Count;
+		public Iterator.Variable? this[int index]
+			=> _variables.Find(v => v.Index == index);
+		public Iterator.Variable? this[string name]
+			=> _variables.Find(v => string.Equals(
+				v.Name, name, StringComparison.OrdinalIgnoreCase));
+
+		internal VariableCollection(Rule rule)
+		{
+			Rule = rule;
+
+			// Find the $This variable, if the rule has one.
+			var thisIndex = ecs_filter_find_this_var(ecs_rule_get_filter(Rule));
+			if (thisIndex >= 0) _variables.Add(new(thisIndex, "This"));
+
+			// Find all the other "accessible" variables.
+			var count = ecs_rule_var_count(Rule);
+			for (var i = 0; i < count; i++) {
+				if ((i == thisIndex) || !ecs_rule_var_is_entity(Rule, i)) continue;
+				var name = ecs_rule_var_name(Rule, i).FlecsToString()!;
+				_variables.Add(new(i, name));
+			}
+		}
+
+		// IEnumerable implementation
+		public IEnumerator<Iterator.Variable> GetEnumerator() => _variables.GetEnumerator();
+		IEnumerator IEnumerable.GetEnumerator() => _variables.GetEnumerator();
+	}
 }