Add SingletonAttribute.AutoAdd

Singletons no longer auto-add themselves,
unless AutoAdd is set to true.

src/gaemstone/ECS/Attributes.cs

@@ -19,6 +19,18 @@ public interface ICreateEntityAttribute {  }
 [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
 public class PrivateAttribute : Attribute {  }
 
+/// <summary>
+/// A singleton is a single instance of a tag or 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
+{
+	/// <summary> Whether to add the entity to itself on registration. </summary>
+	public bool AutoAdd { get; init; } = false;
+}
+
 /// <summary>
 /// Register the proxied type instead of the one marked with this attribute.
 /// This can be used to make types not registered in a module available,

src/gaemstone/ECS/Entity.cs

@@ -21,18 +21,6 @@ public class EntityAttribute : Attribute, ICreateEntityAttribute
 	}
 }
 
-/// <summary>
-/// A singleton is a single instance of a tag or 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 : EntityAttribute
-{
-	public SingletonAttribute() {  }
-	public SingletonAttribute(params string[] path) : base(path) {  }
-}
-
 public readonly struct Entity
 	: IEquatable<Entity>
 {

src/gaemstone/ECS/Universe+Modules.cs

@@ -198,7 +198,7 @@ internal class ModuleInfo
 			foreach (var attr in type.GetMultiple<AddRelationAttribute>())
 				entity.Add(Lookup(attr.Relation), Lookup(attr.Target));
 
-			if (type.Has<SingletonAttribute>()) entity.Add(entity);
+			if (type.Get<SingletonAttribute>()?.AutoAdd == true) entity.Add(entity);
 			if (type.Has<ComponentAttribute>()) entity.CreateComponent(proxyType);
 			else                                entity.CreateLookup(proxyType);
 		}

src/gaemstone/Utility/IL/IterActionGenerator.cs

@@ -271,8 +271,8 @@ public unsafe class IterActionGenerator
 			// Reference types have a backing type of nint - they're pointers.
 			FieldType = underlyingType.IsValueType ? underlyingType : typeof(nint);
 
-			Source = Info.Get<SourceAttribute>()?.Type
-				?? (UnderlyingType.Has<SingletonAttribute>() ? UnderlyingType : null);
+			if (UnderlyingType.Has<SingletonAttribute>()) Source = UnderlyingType;
+			if (Info.Get<SourceAttribute>()?.Type is Type type) Source = type;
 		}
 
 		public static ParamInfo Build(ParameterInfo info)