Rename Iterator.FieldRef to .Field and more

- Add FieldOrEmpty for reference types
- Reorder methods in Iterator class

src/gaemstone.ECS/Iterator.cs

@@ -40,6 +40,7 @@ public unsafe class Iterator
 				ecs_iter_fini(ptr);
 	}
 
+
 	public EntityRef GetVar(Variable var)
 	{
 		fixed (ecs_iter_t* ptr = &Value)
@@ -53,6 +54,7 @@ public unsafe class Iterator
 		return this;
 	}
 
+
 	public bool Next()
 	{
 		fixed (ecs_iter_t* ptr = &Value) {
@@ -68,8 +70,23 @@ public unsafe class Iterator
 		}
 	}
 
-	public EntityRef Entity(int index)
-		=> new(World, new(Value.entities[index]));
+
+	public bool FieldIsSet(int index)
+	{
+		fixed (ecs_iter_t* ptr = &Value)
+			return ecs_field_is_set(ptr, index);
+	}
+
+	public bool FieldIs(int index, Id id)
+	{
+		fixed (ecs_iter_t* ptr = &Value)
+			return ecs_field_id(ptr, index) == id.Value;
+	}
+
+	// TODO: Potentially misleading, doesn't check the field's backing data type.
+	//       The id might be "(Identifier, Name)", but its data type "Identifier".
+	public bool FieldIs<T>(int index)
+		=> FieldIs(index, World.LookupByType<T>());
 
 	public IdRef FieldId(int index)
 	{
@@ -77,6 +94,10 @@ public unsafe class Iterator
 			return new(World, new(ecs_field_id(ptr, index)));
 	}
 
+
+	public EntityRef Entity(int index)
+		=> new(World, new(Value.entities[index]));
+
 	public Span<T> Field<T>(int index)
 		where T : unmanaged
 	{
@@ -88,27 +109,15 @@ public unsafe class Iterator
 		}
 	}
 
-	public Span<T> FieldOrEmpty<T>(int index)
-		where T : unmanaged => FieldIsSet(index) ? Field<T>(index) : default;
+	public SpanToRef<T> Field<T>(int index, T _ = null!) where T : class
+		=> new(Field<ReferenceHandle>(index));
 
-	public SpanToRef<T> FieldRef<T>(int index)
-		where T : class => new(Field<ReferenceHandle>(index));
+	public Span<T> FieldOrEmpty<T>(int index) where T : unmanaged
+		=> FieldIsSet(index) ? Field<T>(index) : Span<T>.Empty;
 
-	public bool FieldIsSet(int index)
-	{
-		fixed (ecs_iter_t* ptr = &Value)
-			return ecs_field_is_set(ptr, index);
-	}
+	public SpanToRef<T> FieldOrEmpty<T>(int index, T _ = null!) where T : class
+		=> FieldIsSet(index) ? Field(index, _) : SpanToRef<T>.Empty;
 
-	// TODO: Potentially misleading, doesn't check the field's backing data type.
-	//       The id might be "(Identifier, Name)", but its data type "Identifier".
-	public bool FieldIs<T>(int index)
-		=> FieldIs(index, World.LookupByType<T>());
-	public bool FieldIs(int index, Id id)
-	{
-		fixed (ecs_iter_t* ptr = &Value)
-			return ecs_field_id(ptr, index) == id.Value;
-	}
 
 	public override string ToString()
 	{
@@ -132,6 +141,7 @@ public unsafe class Iterator
 	public readonly ref struct SpanToRef<T>
 		where T : class
 	{
+		public static SpanToRef<T> Empty => default;
 		private readonly Span<ReferenceHandle> _span;
 		public int Length => _span.Length;
 		public T? this[int index] => (T?)_span[index].Target;