gaemstone.ECS/src/gaemstone.ECS/System.cs

using System;
using System.Runtime.InteropServices;
using gaemstone.Utility;
using static flecs_hub.flecs;

namespace gaemstone.ECS;

public static class SystemExtensions
{
	public static unsafe EntityRef InitSystem(this EntityRef entity,
		Entity phase, QueryDesc query, Action<Iterator> callback)
	{
		var world = entity.World;
		entity.Add(world.DependsOn, phase);
		using var alloc = TempAllocator.Use();
		var desc = new ecs_system_desc_t {
			entity = entity,
			query  = query.ToFlecs(alloc),
			binding_ctx      = (void*)CallbackContextHelper.Create((world, callback)),
			binding_ctx_free = new() { Data = new() { Pointer = &FreeContext } },
			callback         = new() { Data = new() { Pointer = &Callback } },
		};
		return new(world, new(ecs_system_init(world, &desc)));
	}

	[UnmanagedCallersOnly]
	private static unsafe void Callback(ecs_iter_t* iter)
	{
		var (world, callback) = CallbackContextHelper
			.Get<(World, Action<Iterator>)>((nint)iter->binding_ctx);
		callback(new Iterator(world, null, *iter));
	}

	// [UnmanagedCallersOnly]
	// private static unsafe void Run(ecs_iter_t* flecsIter)
	// {
	// 	var (world, callback) = CallbackContextHelper
	// 		.Get<(World, Action<Iterator>)>((nint)flecsIter->binding_ctx);

	// 	// This is what flecs does, so I guess we'll do it too!
	// 	var type = (&flecsIter->next == (delegate*<ecs_iter_t*, Runtime.CBool>)&ecs_query_next)
	// 		? IteratorType.Query : (IteratorType?)null;
	// 	using var iter = new Iterator(world, type, *flecsIter);

	// 	If the method is marked with [Source], set the $This variable.
	// 	if (Method.Get<SourceAttribute>()?.Type is Type sourceType)
	// 		iter.SetThis(World.LookupOrThrow(sourceType));

	// 	if (flecsIter->field_count == 0) callback(iter);
	// 	else while (iter.Next()) callback(iter);
	// }

	[UnmanagedCallersOnly]
	private static unsafe void FreeContext(void* context)
		=> CallbackContextHelper.Free((nint)context);
}