gaemstone.ECS/src/gaemstone.ECS/Observer.cs

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

namespace gaemstone.ECS;

public static class ObserverExtensions
{
	public static unsafe Entity<TContext> InitObserver<TContext>(this Entity<TContext> entity,
		FilterDesc filter, Action<Iterator<TContext>> callback, params Entity[] events)
	{
		if (events.Length == 0) throw new ArgumentException("Must specify at least 1 event", nameof(events));
		if (events.Length >  8) throw new ArgumentException("Must specify at most 8 events", nameof(events));

		var world = entity.World;
		var internalCallback = (nint iterPtr) => callback(new((ecs_iter_t*)iterPtr));
		using var alloc = TempAllocator.Use();
		var desc = new ecs_observer_desc_t {
			entity = entity,
			filter = filter.ToFlecs(alloc),
			binding_ctx      = (void*)CallbackContextHelper.Create(internalCallback),
			binding_ctx_free = new() { Data = new() { Pointer = &FreeContext } },
			callback         = new() { Data = new() { Pointer = &Callback } },
		};
		var span = desc.events;
		for (var i = 0; i < events.Length; i++)
			span[i] = events[i];
		if (ecs_observer_init(world, &desc).Data == default)
			throw new InvalidOperationException();
		return entity;
	}

	[UnmanagedCallersOnly]
	private static unsafe void Callback(ecs_iter_t* iter)
		=> CallbackContextHelper
			.Get<Action<nint>>((nint)iter->binding_ctx)
			.Invoke((nint)iter);

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