gaemstone/src/gaemstone/ECS/Observer.cs

using System;
using System.Linq;
using System.Reflection;
using System.Runtime.InteropServices;
using gaemstone.Utility;
using gaemstone.Utility.IL;
using static flecs_hub.flecs;

namespace gaemstone.ECS;

[AttributeUsage(AttributeTargets.Method)]
public class ObserverAttribute : Attribute
{
	public Type Event { get; }
	internal ObserverAttribute(Type @event) => Event = @event; // Use generic type instead.
}
public class ObserverAttribute<TEvent> : ObserverAttribute
	{ public ObserverAttribute() : base(typeof(TEvent)) {  } }

public static class ObserverExtensions
{
	public static unsafe EntityRef RegisterObserver(this Universe universe,
		string? name, FilterDesc filter, Entity @event, Action<Iterator> callback)
	{
		using var alloc = TempAllocator.Use();
		var desc = new ecs_observer_desc_t {
			filter = filter.ToFlecs(alloc),
			entity = universe.New((name != null) ? EntityPath.Parse(name) : null).Build(),
			binding_ctx = (void*)CallbackContextHelper.Create((universe, callback)),
			callback = new() { Data = new() { Pointer = &Callback } },
		};
		desc.events[0] = @event;
		return new(universe, new(ecs_observer_init(universe, &desc)));
	}

	public static EntityRef RegisterObserver(this Universe universe,
		object? instance, MethodInfo method)
	{
		var attr = method.Get<ObserverAttribute>() ?? throw new ArgumentException(
			"Observer must specify ObserverAttribute", nameof(method));
		var expr = method.Get<ExpressionAttribute>()?.Value;
		FilterDesc filter;
		Action<Iterator> iterAction;

		var param = method.GetParameters();
		if ((param.Length == 1) && (param[0].ParameterType == typeof(Iterator))) {
			filter = new(expr ?? throw new Exception(
				"Observer must specify ExpressionAttribute"));
			if (method.IsStatic) instance = null;
			iterAction = (Action<Iterator>)Delegate.CreateDelegate(
				typeof(Action<Iterator>), instance, method);
		} else {
			var gen = IterActionGenerator.GetOrBuild(universe, method);
			filter  = (expr != null) ? new(expr) : new(gen.Terms.ToArray());
			iterAction = iter => gen.RunWithTryCatch(instance, iter);
		}

		var @event  = universe.LookupOrThrow(attr.Event);
		return universe.RegisterObserver(method.Name, filter, @event, iterAction);
	}

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