gaemstone/src/gaemstone/ECS/Observer.cs

using System;
using System.Reflection;
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; }
	public string? Expression { get; set; }

	public ObserverAttribute(Type @event) => Event = @event;
}

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

	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 filter = new FilterDesc();
		Action<Iterator> iterAction;

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

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