gaemstone/src/gaemstone/ECS/Observer.cs

using System;
using System.Linq;
using System.Reflection;
using gaemstone.Utility;
using gaemstone.Utility.IL;

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 EntityRef InitObserver(this World world,
		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(world, method);
			filter  = (expr != null) ? new(expr) : new(gen.Terms.ToArray());
			iterAction = iter => gen.RunWithTryCatch(instance, iter);
		}

		var @event = world.LookupByTypeOrThrow(attr.Event);
		return world.New(method.Name).Build().InitObserver(@event, filter, iterAction);
	}
}