gaemstone/src/gaemstone/ECS/System.cs

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

namespace gaemstone.ECS;

[AttributeUsage(AttributeTargets.Method)]
public class SystemAttribute : Attribute
{
	public Type Phase { get; }
	public SystemAttribute() : this(typeof(SystemPhase.OnUpdate)) {  }
	internal SystemAttribute(Type phase) => Phase = phase; // Use generic type instead.
}
public class SystemAttribute<TPhase> : SystemAttribute
	{ public SystemAttribute() : base(typeof(TPhase)) {  } }

[AttributeUsage(AttributeTargets.Method)]
public class ExpressionAttribute : Attribute
{
	public string Value { get; }
	public ExpressionAttribute(string value) => Value = value;
}

public static class SystemExtensions
{
	public static EntityRef InitSystem(this World world, Delegate action)
	{
		var attr = action.Method.Get<SystemAttribute>();
		var expr = action.Method.Get<ExpressionAttribute>()?.Value;

		QueryDesc query;
		if (action is Action<Iterator> callback) {
			query = new(expr ?? throw new ArgumentException(
				"System must specify ExpressionAttribute", nameof(action)));
		} else {
			var gen  = IterActionGenerator.GetOrBuild(world, action.Method);
			query    = (expr != null) ? new(expr) : new(gen.Terms.ToArray());
			callback = iter => gen.RunWithTryCatch(action.Target, iter);
		}

		var phase = world.LookupByTypeOrThrow(attr?.Phase ?? typeof(SystemPhase.OnUpdate));
		return world.New(action.Method.Name).Build().InitSystem(phase, query, callback);
	}

	public static EntityRef InitSystem(this World world,
		object? instance, MethodInfo method)
	{
		var attr = method.Get<SystemAttribute>();
		var expr = method.Get<ExpressionAttribute>()?.Value;

		QueryDesc query;
		Action<Iterator> callback;
		var param = method.GetParameters();
		if ((param.Length == 1) && (param[0].ParameterType == typeof(Iterator))) {
			query = new(expr ?? throw new ArgumentException(
				"System must specify ExpressionAttribute", nameof(method)));
			callback = (Action<Iterator>)Delegate.CreateDelegate(typeof(Action<Iterator>), instance, method);
		} else {
			var gen  = IterActionGenerator.GetOrBuild(world, method);
			query    = (expr != null) ? new(expr) : new(gen.Terms.ToArray());
			callback = iter => gen.RunWithTryCatch(instance, iter);
		}

		var phase = world.LookupByTypeOrThrow(attr?.Phase ?? typeof(SystemPhase.OnUpdate));
		return world.New(method.Name).Build().InitSystem(phase, query, callback);
	}
}