gaemstone/src/gaemstone/ECS/Attributes.cs

using System;
using static gaemstone.Flecs.Core;

namespace gaemstone.ECS;

/// <summary>
/// When present on an attribute attached to a type that's part of a module
/// being registered automatically through <see cref="ModuleManager.Register"/>,
/// an entity is automatically created and <see cref="LookupExtensions.CreateLookup"/>
/// called on it, meaning it can be looked up using <see cref="World.LookupByType(Type)"/>.
/// </summary>
public interface ICreateEntityAttribute {  }

[AttributeUsage(AttributeTargets.Struct)]
public class EntityAttribute : Attribute, ICreateEntityAttribute {  }

/// <summary>
/// Use a custom name or path for this entity instead of the type's name.
/// </summary>
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
public class PathAttribute : Attribute, ICreateEntityAttribute
{
	public string Value { get; }
	public PathAttribute(string value) => Value = value;
}

/// <summary>
/// Register the entity under a globally unique symbol.
/// Uses the type's name by default.
/// </summary>
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
public class SymbolAttribute : Attribute, ICreateEntityAttribute
{
	public string? Value { get; }
	public SymbolAttribute() {  }
	public SymbolAttribute(string value) => Value = value;
}

/// <summary>
/// A singleton is a single instance of a tag or component that can be retrieved
/// without explicitly specifying an entity in a query, where it is equivalent
/// to <see cref="SourceAttribute{}"/> with itself as the generic type parameter.
/// </summary>
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
public class SingletonAttribute : Attribute, ICreateEntityAttribute
	{ public bool AutoAdd { get; init; } = true; }

/// <summary>
/// Register the proxied type instead of the one marked with this attribute.
/// This can be used to make types not registered in a module available,
/// including types registered in other assemblies.
/// </summary>
public class ProxyAttribute<T> : ProxyAttribute
	{ public ProxyAttribute() : base(typeof(T)) {  } }


/// <summary>
/// Marked entity automatically has the specified entity added to it when
/// automatically registered. Equivalent to <see cref="EntityBase.Add{T}"/>.
/// </summary>
public class AddAttribute<TEntity> : AddEntityAttribute
	{ public AddAttribute() : base(typeof(TEntity)) {  } }

/// <summary>
/// Marked entity automatically has the specified relationship pair added to it when
/// automatically registered, Equivalent to <see cref="EntityBase.Add{TRelation, TTarget}"/>.
/// </summary>
public class AddAttribute<TRelation, TTarget> : AddRelationAttribute
	{ public AddAttribute() : base(typeof(TRelation), typeof(TTarget)) {  } }

/// <summary>
/// Marked entity represents a relationship type.
/// It may be used as the "relation" in a pair.
/// </summary>
/// <remarks>
/// The relationship may have component data associated with
/// it when added to an entity under these circumstances:
/// <list type="bullet">
///   <item>If marked as a <see cref="TagAttribute"/>, does not carry data.</item>
///   <item>If marked as a <see cref="ComponentAttribute"/>, carries the relation's data.</item>
///   <item>If marked with neither, will carry the target's data, if it's a component.</item>
/// </list>
/// </remarks>
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
public class RelationAttribute : Attribute, ICreateEntityAttribute {  }

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
public class ComponentAttribute : Attribute, ICreateEntityAttribute {  }

/// <seealso cref="Tag"/>
[AttributeUsage(AttributeTargets.Struct)]
public class TagAttribute : AddAttribute<Tag>, ICreateEntityAttribute {  }


/// <seealso cref="IsA"/>
public class IsAAttribute<TTarget> : AddAttribute<IsA, TTarget> {  }

/// <seealso cref="ChildOf"/>
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum)]
public class ChildOfAttribute<TTarget> : AddAttribute<ChildOf, TTarget> {  }

/// <seealso cref="DependsOn"/>
public class DependsOnAttribute<TTarget> : AddAttribute<DependsOn, TTarget> {  }


/// <seealso cref="Exclusive"/>
public class ExclusiveAttribute : AddAttribute<Exclusive> {  }

/// <seealso cref="With"/>
public class WithAttribute<TTarget> : AddAttribute<With, TTarget> {  }


// Base attributes for other attributes.

[AttributeUsage(AttributeTargets.Struct)]
public class ProxyAttribute : Attribute
{
	public Type Type { get; }
	internal ProxyAttribute(Type type) => Type = type;
}

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum, AllowMultiple = true)]
public class AddEntityAttribute : Attribute
{
	public Type Entity { get; }
	internal AddEntityAttribute(Type entity) => Entity = entity;
}

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum, AllowMultiple = true)]
public class AddRelationAttribute : Attribute
{
	public Type Relation { get; }
	public Type Target { get; }
	internal AddRelationAttribute(Type relation, Type target)
		{ Relation = relation; Target = target; }
}