Add some ECS API todo's

libs/ecs/include/psemek/ecs/entity_container.hpp

@@ -19,6 +19,13 @@ namespace psemek::ecs
 {
 
 	using query_cache = std::shared_ptr<detail::query_cache>;
+	using registration_token = std::shared_ptr<void>;
+
+	// TODO:
+	// - Modification callbacks API
+	// - Modification callbacks implementation
+	// - Index API
+	// - Index implementation
 
 	struct entity_container
 	{
@@ -141,6 +148,36 @@ namespace psemek::ecs
 		template <typename ... Components, typename Function>
 		void batch_apply(Function && function, query_cache cache = {});
 
+		/** Register a constructor. Each time an entity is created that has
+		 *    the specified set of components (including attaching components),
+		 *    the constructor is called for this entity immediately after
+		 *    it is created.
+		 *  The constructor function must have the same signature as a function
+		 *    passed to the `apply<Components...>()` call.
+		 *  The returned value is an ownerwhip token. Destroying it removes
+		 *    the constructor from the entity_container.
+		 *  If any two of the passed component types are equal, the call fails with
+		 *    a compilation error.
+		 */
+		// TODO: implement
+		template <typename ... Components, typename Function>
+		registration_token constructor(Function && function);
+
+		/** Register a destructor. Each time an entity is destroyed that has
+		 *    the specified set of components (including detaching components),
+		 *    the destructor is called for this entity immediately before
+		 *    it will be destroyed.
+		 *  The destructor function must have the same signature as a function
+		 *    passed to the `apply<Components...>()` call.
+		 *  The returned value is an ownerwhip token. Destroying it removes
+		 *    the destructor from the entity_container.
+		 *  If any two of the passed component types are equal, the call fails with
+		 *    a compilation error.
+		 */
+		// TODO: implement
+		template <typename ... Components, typename Function>
+		registration_token destructor(Function && function);
+
 	private:
 		detail::entity_list entity_list_;
 		detail::table_container table_container_;