ecs: add callbackFiltered and emitFiltered

ecs/ecs.can

@@ -144,6 +144,10 @@ let copy = (a, b, cache={})
 	end
 end
 
+--- Self descriptive
+let alwaysTrue = () return true end
+let alwaysFalse = () return false end
+
 --- Skip list implementation ---
 -- Well technically it's a conbination of a skip list (for ordering) and a hash map (for that sweet O(1) search). Takes more memory but oh so efficient.
 
@@ -759,14 +763,30 @@ let system_mt = {
 	-- @tparam Entity e the entity to perform the callback on
 	-- @param ... other arguments to pass to the callback
 	callback = :(name, e, ...)
-		-- call callback
-		if @_has[e] and @[name] then
-			@[name](@, e, e[@component], ...)
-		end
-		-- callback on subsystems (if it wasn't removed during the callback)
-		if @_has[e] then
-			for _, ss in ipairs(@systems) do
-				ss:callback(name, e, ...)
+		return @callbackFiltered(alwaysTrue, name, e, ...)
+	end,
+	--- Same as `callback`, but will check every system against a filter before calling the callback.
+	--
+	-- `filter` is a function that receive the arguments `filter(system, name, e, ...)`, and returns a boolean.
+	-- It will be called on each system before emitting the callback on it; if the filter returns false, the callback will not
+	-- be called on this system and its subsystems.
+	--
+	-- Complexity: O(1) per system
+	-- @tparam function filter filter function
+	-- @tparam string name name of the callback
+	-- @tparam Entity e the entity to perform the callback on
+	-- @param ... other arguments to pass to the callback
+	callbackFiltered = :(filter, name, e, ...)
+		if filter(@, name, e, ...) then
+			-- call callback
+			if @_has[e] and @[name] then
+				@[name](@, e, e[@component], ...)
+			end
+			-- callback on subsystems (if it wasn't removed during the callback)
+			if @_has[e] then
+				for _, ss in ipairs(@systems) do
+					ss:callbackFiltered(filter, name, e, ...)
+				end
 			end
 		end
 	end,
@@ -793,19 +813,34 @@ let system_mt = {
 	-- @tparam string name name of the callback
 	-- @param ... other arguments to pass to the callback
 	emit = :(name, ...)
-		-- call event
-		let status
-		if @[name] then
-			status = @[name](@, ...)
-		end
-		-- call event on subsystems (if it wasn't stopped or captured)
-		if status ~= "stop" and status ~= "capture" then
-			for _, s in ipairs(@systems) do
-				status = s:emit(name, ...)
-				if status == "capture" then break end
+		return @emitFiltered(alwaysTrue, name, ...)
+	end,
+	--- Same as `emit`, but will check every system against a filter before calling the event.
+	--
+	-- `filter` is a function that receive the arguments `filter(system, name, ...)`, and returns a boolean.
+	-- It will be called on each system before emitting the event on it; if the filter returns false, the event will not
+	-- be emitted to this system and its subsystems.
+	--
+	-- Complexity: O(1) per system
+	-- @tparam function filter filter function
+	-- @tparam string name name of the callback
+	-- @param ... other arguments to pass to the callback
+	emitFiltered = :(filter, name, ...)
+		if filter(@, name, ...) then
+			-- call event
+			let status
+			if @[name] then
+				status = @[name](@, ...)
 			end
+			-- call event on subsystems (if it wasn't stopped or captured)
+			if status ~= "stop" and status ~= "capture" then
+				for _, s in ipairs(@systems) do
+					status = s:emitFiltered(filter, name, ...)
+					if status == "capture" then break end
+				end
+			end
+			return status
 		end
-		return status
 	end,
 	--- Remove all the entities and subsystems in this system.
 	-- Complexity: O(entityCount) per system
@@ -815,10 +850,6 @@ let system_mt = {
 	end,
 }
 
---- Self descriptive
-let alwaysTrue = () return true end
-let alwaysFalse = () return false end
-
 --- Recursively instanciate a list of systems for a world:
 -- * create their self table with instance fields set
 -- * create a field with their name in world.s (if name defined)