+class WorldEvent(Event):
+ """ Be a hero. Change the world. """
+
+ ENGINE_METHOD = "change_world"
+
+
+class QuitEvent(Event):
+ """ Quit the game. """
+
+ ENGINE_METHOD = "quit_game"
+
+
+class MoveViewportEvent(Event):
+ """ Change to a new scene. """
+
+ ENGINE_METHOD = "move_screen"
+
+
+class Layer:
+ """ A single layer of actors. """
+
+ def __init__(self, name):
+ self.name = name
+ self.actors = []
+
+ def __iter__(self):
+ return iter(self.actors)
+
+ def add(self, actor):
+ self.actors.append(actor)
+ return actor
+
+ def remove(self, actor):
+ self.actors.remove(actor)
+ return actor
+
+
+class Actors:
+ """ Layers of actors.
+
+ Actors may be rendered in different layers. Layers with lower levels
+ are rendered lower than layers with higher ones.
+ """
+
+ def __init__(self):
+ self._ordered_layers = []
+ self._layers = {}
+ self.add_layer("default", 0)
+
+ def __getattr__(self, name):
+ return self._layers[name]
+
+ def add_layer(self, name, level):
+ layer = self._layers[name] = Layer(name)
+ self._ordered_layers.append((level, name))
+ self._ordered_layers.sort()
+ return layer
+
+ def add(self, actor, layer="default"):
+ return self._layers[layer].add(actor)
+
+ def remove(self, actor, layer="default"):
+ return self._layers[layer].remove(actor)
+
+ def draw(self, screen):
+ for lvl, name in self._ordered_layers:
+ for actor in self._layers[name]:
+ actor.draw() # TODO: allow an option screen to be passed in
+
+