+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 Actors:
+ """ A list 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._layer_order = []
+ self._layers = {}
+ self.add_layer("default", 0)
+
+ def add_layer(self, name, level):
+ self._layer_order.append((level, name))
+ self._layer_order.sort()
+ self._layers[name] = []
+
+ def add(self, actor, layer="default"):
+ self._layers[layer].append(actor)
+ return actor
+
+ def remove(self, actor, layer="default"):
+ self._layers[layer].remove(actor)
+ return actor
+
+ def draw(self, screen):
+ for lvl, layer in self._layer_order:
+ for actor in self._layers[layer]:
+ actor.draw() # TODO: allow an option screen to be passed in
+
+