Move all the event handling code from Engine to EventHandler. EventHandler has a
reference to Engine and can deal with entities from its methods.
Refactor Action to take an optional Entity in its initializer. Some actions
don't require an Entity, but many do/will.
Refactor MovePlayerAction into a few different Action subclasses. Move direction
to a parent MoveAction, and create three new subclasses of MoveAction:
- BumpAction: perform the test that an action can be performed in the given direction
- WalkAction, take a step in the given direction
- MeleeAction, attack another Entity in the given direction
Add an ActionResult class that communicates the result of performing an Action.
- ActionResult.succeeded indicates whether the action succeeded.
- ActionResult.done indicates if the action is fully complete or requires followup.
- ActionResult.alternate specifies the follow-up action to perform.
Convert all the key handling actions to BumpActions.
In the Engine's event handler method, loop until an action is completed,
performing specified alternate actions until the result object indicates the
action is done.
