Question

I’m working on a little Java game in which all sorts of events can happen. There are at least a couple of dozen basic events that various event handlers may be interested in. There are also several places in the code where these events might be triggered. Rather than forcing the event listeners to know which class they need to register with, I’d like to create some sort of a centralized message-dispatching system, which some classes would submit events into and interested classes could hook into to listen for certain kinds of events.

But I have some questions. Firstly, this seems like an obvious and common problem. Are there favorite implementations of simple, in-VM messaging systems? Seems like there would be.

Secondly, and more importantly, I’m trying to work out a reasonably elegant way for the dispatching class to know as little as possible about types of messages. I’d love to be able to create new kinds of events without modifying the message dispatcher at all. However, I have an opposite concern. I’d really like for the method signatures of the handling methods to be clear. In other words, I would prefer the following:

public class CollisionConsoleHandler implements CollisionListener {
  @Override
  public void spaceshipCollidedWithMeteor( Spaceship spaceship, Meteor meteor ) {
      //...
  }
}

over something more generic and harder to read:

public class CollisionConsoleHandler implements GameMessageListener {
   @Override
   public void handleMessage( GameMessage message ) {
     if( message instanceof SpaceshipCollisionMessage ) {
        Spaceship spaceship = ((SpaeshipCollisionMessage)message).getSpaceship();
        Meteor meteor = ((SpaeshipCollisionMessage)message).getMeteor();
        //...
     }
   }
}

But I don’t see any good ways to keep type-specific knowledge out of the dispatcher while at the same time keeping the method signatures clean and readable.

Ideas?

Answer 1

If there is a specific listener interface for each event. Each event is able to issue listeners calls itself. Then, the role of the dispatcher is to identify target listeners and to trigger the event notification on them.

For example, a generic event definition can be:

public interface GameEvent<L> {

   public void notify( final L listener);
}

If your CollisionListener is:

public interface CollisionListener {

    public void spaceshipCollidedWithMeteor( Spaceship spaceship, Meteor meteor );

}

Then, the corresponding event can be:

public final class Collision implements GameEvent<CollisionListener> {

   private final Spaceship ship;
   private final Meteor meteor;

   public Collision( final Spaceship aShip, final Meteor aMeteor ) {
      this.ship = aShip;
      this.meteor = aMeteor;
   }

   public void notify( final CollisionListener listener) {
      listener.spaceshipCollidedWithMeteor( ship, meteor );
   }

}

You can imagine a dispatcher which is able to propagate this event on the target listeners like in the following scenario (Events is the dispatcher class):

// A unique dispatcher
final static Events events = new Events();

// Somewhere, an observer is interested by collision events 
CollisionListener observer = ...
events.listen( Collision.class, observer );

// there is some moving parts        
Spaceship aShip = ...
Meteor aMeteor = ...

// Later they collide => a collision event is notified trough the dispatcher
events.notify( new Collision( aShip, aMeteor  ) );

In this scenario, the dispatcher did not require any knowledge on events and listeners. It triggers an individual event notification to each listener, using only the GameEvent interface. Each event/listener pair chooses it’s own dialog modalities (they can exchange many messages if they want).

A typical implementation of a such dispatcher should be something like:

public final class Events {

   /** mapping of class events to active listeners **/
   private final HashMap<Class,ArrayList> map = new HashMap<Class,ArrayList >( 10 );

   /** Add a listener to an event class **/
   public <L> void listen( Class<? extends GameEvent<L>> evtClass, L listener) {
      final ArrayList<L> listeners = listenersOf( evtClass );
      synchronized( listeners ) {
         if ( !listeners.contains( listener ) ) {
            listeners.add( listener );
         }
      }
   }

    /** Stop sending an event class to a given listener **/
    public <L> void mute( Class<? extends GameEvent<L>> evtClass, L listener) {
      final ArrayList<L> listeners = listenersOf( evtClass );
      synchronized( listeners ) {
         listeners.remove( listener );
      }
   }

   /** Gets listeners for a given event class **/
   private <L> ArrayList<L> listenersOf(Class<? extends GameEvent<L>> evtClass) {
      synchronized ( map ) {
         @SuppressWarnings("unchecked")
         final ArrayList<L> existing = map.get( evtClass );
         if (existing != null) {
            return existing;
         }

         final ArrayList<L> emptyList = new ArrayList<L>(5);
         map.put(evtClass, emptyList);
         return emptyList;
      }
   }


   /** Notify a new event to registered listeners of this event class **/
   public <L> void notify( final GameEvent<L> evt) {
      @SuppressWarnings("unchecked")
      Class<GameEvent<L>> evtClass = (Class<GameEvent<L>>) evt.getClass();

      for ( L listener : listenersOf(  evtClass ) ) {
         evt.notify(listener);
      }
   }

}   

I suppose its fulfills your requirements:

• very light,
• fast,
• no casts (at usage),
• Every thing is checked at compile
  time (no possible mistake),
• No API constraints on listeners (each event
  choose it’s own messages),
• Evolutive (no dependencies between
  different events and/or listeners),
• The dispatcher is a generic black
  box,
• Consumers and producers don’t need to
  know each other.