Question

How can you avoid circular dependencies when you’re designing two classes with a producer/consumer relationship? Here ListenerImpl needs a reference to Broadcaster in order to register/unregister itself, and Broadcaster needs a reference back to the Listeners in order to send messages. This example is in Java but it can apply to any OO language.

public interface Listener {   void callBack(Object arg); } public class ListenerImpl implements Listener {   public ListenerImpl(Broadcaster b) { b.register(this); }   public void callBack(Object arg) { ... }   public void shutDown() { b.unregister(this); } } public class Broadcaster {   private final List listeners = new ArrayList();   public void register(Listener lis) { listeners.add(lis); }   public void unregister(Listener lis) {listeners.remove(lis); }   public void broadcast(Object arg) { for (Listener lis : listeners) { lis.callBack(arg); } } } 

Answer 1

I don’t see that being a circular dependency.

Listener depends on nothing.

ListenerImpl depends on Listener and Broadcaster

Broadcaster depends on Listener.

        Listener        ^        ^       /          \      /            \ Broadcaster <--  ListenerImpl 

All arrows end at Listener. There’s no cycle. So, I think you’re OK.