Creating Observable from normal Java events

What is the best way to create an Rx-Java Observable from the classical Java event pattern? That is, given

class FooEvent { ... }

interface FooListener {
  void fooHappened(FooEvent arg);
}

class Bar {
  public void addFooListener(FooListener l);
  public void removeFooListener(FooListener l);
}

I want to implement

Observable<FooEvent> fooEvents(Bar bar);

The implementation I came up with is:

Observable<FooEvent> fooEvents(Bar bar) {
  return Observable.create(new OnSubscribeFunc<FooEvent>() {
    public Subscription onSubscribe(Observer<? super FooEvent> obs) {
      FooListener l = new FooListener() {
        public void fooHappened(FooEvent arg) {
          obs.onNext(arg);
        }
      };

      bar.addFooListener(l);

      return new Subscription() {
        public void unsubscribe() {
          bar.removeFooListener(l);
        }
      };
    }
  }); 
}

However, I don't really like it:

1. it's quite verbose;

2. requires a listener per Observer (ideally there should be no listeners if there are no observers, and one listener otherwise). This can be improved by keeping an observer count as a field in the OnSubscribeFunc, incrementing it on subscribe and decrementing on unsubscribe.

Is there a better solution?

Requirements:

1. Working with existing implementations of event patterns without changing them (if I was controlling that code, I could already write it to return Observable I needed).

2. Getting compiler errors if/when the source API changes. No working with Object instead of actual event argument type or with property name strings.