public class Command<E> {

    public CommandActionObservable execute() {
        final CommandAction<E> command = createCommand();

        final OnSubscribe<CommandAction<E>> onSubscribe = (subscriber) -> {

            /* Create a listener that handles notifications and register it.
             * The idea here is to push the command downstream so it can be re-executed
             */
            final Listener listener = (event) -> {
              subscriber.onNext(command);
            }
            registerListener(listener);

            /* This is where I'm having trouble. The unregister method
             * should be executed when the subscriber unsubscribed,
             * but it never happens
             */
            subscriber.add(Subscriptions.create(() -> {
                unregisterListener(listener);
            }));

            // pass the initial command downstream
            subscriber.onNext(command);

            kickOffBackgroundAction();
        }

        final Observable<CommandAction<E>> actionObservable = Observable.create(onSubscribe)
            .onBackpressureLatest()
            .observeOn(Shedulers.io())
            .onBackpressureLatest();
        return new CommandActionObservable((subscriber) -> {
            actionObservable.unsafeSubscribe(subscriber);
        })
    }

    public class CommandActionObservable extends Observable<CommandAction<E> {

        // default constructor omitted

        public Observable<E> toResult() {
            return lift((Operator) (subscriber) ->  {
                return new Subscriber<CommandAction<E>>() {
                    // delegate onCompleted and onError to subscriber

                    public void onNext(CommandAction<E> action) {
                        // execute the action and pass the result downstream
                        final E result = action.execute();
                        subscriber.onNext(result)
                    }
                }
            }
        }

    }

}

final Observable<SomeType> obs = new Command<SomeType>()
                                       .execute()
                                       .toResult();
subscription.add(obs.subscribe(// impl here));



public void onDestroy() {
    super.onDestroy();
    subscription.unsubscribe();
}