Determine when Subject has no subscribers

Question

I want to create a broadcast system using PublishSubject, a background task will poll some endpoint and broadcast the result periodically using this Subject. I would like to start the polling when the first subscriber subscribes to the Subject, and stop the polling when there are no more subscribers. If a new subscriber subscribes, polling should resume.

The only function I see that is somewhat related is hasObservers() but it doesn't quite fit my needs, I would like to have callbacks for subscription and unsubscription - on the former I would start polling if not stated, and on the latter I would stop polling if there are no more subscribers; how could this be achieved?

Answer 1

You could create a wrapper around a subject that would keep count, but sounds like your problem could be solved with a ConnectableObservable. Consider this:

Observable<PollData> pollData = Observable.interval(1, TimeUnit.SECONDS)
                         .flatMap(i -> api.pollData())
                         .share();

Using the share() operator makes that observable become a ConnectableObservable that will start when the first observer subscribes to it, share all emissions with subsequent subscriptions, and automatically stop when it's last observer unsubscribes. Read more about it here.