How can I receive notifications when Rxjs Subject is subscribed to / unsubscribed from

Question

I'm trying to use Rxjs as a way to manage garbage collection in a large state tree.

How can I create an operator that takes a callback function that is triggered every time the number of subscribers to the observable alters?

Answer 1

Multiple ways, all involve hiding your subject and giving consumers a wrapped observable:

Want to know when something subscribes to your subject?

const subject = new Subject();
const observable = Observable.defer(() => {
  someoneJustSubscribed();
  return subject;
});
return observable;

Want to know when someone unsubscribes?

const subject = new Subject();
const observable = subject.finally(() => someoneJustUnsubscribed());
return observable;

Want to know both?

const subject = new Subject();
const observable = Observable.create(observer => {
    someoneJustSubscribed();
    const sub = subject.subscribe(observer);
    return () => {
        someoneJustUnsubscribed();
        sub.unsubscribe();
    }
});
return observable;