Does a Subject safely unsubscribe its subscribers when it completes?

Question

If I have this class with a subject that emits a single value during its life:

export class MyClass {
  myEmitter$: Subject<void> = new Subject();

  someMethod(){
    this.myEmitter$.next();
    this.myEmitter$.complete();
  }
}

and then in another class:

this.instanceOfMyClass.myEmitter.subscribe();

Should I unsubscribe from instanceOfMyClass.myEmitter$, given that the subject completes after emitting?

Answer 1

When you call complete on the subject, any subscribers will be automatically unsubscribed.

If you look at the source for the subject's complete method:

complete() {
  if (this.closed) {
    throw new ObjectUnsubscribedError();
  }
  this.isStopped = true;
  const { observers } = this;
  const len = observers.length;
  const copy = observers.slice();
  for (let i = 0; i < len; i++) {
    copy[i].complete();
  }
  this.observers.length = 0;
}

You'll see that the subject calls complete on each of its observers. And the Observable Contract states that:

When an Observable issues an [error] or [complete] notification to its observers, this ends the subscription. Observers do not need to issue an [unsubscribe] notification to end subscriptions that are ended by the Observable in this way.