Angular 6 / Rxjs – how to basics: observables success, error, finally

by

I think there’s one key misunderstanding:

You either have someone who wants “success and finally”, or “success and error” but none wants the 3 of them.

This isn’t entirely true. Each Observable can send zero or more next notifications and one error or complete notification but never both. For example when making a successful HTTP call you’ll have one next and one complete notification. On error HTTP request you’ll have only one error notification and that’s all. See http://reactivex.io/documentation/contract.html

This means you’ll never have an Observable emitting both error and complete.

And then there’s the finalize operator. This operator is invoked when disposing the chain (which includes plain unsubscribing as well). In other words it’s called after both error and complete notifications.

So the second example you have is correct. I understand it looks weird that you include finalize before subscribing but in fact each emissions from the source Observable goes first from top to bottom where it reaches subscribers and there if its error or complete notification it triggers dispose handlers bottom up (in opposite order) and at this point finalize is called. See https://github.com/ReactiveX/rxjs/blob/master/src/internal/Subscriber.ts#L150-L152

In your example using finalize is the same as adding the dispose handler yourself into a Subscription objects.

const subscription = this.service.getAll()
  .subscribe(
    (data) => this.onSuccess(data),
    (error) => this.handleError(error)
  );

subscription.add(() => this.stopLoading());

Leave a Comment