CODEWITHSUNDEEP
rxjs
User 5842
User 5842

Reputation: 3029

Does an Observable necessarily complete when it emits data?

When working with rxjs, I often subscribe to my source Observable and using the onData and onError handlers, attach some logic to take care of each case. That looks something like:

this.myService.myObservable().subscribe(
    () => this.onNextData(),
    () => this.onErrorData()
);

Then (and sometimes), inside of each of the handlers, I will do the same logic, say, close a modal on success and on failure.

I saw that there is a finalize operator that I can pipe that will execute some logic based on onComplete and onError. However, are both of these states the same? Does an Observable necessarily complete when it emits data?

Example:

this.myService.myObservable()
    // modalClose always happens on success and on failure (?)
    .pipe(finalize(() => this.modalClose())
    .subscribe(
        () => this.onNextData(),
        () => this.onErrorData()
);

Upvotes: 2

Views: 782

Answers (1)

Vlad274
Vlad274

Reputation: 6844

No, an Observable does not necessarily complete when it emits data.

Observables, unlike promises, are explicitly designed for multiple emissions - so it makes no sense for them to always complete after a single emission.

Think of the complete as the Observable saying "I will never send you more data".

For a very common use-case like making a web request - it is normal that it will either emit a value or an error once and then complete.

For a different use-case such as one part of the application communicating to another - it is normal that it will send multiple messages and only complete when the source is removed.

From the documentation:

Observables are lazy Push collections of multiple values. They fill the missing spot in the following table:

table

Upvotes: 2

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