如何在定义观察者时在第一次发射时取消订阅 Observable/Subject

Question

I am working on some dialogs with angular 9 and I came into a situation where I need to unsubscribe from a Subject after the first emission.

Initially, I thought of piping the subject using the take(1) or first() operator and subscribe to this "transformed finite subject". So the unsubscription will be handled automatically just after the first emissions ocurrs, because when an observable completes the unsubscription logic is called. Example:

const subject = new Subject();

subject.pipe(take(1)).subscribe(console.log);

subject.next("hola");

But then I have also found this "hack" that I think its also working, can someone confirm?

const subject = new Subject();

const subscription = subject.subscribe((value) => {
  console.log(value);
  subscription.unsubscribe(); // <----- this is the magic line
}

subject.next("hola");

I have never seen this before, I think its convenient in some occasions. The thing that confused me is that the observer is calling the subscription defined in the same line. Is this good practice? Is there any other way of achieving this?

Thanks!

Answer 1

You can use first() or take(1) . both unsubscribe automatically when their condition is met.

const { Subject } = require('rxjs/');
const { take, first } = require('rxjs/operators');

const subject = new Subject();

subject.pipe(first()).subscribe(data => {
  console.log({ data });
});

subject.pipe(take(1)).subscribe(data => {
  console.log({ data });
});