CODEWITHSUNDEEP
javascripttypescriptrxjs
Juraj Mlich
Juraj Mlich

Reputation: 688

RxJS - emit only if other does not emit during delay

Suppose I have two observables. Every time the first one emits, I want to wait 2 seconds to see whether during that period the other observable emits something. If yes, do not emit anything. If no, emit.

Example:

const start$ = this.events.pipe(
  delay(1000),
  takeUntil(theSecondObservable) // WHAT SHOULD BE HERE?
).subscribe((value) => {
  console.log(value);
});

Upvotes: 4

Views: 2695

Answers (4)

Trevor Karjanis
Trevor Karjanis

Reputation: 1724

This solution does not complete when the end$ observable emits and uses RxJS 6 syntax.

const start$ = this.router.events.pipe(map(() => true));
const end$ = this.router.events.pipe(map(() => false));

merge(start$, end$)
  .pipe(debounceTime(2000), filter(value => value))
  .subscribe(value => console.log(value));

If you're filtering router events you may not need two observables.

this.router.events
  .pipe(debounceTime(2000), filter(event => event instanceof NavigationEnd))
  .subscribe(value => console.log(value));

Upvotes: 2

Juraj Mlich
Juraj Mlich

Reputation: 688

I was able to solve it with the help of the answers above. However, they did not completely answer my question and there might be a better way. My solution:

const end$ = this.router.events.pipe(
  map(() => false),
);
const start$ = this.router.events.pipe(
  map(() => true),
);

merge(
  end$,
  start$.pipe(
    switchMap(() => {
      return race(end$, of(true).pipe(delay(2000)));
    }),
    filter((value) => value === true) // this really works only with booleans, not sure how I'd do it with more 
                                                            // complex objects
  )
).subscribe((value) => {
  console.log(value);
});

Upvotes: 0

Nuno Sousa
Nuno Sousa

Reputation: 882

Are you interesting in limiting the emitions from the 1st observer or just in the end result?

You can use a race...

let r = race(observer1.pipe(delay(2000)), observer2)
 // r will emmit from observer1 only if observer2 didnt emmit, else it will emmit from observer2

Upvotes: 0

laptou
laptou

Reputation: 6981

Use the race operator. 

const start$ = race(this.events.pipe(delay(1000)), theSecondObservable).subscribe(value => /** ... */);

Once one of the observables emits an object, the other observable will be ignored and only values emitted by the 'winner' will be passed on to the subscriber. In your subscription function, you can perform some logic to figure out which observable your value came from.

Upvotes: 0

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