Operator that skips the next emission from the source whenever another Observable emits

Question

I have a use case where I need an Observable to skip its next emission whenever another notifier Observable emits.

source:    |---X---X---X---X---X---X---X---X---X---X--|>
notifier:  |-------------X---------X----------X-------|>
result:    |---X---X---X-------X---X-------X-------X--|>

Basically, I want an operator called skipNextWhen that takes in the notifier observable and skips the next emission from the source.

I tried using an implementation that uses the pausable operator (re-implemented using switchMap), but couldn't get it to work.

pausable.ts

import { Observable } from 'rxjs/Observable';
import { Subject } from 'rxjs/Subject';
import 'rxjs/add/observable/never';
import 'rxjs/add/operator/startWith';

declare module 'rxjs/Observable' {
    interface Observable<T> {
        pausable: typeof pausable;
    }
}

function pausable<T>(notifier: Observable<boolean>): Observable<T> {
    return notifier.startWith(false).switchMap((paused) => {
        if (paused) {
            return Observable.never();
        } else {
            const source = new Subject();
            this.subscribe(source);
            return source;
        }
    });
}

Observable.prototype.pausable = pausable;

skipNextWhen.ts

import { Observable } from 'rxjs/Observable';
import './pausable';

declare module 'rxjs/Observable' {
    interface Observable<T> {
        skipNextWhen: typeof skipNextWhen;
    }
}

function skipNextWhen<T, R>(other: Observable<T>): Observable<R> {
    const notifier = Observable.merge(this.map(() => false), 
                                      other.map(() => true));
    return this.pausable(notifier);
}

Observable.prototype.skipNextWhen = skipNextWhen;

Is there a more suitable operator that I should consider using instead? The behavior I'm seeing with my current implementation is that the result Observable emits once, and then never again - even if the notifier Observable never emits.

Parallax

What if notifier emits twice between source ? Do you want to skip 2 or one of source ? – Stricklin

No, I don't care about skipping twice. The source should skip the next emission if the notifier emits one or more times between source emissions. – Parallax

Answer 1

I can think of two solutions to this:

1. Using .filter(), .do() and a few side-effects.

   This is mayne easier to understand solution even though it's not that "Rx" way:

   function skipNextWhen(other) {
       let skipNext = false;
   
       return this.merge(other.do(() => skipNext = true).filter(() => false))
           .filter(val => {
               const doSkip = skipNext;
               skipNext = false;
               return !doSkip;
           });
   }
   

   I'm using merge() just to update skipNext, other's value is always ignored.

2. Using .scan():

   This solution is without any state variables and side-effects.

   function skipNextWhen(other) {
       const SKIP = 'skip';
   
       return this.merge(other.mapTo(SKIP))
           .scan((acc, val) => {
               if (acc === SKIP) {
                   return null;
               } else if (val === SKIP) {
                   return SKIP;
               } else {
                   return val;
               }
           }, [])
           .filter(val => Boolean(val) && val !== SKIP);
   }
   

   Basically, when SKIP arrives I return it right away because it's going to be passed again in acc parameter by the scan() operator and later ignored by filter().

   If I receive a normal value but the previous value was SKIP I ignore it and return just null which is later filter away.

Both solutions give the same result:

Observable.prototype.skipNextWhen = skipNextWhen;

const source = Observable.range(1, 10)
    .concatMap(val => Observable.of(val).delay(100));

source
    .skipNextWhen(Observable.interval(350))
    .subscribe(console.log);

This prints the following:

1
2
3
5
6
8
9
10

Just be aware that you're not in fact creating new operator. You just have a shortcut for an operator chain. This for example doesn't let you unsubscribe from other when the source completes.

Oblong

Answer 2

I've started a (very) small library of some rxjs utils I've wanted. It happens to have a function to do exactly what you ask: skipAfter. From the docs:

source: -1-----2-----3-----4-----5-|
skip$:  ----0----------0-0----------

result: -1-----------3-----------5-|

The library is here: https://github.com/simontonsoftware/s-rxjs-utils

Paolo