Add concurrency parameter to switch and switchMap

[benlesh]

Carried over from #215.

The idea is that you could then switch between more than one of the most recently emitted observables.

I think the usefulness of this in comparison to potential performance degradation is dubious, but it's worth trying and comparing numbers, since it should be a non-breaking change.

[staltz]

Is this issue done?

[benlesh]

I don't believe so.

[trxcllnt]

@Blesh isn't this just the difference between popping off the end of the pending queue instead of shifting? Though since this is switchMap we're talking about, we can implement this as a ring buffer instead of an unbounded Array.

[benlesh]

Closing this issue as stale.

[d3lm]

I'd like to bump this issue as I think that there are use cases where a concurrency parameter is very useful for switchMap. We already have such parameter for mergeMap and it would be great to have the same for switchMap. I have implemented switchMapWithConcurrency as a lettable operator. Maybe this can help.

function switchMapWithConcurrency<T>(project: (value) => Observable<T>, concurrent = 1) {
  const subscriptions: Array<Subscription> = [];

  return (source: Observable<any>): Observable<T> => {
    return Observable.create((observer: Observer<T>) => {
      const subscription = source.subscribe((sourceValue) => {
        if (subscriptions.length >= concurrent) {
          let lastSubscription = subscriptions.shift();
          lastSubscription.unsubscribe();
        }

        let innerSubscription = project(sourceValue).subscribe(innerValue => observer.next(innerValue));
        subscription.add(innerSubscription);
        subscriptions.push(innerSubscription);
      });

      return subscription;
    });
  }
}

This can either be used with .let or within .pipe. Here's a working demo on JSBin.

[benlesh]

Yeah... I think this feature has merit... it's just that:

1. It's a low priority
2. switchMap is a very hot-path operator, so whatever we do would have to have minimal impact on both the perf and the size of the operator.

[BioPhoton]

Hi @benlesh

2. switchMap is a very hot-path operator, so whatever we do would have to have minimal impact on both the perf and the size of the operator.

As 2 got even more important (usage of RxJS exploded) and the last activity on this issue was more than 2 years ago I believe it could be closed.

Please let me know your opinion.

[benlesh]

Yeah. It's gone stale again. It's apparently not a feature people are hot to get.