refCount with delay

[OliverJAsh]

Feature Request

Is your feature request related to a problem? Please describe.
I have a Observable<Array<Observable<T>>> which I want to map to Observable<Array<T>>.

When a new array is emitted, the inner observables should unsubscribe/subscribe as follows:

• If Observable exists in previous array and the new/current array, retain pre-existing subscription
• If Observable did not exist in previous array but does exist in new/current array, create new subscription
• If Observable existed in previous array but does not exist in new/current array, unsubscribe from pre-existing subscription

I hoped to achieve this using switchMap on the outer observable and then passing Array<Observable<T>> into combineLatest. However, switchMap will unsubscribe from its previous inner Observable before subscribing to the new inner Observable, which means inner subscriptions are not retained as desired.

Example:

• https://stackblitz.com/edit/typescript-b4wgr1
• https://stackoverflow.com/questions/51875557/combinelatest-switchmap-and-retaining-inner-subscriptions/51901191

Describe the solution you'd like
To offset the fact that switchMap unsubscribes to the previous inner observable before subscribing to the new inner observable, I would like the refCounting to be delayed, thereby ensuring that subscriptions to the inner observables are retained.

This is the solution suggested in this article. An alternative refCount with delay operator can be found here.

Describe alternatives you've considered

(If this is new operator request) describe reason it should be core operator
From my research this seems like something people want but it's not easy to do. I discovered issues/articles/examples on the internet, but there doesn't seem to be anything in the core to help with this.

Additional context

• Related discussion: Proposal: attack and release times for refCount #171
• Issue which needed this: https://stackoverflow.com/questions/51875557/combinelatest-switchmap-and-retaining-inner-subscriptions/51901191#51901191
• Example: refCountWithDelay implementation: https://gist.github.com/marinho/3637210b13c0f298e1692a0b7b104e64
• Example: delayedRefCount implementation: https://medium.com/@volkeron/rxjs-unsubscribe-delay-218a9ab2672e

[aaronjensen]

Thank you for writing this up. I ran into the same thing. For now, I'm actually memoizing the slow operation (it was an http call) and allowing the stream to reset. It's not perfect, but it seems to work in my case.

The operator (or flag to switchMap) you're proposing here would be great.

[OliverJAsh]

@aaronjensen IIRC, I ended up working with @cartant on a combineLatestHigherOrderArray operator, which felt like a better solution at the time: https://github.com/cartant/rxjs-etc/blob/master/source/observable/combineLatestHigherOrderArray-spec.ts. That might help in your case too!

[aaronjensen]

That's helpful, thank you @OliverJAsh. It looks like that package has refCountDelay as well and some other handy operators. I'll try it out.

[benlesh]

This has come up a few times, and now that we've revamped the share-type operators in 7.x it might be time to have a look to see if we can accommodate this with that new API somehow.

Off the top of my head, maybe something like (not at all set on this, it requires some thought):

source$.pipe(
  share({
    resetOnRefCountZero: () => timer(1000) // Where it could be a boolean or a reset timer factory? 
  });
)

I'll add this to agenda items.