test

Cool demo, food for thought... But I'm not sure if I'm understanding the setup you've used.

• Why does the toggleStatus function update its local state BEFORE the delay? That's fine for the setButtonEmoji, but why have a database-tracking variable that doesn't follow the db.

If it wouldn't change your demo to alter this, I think it'd be more believable to do this..

• Why route everything through the dispatcher to begin with? I understand in the real world you might have an already-combined stream, so that's fair. Just curious.

Thanks for the discussion..

Since the custom operator knows everything it need to know when groups can safely be removed, there should be no need for a fixed 15 second timeout.

Instead it could detect if the inner observable inside the switchMap has completed. If so, it can terminate the group.

I gave it a stab in my own branch here.

export function switchMapByKey<T, K, O extends ObservableInput<any>>(
  keySelector: (value: T) => K,
  project: (value: T, index: number) => O
): OperatorFunction<T, ObservedValueOf<O>> {
  return (source) => {
    let vault: any = {}; 
    return source.pipe(
      groupBy(
      (v) => {
        const key = keySelector(v);
        if (!vault[key]) vault[key] = new Subject<void>();
        return key;
      },
      item => item,
      group => vault[group.key]
      ),
      mergeMap((group) =>
        group.pipe(
          switchMap((e, i) =>
            from(project(e, i)).pipe(
              tap({ complete: () => { vault[group.key].complete(); }})
             )
          ),
          finalize(() => { delete vault[group.key]; })
        )
      ) as OperatorFunction<GroupedObservable<K, T>, ObservedValueOf<O>>
    )
  }
}

Edit: This was working in my browser, but TS keeps complaining about types, so it's not perfect yet.
Edit2: I gave TS some hints and now it's happy.