rxjs: Combing two streams, but cancelling the first after the second has sent an event

Question

I am sure that I should be able to find the answer to my problem in the documentation of rxjs, but I am getting nowhere...

Problem: I have two streams, one is more important than the other. Both provide the same information. But in case stream A provides a value for the first time, I want to completely dismiss stream B, because it is now outdated.

It is kind of a cache-thing: provide a value from the cache, but as soon as there is a live value, update the cache and do not read form it anymore.

I have tried several combinations piping the two streams, but I cannot cancel B after A has published...

Which rx-operator could provide the functionality that I need?

Answer 1

It looks like you can just use merge and takeUntil:

merge(
  streamA$.pipe(
    takeUntil(streamB$)
  ),
  streamB$
)

When streamB$ emits anything then streamA$ will be ignored because takeUntil will complete the chain.

Answer 2

If I understand correctly, you can merge the two streams and take the first that answers:

const interval1$ = interval(1000);
const interval2$ = interval(800);

const source1 = interval1$.pipe(
  map(x => { return { id: 1, value: x } }),
  take(5)
);
const source2 = interval2$.pipe(
  map(x => { return { id: 2, value: x } }),
  take(5)
);

const s = merge(source1, source2);

const subscribe = s.pipe(take(1)).subscribe(x => console.log(x));

This prints the first that answers, that is the first element of the second stream in this case.