RxJS Emit a value only if the first (source) observable emits a value and not other combined observables

Question

So what I want to achieve here is to get the result of combineLatest([subject1$, subject2$]) only when source$ emits a value.

The problem is that whichever operator I use the subscription gets lunched any time subject1$ or subject2$ emits a value (that's how switchMap works). Now, I'm having a hard time in finding a suitable operator for this, I guess I'm looking more for some sort of pattern.

    const source$ = fromEvent(document, 'click');
    const subject1$ = new Subject() // can emit one or more values in unknown time
    const subject2$ = new Subject() // can emit one or more values in unknown time

    setInterval(() => { subject1$.next(1) }, 1000)
    setInterval(() => { subject2$.next('a') }, 2000)

    source$
      .pipe(
        switchMap(() => combineLatest([subject1$, subject2$])), // obviously does not work
      )
      .subscribe(([subject1, subject2]) => {
        console.log(subject1, subject2);
      });

Many thanks,

Answer 1

Use withLatestFrom

source$.pipe(
  withLatestFrom(combineLatest([subject1$, subject2$]))
)
.subscribe(([_,[subject1, subject2]]) => {
  console.log(subject1, subject2);
})