How to combine 2 observables and emit values based on specific conditions in Angular 8

Question

I have 2 observables that I want to combine, and use their combined version inside a combineLatest with other observables.

I'm not sure how to combine them correctly for my Use Case.

First of all let's call them ObsA and ObsB, and their combined version ObsCombined.

this.api.ObsA(), 
this.api.ObsB(), 
this.api.ObsCombined()

I need that their combined version only EMITS values following these rules:

• ObsB is a BehaviourSubject and its starting value is Null.
• when ObsA emits, always make ObsCombined emit
• if ObsA emits "null" as next value, then ObsB value must be reset (I will call observable.next(null)), and ObsCombined should then emit both null values

So basically, ObsA is the most important, and the special thing is, when it emits Null, I want to make the other Obs reset and emit Null as well.

If ObsB emits a value, and ObsA last emitted was not Null, then emit this new value and the previous one from ObsA

Answer 1

You can try with RxJs forkJoin

const obA = this.api.ObsA();
const obB = this.api.ObsB();

forkJoin([obA, obB]).subscribe(res=>{
  console.log(res[0]);
  console.log(res[1]);
});

Answer 2

maybe (not checked)

merge(
 obsA.pipe(tap((v)=>{if(v==null) b.next(null)}),
 obsB
).subscribe();