Subscription to Observable.forkJoin is never called

Question

I need to wait for the result of 2 AJAX requests to make some calculation on it.

I have found the Observable.forkJoin method that seems to do the job. So in my component I've added :

import { Observable } from 'rxjs/Rx';

// ...

ngOnInit() {
    Observable.forkJoin(
        this.instrumentService.current,
        this.tuningService.tunings
    ).subscribe(
        data => {
            console.log(data);
        }
    );
}

The AJAX requests are sent but I never pass in the subscribe callback.

If I try to execute each request separatly it works fine :

 this.instrumentService.current.subscribe(instrument => {
     console.log(instrument);
     this.instrument = instrument;
 });

 this.tuningService.tunings.subscribe(tunings => {
     console.log(tunings);
     this.tunings = tunings;
 });

In the data services ( instrumentService and tuningService ), the Observable are declared like this (maybe the source of the bug, don't know) :

private _tunings: BehaviorSubject<Tuning[]> = new BehaviorSubject([]);

/**
 * Class constructor.
 *
 * @param {ApiService} apiService
 */
constructor (private apiService: ApiService) {
    this.getAll().subscribe(tunings => this._tunings.next(tunings));
}

public get tunings() {
    return this._tunings.asObservable();
}

public getAll(): Observable<Tuning[]> {
    return this.apiService.call(this.url);
}

The apiService is responsible of calling the built-in angular http service.

Answer 1

As far as i remember forkJoin currently has a bug, if one of the Observable returns null then the forkJoin subscription is never called. Can you try with .zip instead?

Answer 2

For me, the combineLatest rxjs operator turned out to be the one I was looking for. It works with BehaviorSubjects the way you probably expected forkJoin to work:

When any observable emits a value, emit the last emitted value from each.

(Documentation: https://www.learnrxjs.io/learn-rxjs/operators/combination/combinelatest )