Multiple subscriptions nested into one subscription

user_7635889 0
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I find myself puzzled trying to set a very simple rxjs flow of subscriptions. Having multiple non-related subscriptions nested into another.

I'm in an angular application and I need a subject to be filled with next before doing other subscriptions.

Here would be the nested version of what I want to achieve.

subject0.subscribe(a => {

    this.a = a;

    subject1.subscribe(x => {
        // Do some stuff that require this.a to exists
    });

    subject2.subscribe(y => {
        // Do some stuff that require this.a to exists
    });

});

I know that nested subscriptions are not good practice, I tried using flatMap or concatMap but didn't really get how to realize this.

angular-observable subject rxjs6 observable rxjs
3 Answers

It's always a good idea to separate the data streams per Observable so you can easily combine them later on.

const first$ = this.http.get('one').pipe(
  shareReplay(1)
)

The shareReplay is used to make the Observable hot so it won't call http.get('one') per each subscription.

const second$ = this.first$.pipe(
  flatMap(firstCallResult => this.http.post('second', firstCallResult))
);

const third$ = this.first$.pipe(
  flatMap(firstCallResult => this.http.post('third', firstCallResult))
);

After this you can perform subscriptions to the Observables you need:

second$.subscribe(()=>{}) // in this case two requests will be sent - the first one (if there were no subscribes before) and the second one

third$.subscribe(() => {}) // only one request is sent - the first$ already has the response cached

If you do not want to store the first$'s value anywhere, simply transform this to:

this.http.get('one').pipe(
  flatMap(firstCallResult => combineLatest([
    this.http.post('two', firstCallResult),
    this.http.post('three', firstCallResult)
  ])
).subscribe(([secondCallResult, thirdCallResult]) => {})

Also you can use BehaviorSubject that stores the value in it:

const behaviorSubject = new BehaviorSubject<string>(null); // using BehaviorSubject does not require you to subscribe to it (because it's a hot Observable)
const first$ = behaviorSubject.pipe(
  filter(Boolean), // to avoid emitting null at the beginning
  flatMap(subjectValue => this.http.get('one?' + subjectValue))
)

const second$ = first$.pipe(
  flatMap(firstRes => this.http.post('two', firstRes))
)

const third$ = first$.pipe(
  flatMap(()=>{...})
)

behaviorSubject.next('1') // second$ and third$ will emit new values
behaviorSubject.next('2') // second$ and third$ will emit the updated values again
user_108362200

You can do that using the concat operator. 

const first = of('first').pipe(tap((value) => { /* doSomething */ }));
const second = of('second').pipe(tap((value) => { /* doSomething */ }));
const third = of('third').pipe(tap((value) => { /* doSomething */ }));

concat(first, second, third).subscribe();

This way, everything is chained and executed in the same order as defined.

EDIT

const first = of('first').pipe(tap(value => {
  // doSomething
  combineLatest(second, third).subscribe();
}));
const second = of('second').pipe(tap(value => { /* doSomething */ }));
const third = of('third').pipe(tap(value => { /* doSomething */ }));
first.subscribe();

This way, second and third are running asynchronously as soon as first emits.

user_118812590

You could do something like this:

subject$: Subject<any> = new Subject();
this.subject$.pipe(
        switchMap(() => subject0),
        tap(a => {
            this.a = a;
        }),
        switchMap(() => subject1),
        tap(x => {
            // Do some stuff that require this.a to exists
        }),
        switchMap(() => subject2),
        tap(y => {
            // Do some stuff that require this.a to exists
        })
    );

if you want to trigger this, simply call this.subject$.next();

EDIT: Here is an possible approach with forkJoin, that shout call the subjects parallel.

subject$: Subject<any> = new Subject();
    this.subject$.pipe(
        switchMap(() => subject0),
        tap(a => {
            this.a = a;
        }),
        switchMap(
            () => forkJoin(
                subject1,
                subject2
        )),
        tap([x,y] => {
          // Do some stuff that require this.a to exists
        })
    );
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