CODEWITHSUNDEEP
rxjs
matthias
matthias

Reputation: 2062

how to run multiple observables in sequence

I have an array of observables a$, b$, c$, d$. I want them them to run in sequence. E.g. if a$ finishes, then b$. If b$ finishes then $c ...

I know forkJoin([a$, b$, c$, d$]) except that it will do this in parallel. Is there a corresponding operator? Or can I tell forkJoin to run in sequence?

Upvotes: 1

Views: 4492

Answers (1)

Ingo Bürk
Ingo Bürk

Reputation: 20063

You can use concat to run them in sequence:

concat(a$, b$, c$).subscribe(console.log);

If you also want to skip any intermediate emissions from the source observables on the resulting observer, you can use something like

concat(...[a$, b$, c$].map(source$ => source$.pipe(last())));

… or if you want to only collect the last emission of each source observable into a single emission like forkJoin, you attach toArray to it:

concat(...[a$, b$, c$].map(source$ => source$.pipe(last()))).pipe(
  toArray(),
);

Of course you could extract this into your own operator now if you want:

const concatLast = <T>(...sources: Observable<T>) => concat(
  ...sources.map(source$ => source$.pipe(last()))
).pipe(
  toArray(),
);

There are still fine differences to forkJoin in how it behaves when one of the source observables completes without emitting.

On the rxjs website there's a useful tool called the operator decision tree which helps you figure out the appropriate operator, for example in this case

  • I have some Observables to combine together as one Observable, and
  • I want to subscribe to each in order

Upvotes: 7

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