CODEWITHSUNDEEP
javarx-java
Nilzor
Nilzor

Reputation: 18603

How do I chain execution two indepentent Observables serially without nesting the calls?

Using RxJava I have an Observable<A> and an Observable<B>. I want to start subscription on B as soon as the first (and only) element of A is emitted. I know I can chain it like this:

final Observable<A> obsOfA;
final Observable<B> obsOfB;
obsOfA.subscribe(new Action1<A>() {
        @Override
        public void call(A a) {
            obsOfB.subscribe(...)      
        }
});

..But this will cause a nesting syntax which gets ugly as soon as we introduce Observable<C>. How can I "unwrap" the syntax to a more fluent one - getting one that is more like the javascript Promise.then()-flow?

Upvotes: 3

Views: 207

Answers (2)

Egor Neliuba
Egor Neliuba

Reputation: 15054

You should use flatMap:

obsOfA.flatMap(new Func1<A, Observable<B>>() {
    @Override
    public Observable<B> call(A a) {
        return obsOfB;
    }
})
.subscribe(/* obsOfB has completed */);

Every time obsOfA calls onNext(a), call will be executed with this value a.

Upvotes: 5

Dorus
Dorus

Reputation: 7546

You can use switch, combined with map in switchMap:

obsOfA.switchMap(i -> obsOfB)
.subscribe(/* obsOfB has completed */);

This does almost the same as merge in flatMap as long as obsOfA only yield 1 value, but when it yield more values, flatmap will combine them, while switch will only be subscribed to the last instance of obsOfB. This might be useful when you need to switch to a different stream.

Upvotes: 1

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