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Daniel
Daniel

Reputation: 607

Combining 2 Observables only if the first one fires

I'm quite new to Rx and I have a situation that I don't find anything about (or maybe I don't ask the right question). I have an observable that should trigger a specific method. However to call that method I need the value from a second observable (which will btw have that value always before the first one). How do I combine the two so that the method only gets called when the first observable fires? It's like with combineLatest, but that one fires if either of the Observables fire. Br, Daniel

Upvotes: 0

Views: 3498

Answers (3)

Chandan Bhandari
Chandan Bhandari

Reputation: 217

You can use Flatmap() to do the same if you are using java 8 then : 

Observable.just(function1()).flatMap( dataFromFirstObservable-> 
        Observable.just(function2(dataFromFirstObservable))
    ).subscribe(resultFromSecondObservable -> {
        //perform action here
    });

if using java 7

Observable.just(getData1()).flatMap(new Function<Integer, ObservableSource<?>>() {
        @Override
        public ObservableSource<?> apply(Integer integer) throws Exception {
            return Observable.just(function2(integer));
        }
    }).subscribe(new Consumer<Object>() {
        @Override
        public void accept(Object o) throws Exception {
            //perform action here
        }
    });

Upvotes: 0

Umer Farooq
Umer Farooq

Reputation: 7486

It's quite simple. You just have to call another observable once the first observable is complete or in case the second observable relies on the successive data reception from the first observable, you need to call the second observable in onNext().

The following code tries to illustrate how the observable code would end up looking. I haven't checked the mistakes in syntax etc so use the following code to formulate an idea.

Observable.just(getDataFromServer())
          .subscribeWith(new DisposableObserver() {
            ArrayList<MyData> dataList = new ArrayList<>();
            public void onNext(MyData d) {
                // call second observable here if it relies on batches of data the first observable has
                   dataList.add(d);
            }

            public void onComplete() {
               Observable.from(dataList).map(new Func1<MyData, String>() {
                  @Override public String call(MyData data) {
                       // apply your operations etc
                  }
              });
            }
});

Upvotes: 0

TrevJonez
TrevJonez

Reputation: 959

If I understand the question correctly you can use the withLatestFrom operator so that the combination and subsequent emission is gated by the first observable.

It is listed on the same page with combine latest: http://reactivex.io/documentation/operators/combinelatest.html

Upvotes: 1

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