Is there a way like Promise.all() in RxJava?

Question

I have a question.

I need to get list of some items by a list of the item ids. At first, I tried

Observable.from(itemIds)
  .flatMap(itemId -> requestToServer(itemId))
  .subscribe(item -> { /* do something */ });

But the operator flatMap does not guarantee the order of items. I need to get item in order that ItemIds have.

It would be great if there was the api like Promise.all(). Is there a way like Promise.all() in RxJava? or any other ways?

Answer 1

It sounds like you're looking for the Zip operator

For example:

Observable<Integer> obs1 = Observable.just(1);
Observable<String> obs2 = Observable.just("Blah");
Observable<Boolean> obs3 = Observable.just(true);

Observable.zip(obs1, obs2, obs3, (Integer i, String s, Boolean b) -> i + " " + s + " " + b)
 .subscribe(str -> System.out.println(str));

prints:

1 Blah true

Answer 2

Use concatMap instead. That will concatenate the emitted Observables in order rather than merging their emissions

Returns a new Observable that emits items resulting from applying a function that you supply to each item emitted by the source Observable, where that function returns an Observable, and then emitting the items that result from concatinating those resulting Observables.