Passing composite data through flatMap and similar operators without creating container objects

[jondwillis]

Take the following contrived example:

        Observable
                .just("yo")
                .flatMap(s -> Observable.range(0, 100))
                .subscribe(integer -> Ln.d("Here's an Integer(%s), but how do I access that juicy String?", integer));

I often come up against this problem, where I'd like to act upon multiple chained streams. I can easily imagine a couple of ways to access both the String Observable and the Integer Observable within a downstream subscribe() or some other operator. One is pulling the String Observable into a variable and then accessing it within a lambda. Another is to do something like:

        Observable
                .just("yo")
                .flatMap(s -> 
                        Observable.combineLatest(
                                Observable.just(s),
                                Observable.range(0, 100),
                                Pair::new))
                .subscribe(pair -> Ln.d("Here's an Integer(%s), and here's a juicy String(%s), but isn't this a little hard to follow and annoying?", pair.second, pair.first));

But this necessitates repackaging the original Observable with the dependent Observable into some more complex data structure.

There's gotta be a better way. Help?

[akarnokd]

There is an overload to flatMap that will pair up the original source value and each value returned for it through the Observable returned by the function:

Observable<R> flatMap(
            final Func1<? super T, ? extends Observable<? extends U>> collectionSelector,
            final Func2<? super T, ? super U, ? extends R> resultSelector)

[jondwillis]

Wow, that is definitely cleaner. But it still looks like you must create some sort of container object.

[jondwillis]

It also appears as if switchMap and concatMap do not have the same capabilities.. perhaps that is by design.

[abersnaze]

You might want to checkout this proposal from @stealthcode and I #2452.

[stealthcode]

The proposal @abersnaze mentioned is something like a BiObservable where there are 2 elements of data. In principle there would be a Tri, Quad, etc... Observable. I would love it if this were more of a first class citizen in RxJava since this use case seems to crop up again and again. Here is a unit test that shows the principles of how this would work.

Currently I am working on an RxJava Extensions proposal to make it easier to transform from rx.Observable into other kinds of Observables, for instance from an rx.Observable to an rx.extensions.BiObservable.

[jondwillis]

@akarnokd can you shed any light into why operators like concatMap and switchMap do not have the same resultSelector overload methods? I think I'll try my hand at a pull request.

[headinthebox]

You do not actually need that overload, you can easily achieve the effect using a nested map.

[benjchristensen]

Try this:

import rx.Observable;

public class ComposeViaMap {
    public static void main(String... args) {
        Observable
                .just("a", "b")
                .flatMap(s ->
                        Observable.range(0, 100)
                                .map(integer -> String.format("Here's an Integer(%s), with String(%s)", integer, s)))
                .subscribe(System.out::println);
    }
}

[jondwillis]

@headinthebox @benjchristensen Yup, those two approaches work, but still produce subjectively ugly code which is annoying to compose since I find myself needing to unpackage and repackage the combined objects in each operator:
(contrived example, again)

    .flatMap(session -> service.getVideoPlayerStateSubject(),
                 (Func2<StubSessionImpl, Enums.VideoPlayerState, Pair<StubSessionImpl, Enums.VideoPlayerState>>) Pair::new)
    .distinctUntilChanged()
    .takeUntil(otherThing.skip(1))
    .doOnNext(pair -> {
        StubSessionImpl session = pair.first;
        Enums.VideoPlayerState videoPlayerState = pair.second;
        Ln.d("Here's a session %s and here's a videoPlayerState %s", session, videoPlayerState);
    })
    .subscribe(pair -> {
        StubSessionImpl session = pair.first;
        Enums.VideoPlayerState videoPlayerState = pair.second;
        // do some things with both session and videoPlayerState
    }, Ln::e);

I suppose at this point I'm just looking for some sugar syntax/ container objects to use.

[benjchristensen]

If you want them together then create a tuple or other type to carry them along.

[jondwillis]

For further discussion...

http://java.dzone.com/articles/whats-wrong-java-8-part-v
https://www.reddit.com/r/java/comments/2asgrp/why_doesnt_java_have_tuples/

[benjchristensen]

Yup. It has drawbacks. You've been given a handful of answers. What do you suggest if the answers provided are insufficient?

[jondwillis]

I'm appreciative! I'm still trying to find a good solution, and I'll post here if I ever find anything that isn't kludgy ;)

[benjchristensen]

That would be great to hear what you find.

Also, if you haven't looked at the solution by @stealthcode above, do so. It hurt my head at first but is powerful when passing multiple values.