RxJava: chaining observables

Question

Is it possible to implement something like next chaining using RxJava:

loginObservable()
   .then( (someData) -> {
      // returns another Observable<T> with some long operation
      return fetchUserDataObservable(someData);

   }).then( (userData) -> {
      // it should be called when fetching user data completed (with userData of type T)
      cacheUserData(userData);

   }).then( (userData) -> {
      // it should be called after all previous operations completed
      displayUserData()

   }).doOnError( (error) -> {
      //do something
   })

I found this library very interesting, but can't figure our how to chain requests where each other depends on previous.

score 45 · Accepted Answer · edited Feb 28 '15 at 20:17

45

Sure, RxJava supports .map which does this. From the RxJava Wiki:

map

Basically, it'd be:

loginObservable()
   .switchMap( someData -> fetchUserDataObservable(someData) )
   .map( userData -> cacheUserData(userData) )
   .subscribe(new Subscriber<YourResult>() {
        @Override
        public void onCompleted() {
           // observable stream has ended - no more logins possible
        }
        @Override
        public void onError(Throwable e) {
            // do something
        }
        @Override
        public void onNext(YourType yourType) {
            displayUserData();
        }
    });

edited Feb 28 '15 at 20:17

Ryan

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answered Nov 14 '14 at 18:06

Benjamin Gruenbaum

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1

As i understand map transforms one value to other (for example, int -> String). So even if i return Observable from first map, second map will be called immediately with userData of type Observable, but i want it to be called with type T when fetchUserData completes – Mikhail Nov 14 '14 at 18:17
5

I think [`switchMap`](https://github.com/ReactiveX/RxJava/wiki/Transforming-Observables#switchmap) might be what you want. – Dylan Nov 14 '14 at 18:55
Thanks, seems like switchMap what i want. I would accept it if you put it as answer – Mikhail Nov 14 '14 at 19:23
1

@Mikhail sorry, my bad - was afk and didn't read the comment. `switchMap` is indeed what you need if you're consuming an observable. – Benjamin Gruenbaum Nov 14 '14 at 19:34
9

Also, "flatMap" gives you the typical chaining behavior, comparable to Promises chained as `then`. – André Staltz Nov 14 '14 at 21:02
Can I use chain multiple calls with `switchMap`? – ericn Aug 12 '17 at 07:15
Can someone please answer this question: [Multiple task in an observable](https://stackoverflow.com/questions/51391853/running-two-sequential-task-in-an-observable). – Amit Vikram Singh Jul 18 '18 at 06:41

score 15 · Answer 2 · edited Sep 08 '19 at 20:20

This is the top post when Googling RxJava chain observables so I'll just add another common case where you wouldn't want to transform the data you receive, but chain it with another action (setting the data to a database, for example). Use .flatmap(). Here's an example:

mDataManager
    .fetchQuotesFromApi(limit)
    .subscribeOn(mSchedulerProvider.io())
    .observeOn(mSchedulerProvider.ui())
    // OnErrorResumeNext and Observable.error() would propagate the error to
    // the next level. So, whatever error occurs here, would get passed to
    // onError() on the UI side.
    .onErrorResumeNext(Function { Observable.error<List<Quote>>(it) })
    .flatMap { t: List<Quote> ->
        // Chain observable as such
        mDataManager.setQuotesToDb(t).subscribe(
            {},
            { e { "setQuotesToDb() error occurred: ${it.localizedMessage}" } },
            { d { "Done server set" } }
        )
        Observable.just(t)
    }
    .subscribeBy(
        onNext = {},
        onError = { mvpView?.showError("No internet connection") },
        onComplete = { d { "onComplete(): done with fetching quotes from api" } }
    )

This is RxKotlin2, but the idea is the same with RxJava & RxJava2:

Quick explanation:

we try to fetch some data (quotes in this example) from an api with mDataManager.fetchQuotesFromApi()
We subscribe the observable to do stuff on .io() thread and show results on .ui() thread.
onErrorResumeNext() makes sure that whatever error we encounter from fetching data is caught in this method. I wanna terminate the entire chain when there is an error there, so I return an Observable.error()
.flatmap() is the chaining part. I wanna be able to set whatever data I get from the API to my database. I'm not transforming the data I received using .map(), I'm simply doing something else with that data without transforming it.
I subscribe to the last chain of observables. If an error occurred with fetching data (first observable), it would be handled (in this case, propagated to the subscribed onError()) with onErrorResumeNext()
I am very conscious that I'm subscribing to the DB observable (inside flatmap()). Any error that occurs through this observable will NOT be propagated to the last subscribeBy() methods, since it is handled inside the subscribe() method inside the .flatmap() chain.

The code comes from this project which is located here: https://github.com/Obaied/Sohan/blob/master/app/src/main/java/com/obaied/dingerquotes/ui/start/StartPresenter.kt

Instead of using flatMap() and returning an Observable.just(t) you can use a map() and simply return t. Be aware that both versions dont wait on the result of mDataManager.setQuotesToDb() and return before its completed. — yN., Jun 13 '19 at 14:21

score 0 · Answer 3 · answered Mar 23 '17 at 01:57

0

try using scan()

Flowable.fromArray(array).scan(...).subscribe(...)

answered Mar 23 '17 at 01:57

lingfliu

19
1

RxJava: chaining observables

3 Answers3

Quick explanation:

Linked