rxjava - how to switch to previous stream but also do some work on main thread once first observable is ready?

Question

I have two observables that both make network calls but they depend on each other:

val ob1 = Observable.just(myservice.getNewsArticles())

ob1.flatMap{ newsArticle -> myservice.getCelebrityNamesFromArticle(newsArticle.id)} 
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())

this is what i have so far, but the issue is when ob1 completes i need to immediately update the UI while ob2(thats gets celebrity names) is running. also at the end i need the chain to return ob1 (like a switchMap etc).... so it seems somehow i need to call onNext with a scheduler or something before invoking the flatMap right ?

ps.i noticed that flatMap has a biFunction mapper but i could not get it to work.

Answer 1

If you don't want the UI update to run concurrently with the second call, just use observeOn(mainThread()).doOnNext(article -> updateUI(article)).flatMap.

If you want to update the UI and fire off the second call, flatMap onto a merge of the UI update and the network call:

obs.flatMap(article -> Observable.merge(
    Observable.fromAction(() -> updateUI(article))
       .subscribeOn(mainThread()),
    myservice.getCelebrityNamesFromArticle(article.id)
       .subscribeOn(Schedulers.io())
))
.observeOn(mainThread());

To get both the article and the result of the second call, you can just map over it after the second call, for a pair, etc:

obs.flatMap(article -> Observable.merge(
    Observable.fromAction(() -> updateUI(article))
       .subscribeOn(mainThread()),
    myservice.getCelebrityNamesFromArticle(article.id)
       .subscribeOn(Schedulers.io())
       .map(celebrityNames -> Pair(article, celebrityNames))
))
.observeOn(mainThread());