How to chain asynchronous operations using Java RX Observable?

Question

I want to make a HTTP request repeatedly and act on result. I start with public Observable<NewsItem> fetchItems(NewsFeed feed). One request gets a few news items but I decided to flatten it.

The idea was to use Observable.interval() make the request multiple times, then combine resulting Observables into one.

       Observable
            .interval(timePerItem, TimeUnit.MILLISECONDS)
            .map(i -> feed)
            .map(feed -> fetchItems(feed))
            .subscribe(result -> System.out.println(result));

But the result is Observable<Observable<NewsItem>> not Observable<NewsItem>. How to marge them?

I have found the marge() operator (RX-Java doc: Marge). But it does not seem to fit the use case.

In previous version I used CompletableFuture<List<NewsItem>> fetchNewsItems() but I wasn't able to fit it into Observable chain.

Samuel Gruetter · Accepted Answer · 2014-11-16 20:01:54Z

3

Not sure if I understand the problem, but aren't you just looking for flatMap?

Observable
    .interval(timePerItem, TimeUnit.MILLISECONDS)
    .flatMap(i -> fetchItems(feed))
    .subscribe(result -> System.out.println(result));

answered Nov 16, 2014 at 20:01

Samuel Gruetter

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2 Comments

atok Over a year ago

Looks like that is what I was looking for. Learning rx-java from github.com/ReactiveX/RxJava/wiki is hard. flatMap is not mentioned in the introduction and the way it is described in github.com/ReactiveX/RxJava/wiki/… is strange. Thank you!

Samuel Gruetter Over a year ago

There are many more links to great tutorials at reactivex.io/tutorials.html. And if you find a good tutorial which is not yet mentioned there or you wrote one yourself, make a pull request at github.com/ReactiveX/reactivex.github.io so that the link is included on that page ;-)

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