How to handle paging with RxJava?

Question

I\'m looking into converting my android app to use Rxjava for network requests. I currently access a webservice similar to:

getUsersByKeyword(String query, int l         


        

Answer 1

So, if this is one-way paging, here's a pattern you might try. This code hasn't been run or compiled, but I've tried to over-annotate in order to explain what's going on.

private static final int LIMIT = 50;

// Given: Returns a stream of dummy event objects telling us when
// to grab the next page. This could be from a click or wherever.
Observable getNextPageEvents();  

// Given:
// The search query keywords. Each emission here means a new top-level
// request;
Observable queries;

queries.switchMap((query) -> getNextPageEvents()
        // Ignore 'next page' pokes when unable to take them.
        .onBackPressureDrop()
        // Seed with the first page.
        .startWith(new NextPageEvent())
        // Increment the page number on each request.
        .scan(0, (page, event) -> page + 1) 
        // Request the page from the server.
        .concatMap((page) -> getUsersByKeyword(query, LIMIT, LIMIT * page)
                // Unroll Observable into Observable
                .concatMap((userList) -> Observable.from(userList))
                .retryWhen(/** Insert your favorite retry logic here. */))
        // Only process new page requests sequentially.
        .scheduleOn(Schedulers.trampoline())
        // Trampoline schedules on the 'current thread', so we make sure that's
        // a background IO thread.
        .scheduleOn(Schedulers.io());

That should let the 'next page events' signal trigger a load of the next page's data each time, as well as not jumping pages should it encounter an error loading one. It also restarts completely at the top level if it receives a new search query. If I (or somebody else?) has time, I'd like to check my assumptions about the trampoline and backpressure and make sure it blocks any attempt to prematurely fetch the next page while one is loading.