RxJava2 combine multiple observables to make them return single result

Question

How to combine multiple results emmited by observables into one result and emit it once?

I have a Retrofit service:

public interface MyService {

    @GET("url")
    Observable<UserPostsResult> getUserPosts(@Query("userId") int id);

    @GET("url")
    Observable<UserPostsResult> getUserPosts(@Query("userId") int id, @Query("page") int pageId);
}

And I have a model:

public class UserPostsResult {

   @SerializedName("posts")
   List<UserPost> mPosts;

   @SerializedName("nextPage")
   int mPageId;
}

Also I have ids List<Integer> friendsIds;

My goal is to have a method like this one:

public Observable<Feed> /*or Single<Feed>*/ getFeed(List<Integer> ids) {
    ...
}

It returns one Observable or Single that does the following:

• Combines all getUserPosts(idFromList) to one observable

• For each UserPostsResult must do:

  if (userPostResult.mPageId > -1) getUserPosts(currentUserId, userPostResult.mPageId);

  And merge this result to the previous userPostResult

• Return one single model as result of all operations.

Result class:

public class Feed {
    List<UserPost> mAllPostsForEachUser;
}

EDIT (More details):

My client specifications was that I must take from social network user posts with no logging in, no token requesting. So I must parse HTML pages. That's why I have this complex structure.

EDIT (Partial solution)

public Single<List<Post>> getFeed(List<User> users) {
    return Observable.fromIterable(users)
            .flatMap(user-> mService.getUserPosts(user.getId())
                    .flatMap(Observable::fromIterable))
            .toList()
            .doOnSuccess(list -> Collections.sort(list, (o1, o2) ->
                    Long.compare(o1.getTimestamp(), o2.getTimestamp())
            ));
}

This solution doesn't include pages problem. Thats why it is only partial solution

Answer 1

There are a number of operators which transform things into other things. fromIterable() will emit each item in the iterable, and flatMap() will convert one type of observable into another type of observable and emit those results.

Observable.fromIterable( friendsIds )
  .flatMap( id -> getUserPosts( id ) )
  .flatMap( userPostResult -> userPostResult.mPageId 
            ? getUserPosts(currentUserId, userPostResult.mPageId)
            : Observable.empty() )
  .toList()
  .subscribe( posts -> mAllPostsForEachUser = posts);