MainActivity.java

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    //ui
    private RecyclerView recyclerView;

    // vars
    private CompositeDisposable disposables = new CompositeDisposable();
    private RecyclerAdapter adapter;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        recyclerView = findViewById(R.id.recycler_view);

        initRecyclerView();

        getPostsObservable()
                .subscribeOn(Schedulers.io())
                .flatMap(new Function<Post, ObservableSource<Post>>() {
                    @Override
                    public ObservableSource<Post> apply(Post post) throws Exception {
                        return getCommentsObservable(post);
                    }
                })
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Observer<Post>() {
                    @Override
                    public void onSubscribe(Disposable d) {
                        disposables.add(d);
                    }

                    @Override
                    public void onNext(Post post) {
                        updatePost(post);
                    }

                    @Override
                    public void onError(Throwable e) {
                        Log.e(TAG, "onError: ", e);
                    }

                    @Override
                    public void onComplete() {
                    }
                });
    }

    private Observable<Post> getPostsObservable(){
        return ServiceGenerator.getRequestApi()
                .getPosts()
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .flatMap(new Function<List<Post>, ObservableSource<Post>>() {
                    @Override
                    public ObservableSource<Post> apply(final List<Post> posts) throws Exception {
                        adapter.setPosts(posts);
                        return Observable.fromIterable(posts)
                                .subscribeOn(Schedulers.io());
                    }
                });
    }

    private void updatePost(final Post p){
        Observable
                .fromIterable(adapter.getPosts())
                .filter(new Predicate<Post>() {
                    @Override
                    public boolean test(Post post) throws Exception {
                        return post.getId() == p.getId();
                    }
                })
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Observer<Post>() {
                    @Override
                    public void onSubscribe(Disposable d) {
                        disposables.add(d);
                    }

                    @Override
                    public void onNext(Post post) {
                        Log.d(TAG, "onNext: updating post: " + post.getId() + ", thread: " + Thread.currentThread().getName());
                        adapter.updatePost(post);
                    }

                    @Override
                    public void onError(Throwable e) {
                        Log.e(TAG, "onError: ", e);
                    }

                    @Override
                    public void onComplete() {
                    }
                });
    }

    private Observable<Post> getCommentsObservable(final Post post){
        return ServiceGenerator.getRequestApi()
                .getComments(post.getId())
                .map(new Function<List<Comment>, Post>() {
                    @Override
                    public Post apply(List<Comment> comments) throws Exception {

                        int delay = ((new Random()).nextInt(5) + 1) * 1000; // sleep thread for x ms
                        Thread.sleep(delay);
                        Log.d(TAG, "apply: sleeping thread " + Thread.currentThread().getName() + " for " + String.valueOf(delay)+ "ms");

                        post.setComments(comments);
                        return post;
                    }
                })
                .subscribeOn(Schedulers.io());

    }

    private void initRecyclerView(){
        adapter = new RecyclerAdapter();
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setAdapter(adapter);
    }
    
    @Override
    protected void onDestroy() {
        super.onDestroy();
        disposables.clear();
    }
}