onNext() method is called only once

Question

I am trying to learn RxJava and was implementing a simple app where I am loading a list of posts from JSONPlaceholder and for each post I am loading the comments by calling another api and updating the posts adapter. The problem is onNext() method of the post observable is getting called only once and only one posts comments is getting loaded.

Here is my activity

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    private RecyclerView rv;
    private PostsAdapter adapter;

    private CompositeDisposable disposables = new CompositeDisposable();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        rv = findViewById(R.id.rv);
        rv.setLayoutManager(new LinearLayoutManager(this,
                RecyclerView.VERTICAL, false));
        adapter = new PostsAdapter(this);
        rv.setAdapter(adapter);

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

            @Override
            public void onNext(Post post) {
                Log.d(TAG, "onNext: called");
                updatePost(post);
            }

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

            @Override
            public void onComplete() {
                Log.d(TAG, "onComplete: called");
            }
        });

    }

    private void updatePost(Post post) {
        adapter.updatePost(post);
    }

    private Observable<Post> getPostObservable() {
        return ApiClient.getApi()
                .getPosts()
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .flatMap((Function<List<Post>, ObservableSource<Post>>) posts -> {
                    adapter.setPosts(posts);
                    return Observable.fromIterable(posts)
                            .subscribeOn(Schedulers.io());
                });
    }

    private Observable<Post> getCommentsObservable(final Post post) {
        return ApiClient.getApi()
                .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;
                        Thread.sleep(delay);
                        post.setComments(comments);
                        return post;
                    }
                })
                .subscribeOn(Schedulers.io());
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        disposables.clear();
    }
}

Here is the Adapter

public class PostsAdapter extends RecyclerView.Adapter<PostsAdapter.ViewHolder> {

    private final String TAG = "nexa_" + this.getClass().getSimpleName();

    private Context context;
    private List<Post> dataList = new ArrayList<>();

    public PostsAdapter(Context context) {
        this.context = context;
    }

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        return new ViewHolder(
                LayoutInflater.from(parent.getContext())
                        .inflate(R.layout.item_posts, parent, false)
        );
    }

    public void setPosts(List<Post> posts) {
        this.dataList = posts;
        notifyDataSetChanged();
    }

    public void updatePost(Post post) {
        dataList.set(dataList.indexOf(post), post);
        notifyItemChanged(dataList.indexOf(post));
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {

        if (dataList.isEmpty())
            return;
        holder.bind(dataList.get(position));
    }

    @Override
    public int getItemCount() {
        return dataList.size();
    }

    public class ViewHolder extends RecyclerView.ViewHolder {

        TextView tvNumber, tvDesc;
        ProgressBar progressBar;

        public ViewHolder(@NonNull View itemView) {
            super(itemView);

            tvNumber = itemView.findViewById(R.id.tv_number);
            tvDesc = itemView.findViewById(R.id.tv_desc);
            progressBar = itemView.findViewById(R.id.progressbar);
        }

        public void bind(Post post) {
            tvDesc.setText(post.getBody());

            if (post.getComments() == null) {
                toggleProgressbar(true);
                tvNumber.setText("");
            } else {
                toggleProgressbar(false);
                tvNumber.setText(String.valueOf(post.getComments().size()));
            }


        }

        void toggleProgressbar(boolean show) {
            if (show) {
                progressBar.setVisibility(View.VISIBLE);
            } else {
                progressBar.setVisibility(View.GONE);
            }
        }
    }
}

ApiClient class

public class ApiClient {

    public static final String BASE_URL = "https://jsonplaceholder.typicode.com/";
    private static Retrofit retrofit = null;

    public static ApiInterface getApi() {

        ConnectionSpec spec = new ConnectionSpec.Builder(ConnectionSpec.COMPATIBLE_TLS)
                .tlsVersions(TlsVersion.TLS_1_2, TlsVersion.TLS_1_1, TlsVersion.TLS_1_0)
                .cipherSuites(
                        CipherSuite.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
                        CipherSuite.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256,
                        CipherSuite.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA,
                        CipherSuite.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA)
                .build();

        final OkHttpClient okHttpClient = new OkHttpClient.Builder()
                .connectionSpecs(Collections.singletonList(spec))
                .connectTimeout(60, TimeUnit.SECONDS)
                .writeTimeout(30, TimeUnit.SECONDS)
                .readTimeout(30, TimeUnit.SECONDS)
                .addInterceptor(new Interceptor() {
                    @Override
                    public Response intercept(Chain chain) throws IOException {
                        Request original = chain.request();

                        // Request customization: add request headers
                        Request.Builder requestBuilder = original.newBuilder()
                                .header("Authorization", "");

                        Request request = requestBuilder.build();
                        return chain.proceed(request);
                    }
                })
                .build();

        if (retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                    .client(okHttpClient)
                    .build();
        }
        return retrofit.create(ApiInterface.class);
    }
}

ApiInterface class

public interface ApiInterface {

    @GET("posts")
    Observable<List<Post>> getPosts();

    @GET("posts/{id}/comments")
    Observable<List<Comment>> getComments(
            @Path("id") int id
    );
}

Answer 1

The problem was i was calling getCommentsObservable() which is a network call from background thread. I added observeOn(AndroidSchedulers.mainThread()) and it is working now.

private Observable<Post> getCommentsObservable(final Post post) {
        return ApiClient.getApi()
                .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;
                        Thread.sleep(delay);
                        post.setComments(comments);
                        return post;
                    }
                })
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread()); // ADDED THIS LINE
    }