Ytukyg

Actually, I am new to Paging Library. Here is situation, i'm observing PagedList from my ViewModel which is always returning zero even news list appeared on UI.

 viewModel.getNews().observe(this, news -> {

                    swipeRefreshLayout.setRefreshing(false);

                    Timber.d("news size is %s",news.size());// **news size is 0**

                    adapter.submitList(news);

                });

My DataSource is

public class NewsDataSource extends ItemKeyedDataSource<String, News> {



    private MutableLiveData<Resource.Status> netStatusLive = new MutableLiveData<>();



   /*Hide Logic for clean purpose*/



    private List<News> localNewsList = new ArrayList<>();



    public NewsDataSource( /*Hide Logic for clean purpose*/) {

         /*Hide Logic for clean purpose*/

    }





    @Override

    public void loadInitial(@NonNull LoadInitialParams<String> params, @NonNull LoadInitialCallback<News> callback) {

        netStatusLive.postValue(Resource.Status.LOADING);

        localNewsList.clear();

        Disposable disposable =  /*Hide Logic for clean purpose*/



        compositeDisposable.add(disposable);

    }



    @Override

    public void loadAfter(@NonNull LoadParams<String> params, @NonNull LoadCallback<News> callback) {

        if (localNewsList.size() > 19) {

            netStatusLive.postValue(Resource.Status.LOADING);

            Disposable disposable =  /*Hide Logic for clean purpose*/

            compositeDisposable.add(disposable);

        }



    }



    private String getLastIdOfNews(List<News> localNewsList) {

        if (localNewsList.size() <= 0) {

            return "android";

        } else {

            return localNewsList.get(localNewsList.size() - 1).getId();

        }

    }



    @Override

    public void loadBefore(@NonNull LoadParams<String> params, @NonNull LoadCallback<News> callback) {

        //do nothing...

    }



    @NonNull

    @Override

    public String getKey(@NonNull News item) {

        return item.getId();

    }



    private void onPaginationError(Throwable throwable) {

        netStatusLive.postValue(Resource.Status.ERROR);

        Timber.e(throwable);

    }



    public LiveData<Resource.Status> getNetworkState() {

        return netStatusLive;

    }



}

In this case localNewsList is require to give last id of News on loadAfter which is like

Observable<List<News>> getNewsList(@Query("skip") int skip,

                                            @Query("limit") int limit,

                                            @Query("lastNewsId") String lastNewsId);

My DataSourceFactory is

    public class NewsDataSourceFactory extends DataSource.Factory<String, Journal> {

        private MutableLiveData<JournalDataSource> dataSourceLive = new MutableLiveData<>();



         /*Hide Logic for clean purpose*/



        public NewsDataSourceFactory( /*Hide Logic for clean purpose*/) {

             /*Hide Logic for clean purpose*/

        }



        @Override

        public DataSource<String, Journal> create() {

            JournalDataSource dataSource = new JournalDataSource( /*Hide Logic for clean purpose*/);

            dataSourceLive.postValue(dataSource);

            return dataSource;



        }

 public void prepareData(String coursId, String studnetId) {

        this.coursId = coursId;

        this.studnetId = studnetId;

    }





        public MutableLiveData<JournalDataSource> getDataSource() {

            return dataSourceLive;

        }

    }

ViewModle is

public class NewsViewModel extends ViewModel {



    private LiveData<PagedList<News>> newslist = new MutableLiveData<>();

    private LiveData<Resource.Status> networkState = new MutableLiveData<>();



    private NewsDataSourceFactory factory;



    private PagedList.Config config;



    private Executor executor;





    NewsViewModel(NewsDataSourceFactory factory, PagedList.Config config) {

        this.factory = factory;

        this.config = config;



        executor = Executors.newFixedThreadPool(5);





        networkState = Transformations.switchMap(factory.getDataSource(), source -> {

                    Timber.d("network status get");

                    return source.getNetworkState();

                }

        );

    }



    public void getNews(String courseId, String studentId) {

        //Newss = repo.getNewss(auth, courseId, studentId, 0, 20, "android");

        factory.prepareData(courseId, studentId);

        newslist = new LivePagedListBuilder<>(factory, config).setFetchExecutor(executor).build();

    }



    public void refresh() {

        factory.getDataSource().getValue().invalidate();

    }



    public LiveData<PagedList<News>> getNews() {

        return newslist;

    }



    public LiveData<Resource.Status> getNetworkState() {

        return networkState;

    }

I think, my problem may be mismatch my datasource use case and my datasource type.So PageList doesn't work properly as we expected.

According to Android Paging Library Doc, You should call Api synchronous !
If you use RxRetrofit just remove observeOn() and subscribeOn() to run api on current thread that paging library use .
I tested it and it works fine for me .