PageList.size is always zero

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 JournalDataSourceFactory extends DataSource.Factory<String, Journal> {
    private MutableLiveData<JournalDataSource> dataSourceLive = new MutableLiveData<>();

     /*Hide Logic for clean purpose*/

    public JournalDataSourceFactory( /*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 MutableLiveData<JournalDataSource> getDataSource() {
        return dataSourceLive;
    }
}

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

from PageList.size is always zero