Odtnhj

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 .