在分页库android错误中实现重试-空指针异常[重复]

Question

我正在尝试在使用分页库android时实现“重试”机制，以防在加载项目时没有互联网或设备离线。

当我单击“重试”图像时出现错误 - “尝试在空对象引用上调用虚拟方法 'void com.myapp.myapp.repository.FeedDataSource.retryPagination()'”

FeedDataSource.java

public class FeedDataSource extends PageKeyedDataSource<Long, Feed> {
.....
//for retry
private LoadParams<Long> params;
private LoadCallback<Long, Feed> callback;
........
 @Override
public void loadAfter(@NonNull LoadParams<Long> params, @NonNull LoadCallback<Long, Feed> callback) {

    this.params = params;
    this.callback = callback;

    Log.e(TAG, "Loading rage" + params.key + " Count" + params.requestedLoadSize);
    networkState.postValue(NetworkState.LOADING);
    RestApi restApi = RetrofitApi.create();
    Call<FeedResponse> call = restApi.fetchFeed(params.key, params.requestedLoadSize, username);
    try
    {
        Response<FeedResponse> response = call.execute();
        if(response.isSuccessful()){
            FeedResponse feedResponse = response.body();
            if(feedResponse !=null){
                networkState.postValue(NetworkState.LOADED);
                List<Feed> responseItems = feedResponse.getFeeds();
                callback.onResult(responseItems, params.key + 1);
            }
      }
         ........
 }

  public void retryPagination()
{
    loadAfter(params, callback);
}

 }

FeedDataFactory.java

public class FeedDataFactory extends DataSource.Factory {

private MutableLiveData<FeedDataSource> mutableLiveData;
private FeedDataSource feedDataSource;
private AppController appController;

public FeedDataFactory(AppController appController){
    this.appController = appController;
    this.mutableLiveData = new MutableLiveData<FeedDataSource>();
}

@Override
public DataSource create() {
    feedDataSource = new FeedDataSource(appController);
    mutableLiveData.postValue(feedDataSource);
    return feedDataSource;
}

public MutableLiveData<FeedDataSource> getMutableLiveData(){
    return mutableLiveData;
   }
}

FeedViewModel.java

public class FeedViewModel extends ViewModel {

..............

public void retry() {
    FeedDataFactory feedDataFactory = new FeedDataFactory(appController);
    feedDataFactory.getMutableLiveData().getValue().retryPagination();                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
  } 

FeedAdapter.java

public class FeedListAdapter extends PagedListAdapter<Feed, RecyclerView.ViewHolder> {
    ......
    private final Callback callback;

     public FeedListAdapter(@NonNull DiffUtil.ItemCallback<Feed> diffCallback, Callback callback, Context context) {
    super(Feed.DIFF_CALLBACK);
    this.context = context;
    this.callback = callback;
     }

    public void bindView(NetworkState networkState) {

            if (networkState != null && networkState.getStatus() == NetworkState.Status.FAILED) {
            binding.refresh.setVisibility(View.VISIBLE);
            binding.refresh.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    binding.refresh.setVisibility(View.GONE);
                    binding.progressBar.setVisibility(View.VISIBLE);
                    callback.onRetryClicked();
                }
            });
        } 
    }
     public interface Callback {
      void onRetryClicked();
      }
  }

HomeFragment.java

public class HomeFragment extends Fragment implements ClickHandler, FeedListAdapter.Callback {
  private FeedListAdapter feedListAdapter;
  private FeedViewModel feedViewModel;
   ..............
   @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
        feedViewModel = new ViewModelProvider(this).get(FeedViewModel.class);
       feedViewModel.init(appController);

        feedListAdapter = new FeedListAdapter(Feed.DIFF_CALLBACK, this, getContext());

        recyclerView.setAdapter(feedListAdapter);

        feedViewModel.getArticleLiveData().observe(getViewLifecycleOwner(), pagedList -> {
        feedListAdapter.submitList(pagedList);
      });
      }
  }

请帮忙，我做错了什么！

Answer 1

首先，我要感谢你，因为你的解决方案是对我实现网络重试的提示，我真的很感激。

其次，关于你得到的 NullPointerException，这是因为在你的 FeedViewModel.java 中，当重试按钮被触发时，你为数据源创建了一个全新的工厂。这既不是必要的，也不是正确的，因为工厂不会创建任何新的数据源，除非前一个数据源无效。您应该通过您当前的工厂检索数据源。

我要提到的另一个问题是，如果您的请求失败，您不会在回调中调用 onResult(...)。根据文档：

回调只能被调用一次，如果再次调用就会抛出。它 对于接受回调的 DataSource 加载方法始终有效 隐藏回调并稍后调用它。这使数据源能够 完全异步，并处理临时的、可恢复的错误 状态（比如可以重试的网络错误）

当我在 Retrofit 中调用 onFailure() 时，这是我在 DataSource 中的实现：

@Override
        public void onFailure(Call<HotItemsResponse> call, Throwable t) {
            loadingState.postValue(Contants.LoadingState.SUB_LOAD_ERROR);
            //when the load is fail, dont call onResult() on the call back,
            //just ignore it, update the loading state for the UI to handle reload

            //callback.onResult(new ArrayList<ProductItem>(), currentPage);
        }