java - 在分页库android错误中实现重试 - 空指针异常
问题描述
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!
解决方案
首先,我要感谢您,因为您的解决方案是对我实施网络重试的提示,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:
回调只能调用一次,如果再次调用将抛出。对于接受回调来存储回调并稍后调用它的 DataSource 加载方法始终有效。这使 DataSource 能够完全异步,并能够处理临时的、可恢复的错误状态(例如可以重试的网络错误)
当调用 Retrofit 中的 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);
}
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