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(('
源数据源.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;
}
}
馈视图模型.java
public class FeedViewModel extends ViewModel {
..............
public void retry() {
FeedDataFactory feedDataFactory = new FeedDataFactory(appController);
feedDataFactory.getMutableLiveData().getValue().retryPagination();
}
馈送适配器.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();
}
}
首页片段.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！
首先，我要感谢您，因为您的解决方案是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，你会为数据源创建一个全新的工厂。这不是必需的，也不是正确的，因为除非前一个数据源无效，否则工厂不会创建任何新的数据源。应通过当前工厂检索数据源。
我想提到的另一个问题是，如果你的请求失败，你不会在回调上调用 onResult(...(。根据文档：
一个回调只能调用一次，如果再次调用，将引发。它 对于采用回调的数据源加载方法始终有效 以存储回调并在以后调用它。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(例如可以重试的网络错误(
以下是我在 DataSource 中调用 Retrofit 中的 onFailure(( 时的实现：
@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);
}