Java 在分页库中实现重试android错误-空指针异常
我试图在使用android分页库时实现“重试”机制，以防在加载项目时没有互联网或设备脱机 当我单击“重试”映像时，我得到错误-“尝试在空对象引用上调用虚拟方法'void com.myapp.myapp.repository.FeedDataSource.retryPagination（）”” FeedDataSource.javaJava 在分页库中实现重试android错误-空指针异常,java,android,android-studio,mvvm,pagination,Java,Android,Android Studio,Mvvm,Pagination,我试图在使用android分页库时实现“重试”机制，以防在加载项目时没有互联网或设备脱机 当我单击“重试”映像时，我得到错误-“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> { ..... /
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);
}
}
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;
}
}
public class FeedViewModel extends ViewModel {
..............
public void retry() {
FeedDataFactory feedDataFactory = new FeedDataFactory(appController);
feedDataFactory.getMutableLiveData().getValue().retryPagination();
}
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();
}
}
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);
});
}
}
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);
}
}
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;
}
}
public class FeedViewModel extends ViewModel {
..............
public void retry() {
FeedDataFactory feedDataFactory = new FeedDataFactory(appController);
feedDataFactory.getMutableLiveData().getValue().retryPagination();
}
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();
}
}
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);
});
}
}
请帮帮我，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一个提示，我非常感谢 其次，关于您得到的NullPointerException，这是因为在FeedViewModel.java中，当触发重试按钮时，您为datasource创建了一个全新的工厂。这既不必要也不正确，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。您应该通过当前工厂检索数据源 我想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（…）。根据文件： 回调只能调用一次，如果再次调用将抛出。信息技术 对于接受回调的数据源加载方法始终有效 隐藏回调并稍后调用。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） 以下是我在改型中调用onFailure（）时在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);
}
@覆盖
失败时公共无效（调用调用，可丢弃的t）{
loadingState.postValue（Contants.loadingState.SUB\u加载错误）；
//加载失败时，不要在回调时调用onResult（），
//忽略它，更新UI的加载状态以处理重载
//onResult（新的ArrayList（），currentPage）；
}
@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);
}
@覆盖
失败时公共无效（调用调用，可丢弃的t）{
loadingState.postValue（Contants.loadingState.SUB\u加载错误）；
//加载失败时，不要在回调时调用onResult（），
//忽略它，更新UI的加载状态以处理重载
//onResult（新的ArrayList（），currentPage）；
}