java - 如何修复 Android 中 fragment 中未找到 View 的问题

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 5 年前。

在我的应用程序中，我想从服务器获取一些数据并将其显示到RecyclerView中为此我应该使用 Fragment .

我希望用户看到 fragment 时调用我的API调用方法。但是当使用下面的代码时，显示强制关闭错误。

代码:

@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
    super.setUserVisibleHint(isVisibleToUser);
    if (isVisibleToUser){
        getData(getActivity());
    }
}

错误:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference
    at com.app.test.Fragments.MainNotificationFrags.NotificationAllFrag.getData(NotificationAllFrag.java:157)
    at com.app.test.Fragments.MainNotificationFrags.NotificationAllFrag.setUserVisibleHint(NotificationAllFrag.java:142)
    at android.support.v4.app.FragmentStatePagerAdapter.setPrimaryItem(FragmentStatePagerAdapter.java:157)
    at android.support.v4.view.ViewPager.populate(ViewPager.java:1266)
    at android.support.v4.view.ViewPager.populate(ViewPager.java:1116)
    at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1642)
    at android.view.View.measure(View.java:17496)
    at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:728)
    at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:464)
    at android.view.View.measure(View.java:17496)
    at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1658)
    at android.view.View.measure(View.java:17496)
    at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:728)
    at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:464)
    at android.view.View.measure(View.java:17496)
    at android.support.v4.widget.DrawerLayout.onMeasure(DrawerLayout.java:1081)
    at android.view.View.measure(View.java:17496)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
    at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
    at android.view.View.measure(View.java:17496)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1438)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:724)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:615)
    at android.view.View.measure(View.java:17496)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
    at android.view.View.measure(View.java:17496)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1438)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:724)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:615)
    at android.view.View.measure(View.java:17496)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
    at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2636)
    at android.view.View.measure(View.java:17496)
    at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2031)
    at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1193)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1400)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1078)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5875)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
    at android.view.Choreographer.doCallbacks(Choreographer.java:580)
    at android.view.Choreographer.doFrame(Choreographer.java:550)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5349)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)

fragment 完整代码:

public class NotificationAllFrag extends Fragment {

    @BindView(R.id.mainNotification_recyclerView)
    RecyclerView mainNotification_recyclerView;
    @BindView(R.id.mainNotification_noNotificationTxt)
    TextView mainNotification_noNotificationTxt;
    @BindView(R.id.mainNotification_progressBar)
    ProgressBar mainNotification_progressBar;
    @BindView(R.id.newsPageLoadLay)
    RelativeLayout newsPageLoadLay;
    private Context context;
    private List<Datum> model = new ArrayList<>();
    public static NotificationListAdapter notificationListAdapter;
    private SharedPrefrencesHandler prefrencesHandler;
    private String token = "";
    private LinearLayoutManager layoutManager;

    public NotificationAllFrag() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_notification_all, container, false);

        //Initialize
        ButterKnife.bind(this, view);
        context = getActivity();
        notificationListAdapter = new NotificationListAdapter(context, model);
        prefrencesHandler = new SharedPrefrencesHandler(context);
        layoutManager = new LinearLayoutManager(context);
        //RecyclerView
        mainNotification_recyclerView.setLayoutManager(layoutManager);
        mainNotification_recyclerView.setHasFixedSize(true);

        //Get token
        token = prefrencesHandler.getFromShared(SharedPrefrencesKeys.TOKEN.name());
        // Lazy loader
        newsPageLoadLay.setVisibility(View.GONE);
        mainNotification_recyclerView.setOnScrollListener(new EndlessRecyclerLinearPage1(layoutManager) {
            @Override
            public void onLoadMore(int current_page) {
                ExploreSendData sendData = new ExploreSendData();
                sendData.setPageIndex(current_page);
                sendData.setPageSize(10);
                sendData.setShowFollows(true);
                sendData.setShowMovies(false);
                sendData.setShowNews(false);
                sendData.setShowReplies(true);
                sendData.setShowSeries(false);
                sendData.setShowSuggestions(true);

                newsPageLoadLay.setVisibility(View.VISIBLE);

                InterfaceApi api = ApiClient.getClient().create(InterfaceApi.class);
                Call<ExploreResponse> call = api.getExplore(token, sendData);

                call.enqueue(new Callback<ExploreResponse>() {
                    @Override
                    public void onResponse(Call<ExploreResponse> call, Response<ExploreResponse> response) {
                        if (response.body().getData() != null && response.body().getStatusCode() != 401
                                && response.body().getStatusCode() != 402) {
                            if (response.body().getData().size() > 0) {
                                notificationListAdapter.addNewItem(response.body().getData());
                                //Gone no explore
                                newsPageLoadLay.setVisibility(View.GONE);
                            }
                        } else {
                            prefrencesHandler.remove(SharedPrefrencesKeys.TOKEN.name());
                            startActivity(new Intent(context, LoginActivity.class));
                        }

                        newsPageLoadLay.setVisibility(View.GONE);
                    }

                    @Override
                    public void onFailure(Call<ExploreResponse> call, Throwable t) {
                        newsPageLoadLay.setVisibility(View.GONE);
                    }
                });
            }
        });

        getData(getActivity());

        return view;
    }

    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        if (isVisibleToUser){
            getData(getActivity());
        }
    }

    public void getData(final Context context) {
        ExploreSendData sendData = new ExploreSendData();
        sendData.setPageIndex(1);
        sendData.setPageSize(10);
        sendData.setShowFollows(true);
        sendData.setShowMovies(false);
        sendData.setShowNews(false);
        sendData.setShowReplies(true);
        sendData.setShowSeries(false);
        sendData.setShowSuggestions(true);

        mainNotification_progressBar.setVisibility(View.VISIBLE);

        InterfaceApi api = ApiClient.getClient().create(InterfaceApi.class);
        Call<ExploreResponse> call = api.getExplore(
                new SharedPrefrencesHandler(context).getFromShared(SharedPrefrencesKeys.TOKEN.name()), sendData);

        call.enqueue(new Callback<ExploreResponse>() {
            @Override
            public void onResponse(Call<ExploreResponse> call, Response<ExploreResponse> response) {
                if (response.body().getData() != null && response.body().getStatusCode() != 401
                        && response.body().getStatusCode() != 402) {
                    if (response.body().getData().size() > 0) {
                        model.clear();
                        model.addAll(response.body().getData());
                        notificationListAdapter.notifyDataSetChanged();
                        mainNotification_recyclerView.setAdapter(notificationListAdapter);
                        //Gone no explore
                        mainNotification_noNotificationTxt.setVisibility(View.GONE);

                    } else {
                        mainNotification_noNotificationTxt.setVisibility(View.VISIBLE);
                        mainNotification_recyclerView.setVisibility(View.GONE);
                    }
                } else {
                    prefrencesHandler.remove(SharedPrefrencesKeys.TOKEN.name());
                    startActivity(new Intent(context, LoginActivity.class));
                }

                mainNotification_progressBar.setVisibility(View.GONE);
            }

            @Override
            public void onFailure(Call<ExploreResponse> call, Throwable t) {
                mainNotification_progressBar.setVisibility(View.GONE);
            }
        });
    }

我该如何解决这个问题？请帮忙

Answer 1

这不是正确的解决方案，但它 100% 有效

 @Override
 public void setUserVisibleHint(boolean isVisibleToUser) {
     super.setUserVisibleHint(isVisibleToUser);
     if (isVisibleToUser && mainNotification_recyclerView!=null){
         getData(getActivity());
     }
 }



 @Override
 public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
     super.onViewCreated(view, savedInstanceState);
     if (getUserVisibleHint()) {
         getData(getActivity());
     }
 }

it was work with multiple or single tab when user was go on tab that time initialize data

it can help to load data faster even load in all fragment at same time