访问片段视图时偶尔会出现NPE

Question

我偶尔会NullPointerException进入片段.它发生在应用程序在后台很长一段时间然后我打开它并滑动到这个片段.

public class SummaryFragment extends Fragment implements FragmentLifecycle {

    private static final String TAG = "DTAG";
    private DateFormat dateFormatName;
    private Preference prefs;
    private List<String> monthList;
    private TextView totalTimeFullTv;
    private TextView totalTimeNetTv;
    private TextView averageTimeTv;
    private TextView overUnderTv;
    private TextView minTimeTv;
    private TextView maxTimeTv;
    private TextView vacationsTv;
    private TextView sickTv;
    private TextView headlineTv;
    private TextView overUnderTvH;
    private OnFragmentInteractionListener mListener;

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


    public static SummaryFragment newInstance(String param1, String param2) {
        SummaryFragment fragment = new SummaryFragment();
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View RootView = inflater.inflate(R.layout.fragment_summary, container, false);

        dateFormatName = new SimpleDateFormat(getResources().getString(R.string.month_text));
        monthList = Arrays.asList(new DateFormatSymbols().getMonths());
        prefs = new Preference(GeneralAdapter.getContext());


        totalTimeFullTv = RootView.findViewById(R.id.textView_sum_ttf);
        totalTimeNetTv = RootView.findViewById(R.id.textView_sum_ttn);
        averageTimeTv = RootView.findViewById(R.id.textView_sum_av);
        overUnderTv = RootView.findViewById(R.id.textView_sum_ou);
        overUnderTvH = RootView.findViewById(R.id.textView_sum_ou_h);


        minTimeTv = RootView.findViewById(R.id.textView_sum_min);
        maxTimeTv = RootView.findViewById(R.id.textView_sum_max);
        vacationsTv = RootView.findViewById(R.id.textView_sum_vac);
        sickTv = RootView.findViewById(R.id.textView_sum_sick);
        headlineTv= RootView.findViewById(R.id.textView_sum_headline);

        return RootView;
    }

    private void refreshData() {

        if (prefs == null)
        {
            prefs = new Preference(GeneralAdapter.getContext());
        }

        String month = prefs.getString(Preference.CURRENT_MONTH);

        MonthData monthData = Calculators.CalculateLocalData(MainActivity.db.getAllDays(month));

        totalTimeFullTv.setText(monthData.getTotalTimeFull()); //Crash here
        totalTimeNetTv.setText(monthData.getTotalTimeNet());
        averageTimeTv.setText(monthData.getAverageTime());
        overUnderTv.setText(monthData.getOverUnder());
        if (monthData.getOverUnderFloat()<0)
        {
            overUnderTvH.setText(R.string.sum_over_time_neg);
            overUnderTv.setTextColor(ContextCompat.getColor(GeneralAdapter.getContext(),R.color.negative_color));
        }
        else
        {
            overUnderTvH.setText(R.string.sum_over_time_pos);
            overUnderTv.setTextColor(ContextCompat.getColor(GeneralAdapter.getContext(),R.color.positive_color));
        }

        minTimeTv.setText(monthData.getMinTime());
        maxTimeTv.setText(monthData.getMaxTime());
        vacationsTv.setText(""+monthData.getVacations());
        sickTv.setText(""+monthData.getSick());
        headlineTv.setText(month);
    }

    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onFragmentInteraction(uri);
        }
    }

    @Override
    public void onAttachFragment(Fragment childFragment) {
        super.onAttachFragment(childFragment);

    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    @Override
    public void onPauseFragment() {

    }

    @Override
    public void onResumeFragment()
    {
        refreshData();
    }


    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }
}

MainActivity viewPager:

viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            int currentPosition = 0;

            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            }

            @Override
            public void onPageSelected(int position) {

                FragmentLifecycle fragmentToHide = (FragmentLifecycle) adapter.getItem(currentPosition);
                fragmentToHide.onPauseFragment();

                FragmentLifecycle fragmentToShow = (FragmentLifecycle) adapter.getItem(position);
                fragmentToShow.onResumeFragment(); //Crash start

                currentPosition = position;
            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });

日志:

                                            E/AndroidRuntime: FATAL EXCEPTION: main
                                               Process: michlind.com.workcalendar, PID: 25038
                                               java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                                                   at michlind.com.workcalendar.mainfragments.SummaryFragment.refreshData(SummaryFragment.java:99)
                                                   at michlind.com.workcalendar.mainfragments.SummaryFragment.onResumeFragment(SummaryFragment.java:147)
                                                   at michlind.com.workcalendar.activities.MainActivity.onPageSelected(MainActivity.java:84)
                                                   at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1941)
                                                   at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:680)
                                                   at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:664)
                                                   at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:2257)
                                                   at android.view.View.dispatchTouchEvent(View.java:11776)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2962)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2643)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:448)
                                                   at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1829)
                                                   at android.app.Activity.dispatchTouchEvent(Activity.java:3307)
                                                   at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:68)
                                                   at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:410)
                                                   at android.view.View.dispatchPointerEvent(View.java:12015)
                                                   at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4795)
                                                   at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4609)
                                                   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4147)
                                                   at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4200)
                                                   at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4166)
                                                   at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4293)
                                                   at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4174)
                                                   at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4350)
                                                   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4147)
                                                   at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4200)
                                                   at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4166)
                                                   at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4174)
                                                   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4147)
                                                   at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6661)
                                                   at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6635)
                                                   at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6596)
                                                   at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6764)
                                                   at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:186)
                                                   at android.os.MessageQueue.nativePollOnce(Native Method)
                                                   at android.os.MessageQueue.next(MessageQueue.java:325)
                                                   at android.os.Looper.loop(Looper.java:142)
                                                   at android.app.ActivityThread.main(ActivityThread.java:6494)

更新:

我最终用过:

@Override
public void onPageSelected(int position) {

    Fragment fragment = adapter.getFragment(position);
    if (fragment != null) {
        fragment.onResume();
    }
}

在我的MainActivity中,并在每个片段上使用onResume().适用于适配器的解决方案:http: //thedeveloperworldisyours.com/android/update-fragment-viewpager/

Answer 1

问题是，您尝试过早访问视图：此时尚未创建视图层次结构。

如果您发布一个事件，该事件将在下一帧发生，那么您可以保证，该视图层次结构将已经设置：

@Override
public void onResumeFragment() {
    new Handler().post(new Runnable() {
        @Override
        public void run() {
            refreshData();
        }
    });
}