Android 为什么RecycleView适配器在查看寻呼机片段时有时为空

我仍然无法复制确切的步骤，但ViewPager片段上承载的RecycleView适配器有时为空

崩溃发生在android 8或更高版本上。我试着在没有后台进程的情况下运行它，或者在开发人员选项上“不检查活动”，以防它受到配置更改的影响，但我没有遇到任何崩溃

片段上的RecycleView适配器设置

public class FragmentExploreEvents extends InFragment implements IFragmentExploreEvents {

private String TAG = FragmentExploreEvents.class.getSimpleName();

private RecyclerView mRecyclerView;
private GenericAdapter mAdapter;
protected List<GenericBean> mData;
Calendar dateToday = DateUtils.getCalendarToday();
Calendar selectedDate = Calendar.getInstance();


// initializing the RecyclerView and Adapter
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_explore_events, container, false);

    mProgressBar = view.findViewById(R.id.progress_bar);

    mRecyclerView = view.findViewById(R.id.rv_content);



mData = new ArrayList<>();
    LinearLayoutManager mLayoutManager = new LinearLayoutManager(mActivity, RecyclerView.VERTICAL, false);
    mRecyclerView.setLayoutManager(mLayoutManager);
    mAdapter = new GenericAdapter(mActivity, mData, this, true);

    mRecyclerView.setAdapter(mAdapter);


    return view;
}


   // method that I am calling on activity to refresh the recyclerview data
   @Override
    public void onLoadList(List<GenericBean> data) {
          if (mAdapter != null) {
             // Null pointer exception is occurring on this method 
             mAdapter.addData(data);
          } else {
             Crashlytics.log("Adapter was null");
         }

}

---

您应该在

onViewCreated

上初始化适配器和回收器视图，该视图在

onCreateView

之后调用。通过这样做，您可以确保视图已经膨胀，并且可以安全地设置适配器。

但是视图已经在onLoadList上膨胀，因为我正在对初始化和存储的片段调用该方法

public class ExploreClassesActivity extends NewBasicDisplayActivity implements BookingCallBackListener, CustomViewPagerListener {

WeekDaysSelector weekDaysSelector;

CustomViewPager mPager;
private MyPagerAdapter mPagerAdapter;
List<GenericBean> mData = new ArrayList<>();
private int prevPagePosition = 0;
private boolean enableUpdatePageOnSwipe = false;
Calendar dateToday;
Calendar selectedDate;


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

    setContentView(R.layout.activity_swipe_explore_events);

    weekDaysSelector = findViewById(R.id.activity_explore_event_day_selector);
    mPager = findViewById(R.id.view_pager);
    mToolbar = findViewById(R.id.toolbar);

    setupViewPager();
    setupView();

}



// setting up view pager with a listener to call loadData on page selection change
private void setupViewPager() {

    mPagerAdapter =
            new MyPagerAdapter(getSupportFragmentManager());

    mPager.setOffscreenPageLimit(6);
    mPager.setAdapter(mPagerAdapter);

    mPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {

        @Override
        public void onPageSelected(int position) {
                selectedDate.add(Calendar.DAY_OF_MONTH,1);
                position = weekDaysSelector.highlightDate(selectedDate);
                prevPagePosition = position;
                loadData();
        }


    });
}

}

//setting up a day selection listener to load data into the current viewpager fragment
protected void setupView() {


    weekDaysSelector.setListener(new WeekDaySelectorListener() {
        @Override
        public void onDateSelection(Calendar date, int position) {
            selectedDate = date;
            prevPagePosition = position;
            loadData();

        }


    });
    loadData();
}




// updating the fragment with the current fragment with the received data
private void updateFragmentList(List<GenericBean> mData) {
    mPager.setCurrentItem(prevPagePosition);
    // onLoadList is called on the current display view pager fragment
    IFragmentExploreEvents fragmentList = (IFragmentExploreEvents) mPagerAdapter.getItem(mPager.getCurrentItem());
    fragmentList.onLoadList(mData); // Null pointer is occurring 



protected void loadData() {
    String fromDate = DateUtils.getStringDateFromDate(selectedDate.getTime());
    selectedDate.set(Calendar.HOUR_OF_DAY, 23);
    String toDate = DateUtils.getStringDateFromDate(selectedDate.getTime());

    getClasses(fromDate, toDate);
}


private void getClasses(String startDate, String endDate) {
   // preparing displayed fragment before calling onLoadList when api data is received
    mPager.setCurrentItem(prevPagePosition);
    IFragmentExploreEvents fragmentList = (IFragmentExploreEvents) mPagerAdapter.getItem(mPager.getCurrentItem());
    fragmentList.onUpdate(selectedDate);

    API.getOnlineClasses(InPrefs.getToken(this), startDate, endDate, new Response.Listener<ClassesResponseBean>() {
        @Override
        public void onResponse(ClassesResponseBean response) {
            if (CodeUtils.isValidResponse(response)) {

                List<GenericBean> classes = response.getClassesByHour();
                updateFragmentList(classes);
            }
        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            handleError(error);
            updateFragmentList(new ArrayList<GenericBean>());
        }
    }, TAG);
}



// ViewPager adapter class extends CachingFragmentStatePagerAdapter methods to reuse viewpager 
fragments
public static class MyPagerAdapter extends CachingFragmentStatePagerAdapter {

    public MyPagerAdapter(FragmentManager fragmentManager) {
        super(fragmentManager);
    }

    @Override
    public int getCount() {
        return DateUtils.DAYS_OF_TIME;
    }

    // Created fragments are reused if they were already initialized
    @Override
    public Fragment getItem(int position) {
        if (getRegisteredFragment(position) != null) {
            return getRegisteredFragment(position);
        }
        return FragmentExploreEvents.newInstance();
    }


}

 public abstract class CachingFragmentStatePagerAdapter extends FragmentStatePagerAdapter {
         // Sparse array to keep track of registered fragments in memory
          private SparseArray<Fragment> registeredFragments = new SparseArray<Fragment>();

       public CachingFragmentStatePagerAdapter(FragmentManager fragmentManager) {
           super(fragmentManager,FragmentStatePagerAdapter.BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT);
        }

   // Register the fragment when the item is instantiated
   @Override
   public Object instantiateItem(@NonNull ViewGroup container, int position) {
    Fragment fragment = (Fragment) super.instantiateItem(container, position);
    registeredFragments.put(position, fragment);
    return fragment;
    }

   // Unregistered when the item is inactive
   @Override
   public void destroyItem(@NonNull ViewGroup container, int position, Object object) {
    registeredFragments.remove(position);
    super.destroyItem(container, position, object);
    }

    // Returns the fragment for the position (if instantiated)
     public Fragment getRegisteredFragment(int position) {
    return registeredFragments.get(position);
     }
}

   Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.codefish.in.adapters.GenericAdapter.addData(java.util.List)' on a null object reference
   at com.codefish.in.fragments.FragmentExploreEvents.onLoadList + 2(FragmentExploreEvents.java:2)
   at com.codefish.in.activities.ExploreClassesActivity.updateFragmentList + 21(ExploreClassesActivity.java:21)
   at com.codefish.in.activities.ExploreClassesActivity.access$300(ExploreClassesActivity.java)
   at com.codefish.in.activities.ExploreClassesActivity$3.onResponse + 69(ExploreClassesActivity.java:69)
   at com.codefish.in.activities.ExploreClassesActivity$3.onResponse + 2(ExploreClassesActivity.java:2)
   at com.codefish.in.api.InPostRequest.deliverResponse + 2(InPostRequest.java:2)
   at com.android.volley.ExecutorDelivery$ResponseDeliveryRunnable.run + 30(ExecutorDelivery.java:30)
   at android.os.Handler.handleCallback + 883(Handler.java:883)
   at android.os.Handler.dispatchMessage + 100(Handler.java:100)
   at android.os.Looper.loop + 237(Looper.java:237)
   at android.app.ActivityThread.main + 7762(ActivityThread.java:7762)
   at java.lang.reflect.Method.invoke(Method.java)
   at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run + 493(RuntimeInit.java:493)
   at com.android.internal.os.ZygoteInit.main + 1047(ZygoteInit.java:1047)