Исключение нулевого указателя, когда LayoutManager.canScrollVertically()

Question

Я пытаюсь включить SwipeRefreshLayout в мое приложение. Он работает, но когда я переключаю свой WIFI (мое приложение использует его, и я хотел включить предупреждения, если нет Wi-Fi). Кто-нибудь есть какие-либо предложения, почему я получаю это исключение нулевого указателя:

NullPointerException: Attempt to invoke virtual method 'boolean android.support.v7.widget.RecyclerView$LayoutManager.canScrollVertically()' on a null object reference 
                     at android.support.v7.widget.RecyclerView.computeVerticalScrollOffset(RecyclerView.java:1540) 
                     at android.view.View.canScrollVertically(View.java:13757) 
                     at android.support.v4.view.ViewCompatICS.canScrollVertically(ViewCompatICS.java:35) 
                     at android.support.v4.view.ViewCompat$ICSViewCompatImpl.canScrollVertically(ViewCompat.java:1161) 
                     at android.support.v4.view.ViewCompat.canScrollVertically(ViewCompat.java:1575) 
                     at android.support.v4.widget.SwipeRefreshLayout.canChildScrollUp(SwipeRefreshLayout.java:643) 
                     at android.support.v4.widget.SwipeRefreshLayout.onInterceptTouchEvent(SwipeRefreshLayout.java:657) 
                     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2108) 
                     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553) 
                     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2197) 
                     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553) 
                     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2197) 

я должен указать на то, что перед добавлением SwipeFredreshLayput в моем приложении у меня не был такая проблема.

Это мой XML: Hourly_Fragment:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
      xmlns:tools="http://schemas.android.com/tools" 
      android:layout_width="match_parent" 
      android:layout_height="match_parent" 
      android:paddingLeft="@dimen/activity_horizontal_margin" 
      android:paddingRight="@dimen/activity_horizontal_margin" 
      android:paddingTop="@dimen/activity_vertical_margin" 
      android:paddingBottom="@dimen/activity_vertical_margin" 
      android:background="@drawable/bg_gradient" 
      tools:context="koemdzhiev.com.stormy.ui.HourlyForecastActivity"> 

<android.support.v4.widget.SwipeRefreshLayout 
    android:id="@+id/hourly_swipe_refresh_layout" 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content"> 

     <android.support.v7.widget.RecyclerView 
      android:layout_width="match_parent" 
      android:layout_height="match_parent" 
      android:layout_centerHorizontal="true" 
      android:id="@+id/recyclerView"/> 

</android.support.v4.widget.SwipeRefreshLayout> 

</RelativeLayout> 

Это мой Hourly_fragment код Java:

public class Hourly_forecast_fragment extends Fragment { 
private Hour[] mHours; 
private MainActivity mActivity; 
@InjectView(R.id.hourly_swipe_refresh_layout) 
SwipeRefreshLayout mSwipeRefreshLayout; 
RecyclerView.LayoutManager layoutManager; 
//inject the RecyclerView as member variable 
@InjectView(R.id.recyclerView) 
RecyclerView mRecyclerView; 

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

    mActivity = ((MainActivity) getActivity()); 


} 

@Override 
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { 
    View v =inflater.inflate(R.layout.hourly_forecast_fragment,container,false); 
    ButterKnife.inject(this, v); 
    mSwipeRefreshLayout.setColorSchemeColors(R.color.orange,R.color.green,R.color.blue); 
    if (mRecyclerView != null) 
     mSwipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() { 
      @Override 
      public void onRefresh() { 
       if (mActivity.isNetworkAvailable()) { 
        mActivity.getLocation(); 
       } else { 
        mActivity.alertForNoInternet(); 
        mSwipeRefreshLayout.setRefreshing(false); 
       } 
      } 
     }); 
    Log.e("Forecast_fragment", "onCreateView"); 
    return v; 
} 

public void setUpHourlyFragment(){ 
    if (mActivity.mForecast != null) { 

     Hour[] hourlyForecast = mActivity.mForecast.getHourlyForecast(); 
     mHours = Arrays.copyOf(hourlyForecast, hourlyForecast.length, Hour[].class); 

     HourAdapter adapter = new HourAdapter(mActivity, mHours); 
     mRecyclerView.setAdapter(adapter); 
     mRecyclerView.setHasFixedSize(true); 
     layoutManager = new LinearLayoutManager(mActivity); 
     mRecyclerView.setLayoutManager(layoutManager); 
     //if dealing with fixed size data, it is recommended to do the following... 
     mRecyclerView.setHasFixedSize(true); 
    } 
} 
} 

Основной код вида деятельности:

public class MainActivity extends AppCompatActivity { 
ViewPager pager; 
ViewPagerAdapter adapter; 
SlidingTabLayout tabs; 
CharSequence Titles[]={"Current","Hourly","Daily"}; 
int Numboftabs =3; 
Current_forecast_fragment mCurrent_forecast_fragment; 
Hourly_forecast_fragment mHourly_forecast_fragment; 
Daily_forecast_fragment mDaily_forecast_fragment; 

public static final String TAG = MainActivity.class.getSimpleName(); 
public static final String LOCATION_KEY = "location_key"; 
public Forecast mForecast; 
public static final String DAILY_FORECAST = "DAILY_FORECAST"; 
public static final String HOURLY_FORECAST = "HOURLY_FORECAST"; 
//default coordinates - Gotse Delchev, UK Lati:57.156866 ; Long: 
private double latitude = 41.5667; 
private double longitude = 23.7333; 
private LocationManager locationManager; 


@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    //-----------MY CODE STARTS HERE----------------- 
    changeWindowTopColor(); 
    this.mCurrent_forecast_fragment = new Current_forecast_fragment(); 
    this.mHourly_forecast_fragment = new Hourly_forecast_fragment(); 
    this.mDaily_forecast_fragment = new Daily_forecast_fragment(); 
    getLocation(); 

    // Creating The ViewPagerAdapter and Passing Fragment Manager, Titles fot the Tabs and Number Of Tabs. 
    adapter = new ViewPagerAdapter(getSupportFragmentManager(),Titles,Numboftabs,mCurrent_forecast_fragment, 
      mHourly_forecast_fragment,mDaily_forecast_fragment); 

    // Assigning ViewPager View and setting the adapter 
    pager = (ViewPager) findViewById(R.id.pager); 
    pager.setOffscreenPageLimit(3); 
    pager.setAdapter(adapter); 

    // Assiging the Sliding Tab Layout View 
    tabs = (SlidingTabLayout) findViewById(R.id.tabs); 
    tabs.setDistributeEvenly(true); // To make the Tabs Fixed set this true, This makes the tabs Space Evenly in Available width 

    // Setting Custom Color for the Scroll bar indicator of the Tab View 
    tabs.setCustomTabColorizer(new SlidingTabLayout.TabColorizer() { 
     @Override 
     public int getIndicatorColor(int position) { 
      return ContextCompat.getColor(MainActivity.this,R.color.tabsScrollColor); 
     } 
    }); 

    // Setting the ViewPager For the SlidingTabsLayout 
    tabs.setViewPager(pager); 


} 
private void getForecast(double latitude, double longitude) { 
if (isNetworkAvailable()) { 


     OkHttpClient client = new OkHttpClient(); 
     Request request = new Request.Builder() 
       .url(forecast) 
       .build(); 

     Call call = client.newCall(request); 

     call.enqueue(new Callback() { 
      @Override 
      public void onFailure(Request request, IOException e) { 
       runOnUiThread(new Runnable() { 
        @Override 
        public void run() { 
         mCurrent_forecast_fragment.toggleRefresh(); 
         mHourly_forecast_fragment.mSwipeRefreshLayout.setRefreshing(false); 
        } 
       }); 
       alertUserAboutError(); 
      } 

      //when the call to the Okhttp library finishes, than calls this method: 
      @Override 
      public void onResponse(Response response) throws IOException { 
       runOnUiThread(new Runnable() { 
        @Override 
        public void run() { 
         mCurrent_forecast_fragment.toggleRefresh(); 
         mHourly_forecast_fragment.mSwipeRefreshLayout.setRefreshing(false); 
        } 
       }); 
       try { 
        String jsonData = response.body().string(); 
        //Log.v(TAG, jsonData); 
        if (response.isSuccessful()) { 
         mForecast = parseForecastDetails(jsonData); 
         runOnUiThread(new Runnable() { 
          @Override 
          public void run() { 
           mCurrent_forecast_fragment.updateDisplay(); 
           mHourly_forecast_fragment.setUpHourlyFragment(); 
           mDaily_forecast_fragment.setUpDailyFragment(); 

          } 
         }); 


        } else { 
         alertUserAboutError(); 
        } 
       } catch (IOException | JSONException e) { 
        Log.e(TAG, "Exception caught:", e); 
       } 
      } 
     }); 
    } else { 
     mCurrent_forecast_fragment.toggleRefresh(); 
     mHourly_forecast_fragment.mSwipeRefreshLayout.setRefreshing(false); 
     Toast.makeText(this,getString(R.string.network_unavailable_message), Toast.LENGTH_LONG).show(); 
     alertForNoInternet(); 
    } 
} 

public void alertForNoInternet() { 
    WIFIDialogFragment dialog = new WIFIDialogFragment(); 
    dialog.show(getFragmentManager(), getString(R.string.error_dialog_text)); 
} 

Answer 1

Помещенный эти строки:

layoutManager = new LinearLayoutManager(mActivity); 
mRecyclerView.setLayoutManager(layoutManager); 

после этой строки в onCreateView:

ButterKnife.inject(this, v); 

Вы должны вообще избежать иметь RecyclerView без LayoutManager прилагается, это приводит к странному исключению, как этот, так как большинство из RecyclerView признаков, даже не-чертежа связанный с отношением LayoutManager. Тем не менее, вполне нормально иметь RecyclerView без Adapter (потому что у вас нет данных еще или данных). Но вы должны приложить LayoutManager asap после создания RecyclerView :-)