TextView von ViewPager wirft Null Pointer Exception

Question

Ich habe ein Fragment mit einem ViewPager drin. Im ViewPager habe ich 4 Inhaltsbildschirme, also wische herum. Der Inhalt hat ein paar TextView-Komponenten und ich versuche auf sie vom ersten Fragment zuzugreifen, aber wenn ich setText oder ähnliches benutze, bekomme ich eine Nullzeiger-Ausnahme. Ich habe überprüft und die ID für die TextView ist gültig, und ich habe einen sauberen Build erstellt.

Hier ist ein Ausschnitt aus meinem entsprechenden Code:

private TextView textViewOne; 

private PagerAdapter mPagerAdapter; 
private ViewPager pager; 
View rootView; 

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
    rootView = inflater.inflate(R.layout.fragment_layout_viewpager, container, false); 

    InitializePaging(); 

    textViewOne = (TextView) rootView.findViewById(R.id.create_name); 

    //would return null if I did this: 
    textViewOne.setText("test"); 
    return rootView; 
} 

private void InitializePaging() { 
    mPagerAdapter = new ViewPagerAdapter(getContext(), getChildFragmentManager(), 
      Arrays.asList(PageOne.class, PageTwo.class, PageThree.class, PageFour.class)); 

    pager = (ViewPager) rootView.findViewById(R.id.content_pager); 
    pager.setAdapter(mPagerAdapter); 
} 

Die PageOne Klasse:

public class PageOne extends Fragment { 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
    if(container == null) { 
     return null; 
    } 
     return (LinearLayout) inflater.inflate(R.layout.fragment_page_one, container, false); 
    } 
} 

Dies ist fragment_layout_viewpager xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:orientation="vertical"> 
     <android.support.v4.view.ViewPager 
      android:id="@+id/content_pager" 
      android:layout_width="fill_parent" 
      android:layout_height="fill_parent"/> 

</LinearLayout> 

Answer 1

Sie fragment_layout_viewpager aufblasen, so Die create_name sollte in diesem Layout existieren, sonst wird es Wirf einen Nullzeiger.

Answer 2

Create an interface with 1 abstract method. Let the Fragment implement the interface and call the callback method from Activity.In the below example, I try to pass value on click of button from MainActivity to ThirdFragment. 

public interface FragmentCommunicator { 
    public void passDataToFragment(String someValue); 
} 

    public class MainActivity extends FragmentActivity { 
    FragmentCommunicator fragmentCommunicator; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     final Button btn= (Button)findViewById(R.id.button) ; 
     btnsetOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       randomvalue= getRansomString(); 
       if(fragmentCommunicator!=null) 
       fragmentCommunicator.passDataToFragment(randomvalue); 
      } 
     }); 
     ViewPager pager = (ViewPager) findViewById(R.id.viewPager); 
     pager.setAdapter(new MyPagerAdapter(getSupportFragmentManager())); 
    } 
    private class MyPagerAdapter extends FragmentPagerAdapter { 
     public MyPagerAdapter(FragmentManager fm) { 
      super(fm); 
     } 
     @Override 
     public Fragment getItem(int pos) { 
      switch(pos) { 
       case 0: return FirstFragment.newInstance("FirstFragment, Instance 1"); 
       case 1: return ThirdFragment.newInstance("ThirdFragment, Instance 1"); 
       default: return FirstFragment.newInstance("FirstFragment,Instance 1"); 
} 
     } 
     @Override 
     public int getCount() { 
      return 2; 
     } 
    } 
    public static class FirstFragment extends Fragment { 
     @Override 
     public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
      View v = inflater.inflate(R.layout.first_frag, container, false); 
      TextView tv = (TextView) v.findViewById(R.id.tvFragFirst); 
      tv.setText(getArguments().getString("msg")); 
      return v; 
     } 
     public static FirstFragment newInstance(String text) { 
      Log.e("FirstFragment","newInstance called"); 
      FirstFragment f = new FirstFragment(); 
      Bundle b = new Bundle(); 
      b.putString("msg", text); 
      f.setArguments(b); 
      return f; 
     } 
    } 
    public class ThirdFragment extends Fragment implements FragmentCommunicator { 
    TextView tv; 
    String value=""; 
    public FragmentCommunicator fragmentCommunicator; 
    @Override 
    public void passDataToFragment(String someValue) { 
     this.value = someValue; 
     tv.setText(value); 
    } 
    @Override 
    public void onAttach(Activity activity) { 
     super.onAttach(activity); 
     try {   
      ((MainActivity)activity).fragmentCommunicator = this; 
     } catch (ClassCastException e) { 
      throw new ClassCastException(activity.toString() 
        + " must implement FragmentCommunicator"); 
     } 
    } 
    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
     View v = inflater.inflate(R.layout.first_frag, container, false); 
     value=getArguments().getString("msg"); 
     tv = (TextView) v.findViewById(R.id.tvFragFirst); 
     tv.setText(value); 
     return v; 
    } 
    public static ThirdFragment newInstance(String text) { 
     ThirdFragment f = new ThirdFragment(); 
     Bundle b = new Bundle(); 
     b.putString("msg",text); 
     f.setArguments(b); 
     return f; 
    } 
}