麻煩與片段TextWatcher到活動通信

Question

有問題越來越片段到活動通信正常工作..

我有一個片段編輯框，我需要它給每個編輯後的值發送回主要活動。

片段類：

public class FragA extends Fragment { 

    FragAInt mCallback; 

    public interface FragAInt{ 
     public void onEditBoxEdited(String boxContent); 
    } 


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

     ((EditText) et).addTextChangedListener(new TextWatcher() { 
      @Override 
      public void onTextChanged(CharSequence s, int start, int before, int count) { 

       // TODO Auto-generated method stub 

      } 
      @Override 
      public void beforeTextChanged(CharSequence s, int start, int count, int after) { 

       // TODO Auto-generated method stub 
      } 

      @Override 
      public void afterTextChanged(Editable s) { 
       mCallback.onEditBoxEdited(((EditText) et).getText().toString()); 
      } 
     }); 
     return view; 
    } 
} 

MainActivity

public class MainActivity extends FragmentActivity implements FragA.FragAInt{ 

    private String editBoxVal; 

    public void onEditBoxEdited(String editBoxContent){ 
     this.editBoxVal= editBoxContent; 
    } 
} 

該應用程序將運行，但它會只要你在一個空指針異常退出框中鍵入崩潰。

我哪裏出錯了？

完整的錯誤：

java.lang.NullPointerException: 
Attempt to invoke interface method 'void nemo.myapplication.FragA$FragAInt.onEditBoxEdited(java.lang.String)' on a null object reference 
at nemo.myapplication.FragA$1.afterTextChanged(FragA.java:49) 
at android.widget.TextView.sendAfterTextChanged(TextView.java:8017) 
at android.widget.TextView$ChangeWatcher.afterTextChanged(TextView.java:10182) 
at android.text.SpannableStringBuilder.sendAfterTextChanged(SpannableStringBuilder.java:1043) 
at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:560) 
at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:492) 
at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:34) 
at android.view.inputmethod.BaseInputConnection.replaceText(BaseInputConnection.java:685) 
at android.view.inputmethod.BaseInputConnection.setComposingText(BaseInputConnection.java:445) 
at com.android.internal.view.IInputConnectionWrapper.executeMessage(IInputConnectionWrapper.java:340) 
at com.android.internal.view.IInputConnectionWrapper$MyHandler.handleMessage(IInputConnectionWrapper.java:78) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:234) 
at android.app.ActivityThread.main(ActivityThread.java:5526) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

Answer 1

您需要分配mCallback變量。我想你錯過了那部分，你可以使用onAttach事件。

public class FragA extends Fragment { 

    FragAInt mCallback; 

    public interface FragAInt{ 
     public void onEditBoxEdited(String boxContent); 
    } 

    public FragA(FragAInt mCallback) { 
     this.mCallback = mCallback; 
    } 

    @Override 
    public void onAttach(Activity activity) { 
     super.onAttach(activity); 

     // This makes sure that the container activity has implemented 
     // the callback interface. If not, it throws an exception 
     try { 
      mCallback = (FragAInt) activity; 
     } catch (ClassCastException e) { 
      throw new ClassCastException(activity.toString() 
        + " must implement OnHeadlineSelectedListener"); 
     } 
    } 

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

     ((EditText) et).addTextChangedListener(new TextWatcher() { 
      @Override 
      public void onTextChanged(CharSequence s, int start, int before, int count) { 

       // TODO Auto-generated method stub 

      } 
      @Override 
      public void beforeTextChanged(CharSequence s, int start, int count, int after) { 

       // TODO Auto-generated method stub 
      } 

      @Override 
      public void afterTextChanged(Editable s) { 
       mCallback.onEditBoxEdited(((EditText) et).getText().toString()); 
      } 
     }); 
     return view; 
    } 
}