片段中的空指针异常

Question

我从其他片段开始一个片段时得到NullPointerException.我试图在动态启动片段后调用第二个片段中的方法.

这是我的logcat:

05-20 09:58:31.907: E/AndroidRuntime(2585): FATAL EXCEPTION: main
05-20 09:58:31.907: E/AndroidRuntime(2585): java.lang.NullPointerException
05-20 09:58:31.907: E/AndroidRuntime(2585):     at com.exercise.FragmentTest.MyFragment3.setImage(MyFragment3.java:22)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at com.exercise.FragmentTest.FragmentTestActivity$1.onClick(FragmentTestActivity.java:48)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.view.View.performClick(View.java:4204)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.view.View$PerformClick.run(View.java:17355)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.os.Handler.handleCallback(Handler.java:725)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.os.Handler.dispatchMessage(Handler.java:92)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.os.Looper.loop(Looper.java:137)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.app.ActivityThread.main(ActivityThread.java:5041)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at java.lang.reflect.Method.invokeNative(Native Method)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at java.lang.reflect.Method.invoke(Method.java:511)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at dalvik.system.NativeStart.main(Native Method)

和我的主要活动代码:

public class FragmentTestActivity extends FragmentActivity{

    Fragment fragment;
    Button btnFragment1, btnFragment2, btnFragment3;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btnFragment1 = (Button)findViewById(R.id.displayfragment1);
        btnFragment2 = (Button)findViewById(R.id.displayfragment2);
        btnFragment3 = (Button)findViewById(R.id.displayfragment3);

       // get an instance of FragmentTransaction from your Activity 
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

        //add a fragment 
        MyFragment myFragment = new MyFragment();
        fragmentTransaction.add(R.id.myfragment, myFragment);
        fragmentTransaction.commit();

        btnFragment1.setOnClickListener(btnFragmentOnClickListener);
        btnFragment2.setOnClickListener(btnFragmentOnClickListener);
        btnFragment3.setOnClickListener(btnFragmentOnClickListener);
    }

    Button.OnClickListener btnFragmentOnClickListener
    = new Button.OnClickListener(){

        @Override
        public void onClick(View v) {


            if(v == btnFragment3){
                MyFragment3 newfragment = new MyFragment3();

                FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

                // Replace whatever is in the fragment_container view with this fragment,
                // and add the transaction to the back stack
                transaction.replace(R.id.myfragment, newfragment);
                transaction.addToBackStack(null);

                // Commit the transaction
                transaction.commit();
                            newfragment.setImage();
                return;

            }


            // TODO Auto-generated method stub
            Fragment newFragment = null;

            // Create new fragment
            if(v == btnFragment1){
                newFragment = new MyFragment();
            }
                else if(v == btnFragment2){
                    newFragment = new MyFragment2();
                }


            FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

            // Replace whatever is in the fragment_container view with this fragment,
            // and add the transaction to the back stack
            transaction.replace(R.id.myfragment, newFragment);
            transaction.addToBackStack(null);

            // Commit the transaction
            transaction.commit();

            // Create new transaction

        }};

}

而我的Fragment3类是:

public class MyFragment3 extends Fragment {
    ImageView iv;
     @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container,
       Bundle savedInstanceState) {
      // TODO Auto-generated method stub
      View myFragmentView = inflater.inflate(R.layout.fragmentlayout3, container, false);
      iv = (ImageView)myFragmentView.findViewById(R.id.image); 

      return myFragmentView;
     }
     public void setImage(){
         iv.setImageResource(R.drawable.penguins);
     }
}

请帮我克服这个问题.

Answer 1

你的错误发生在这里：

FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
newfragment.setImage();

它之所以发生是因为变量iv仍然是null。

您必须等到片段显示（膨胀）之后才能访问膨胀到其中的视图。

有关更多信息，请尝试阅读Fragment 生命周期。

onCreateView()
当片段第一次绘制其用户界面时，系统会调用此方法。要为片段绘制 UI，您必须从此方法返回一个视图，该视图是片段布局的根。如果片段不提供 UI，则可以返回 null。

创建它的一个非常好的方法是使用该setArgs方法并传递Bundle带有有关图像的信息。

尝试使用此代码：

public MyFragment3 extends Fragment {

    public static final IMAGE_RES = "IMAGE_RES";
    private int imageRes;
    private ImageView iv;

    public static MyFragment3 init(int imageRes) {
        MyFragment3 frag = new MyFragment3();

        Bundle args = new Bundle();
        args.putInt(IMAGE_RES, imageRes);
        frag.setArguments(args);

        return frag;
    }

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(SavedInstanceState);

        imageRes = (getArguments() != null)? getArguments().getInt(IMAGE_RES) : 0;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
       Bundle savedInstanceState) {
          View myFragmentView = inflater.inflate(R.layout.fragmentlayout3, container, false);
         iv = (ImageView)myFragmentView.findViewById(R.id.image); 
         iv.setImageResource(imageRes);

         return myFragmentView;
    }
}