android - getSupportFragmentManager().findFragmentById() 返回 null

Question

我正在尝试在 android 中实现 fragment 通信，就像 android 指南 http://developer.android.com/guide/components/fragments.html#CommunicatingWithActivity 中的那样。

但由于 getSupportFragmentManager().findFragmentById() 返回 null，我的应用程序崩溃了。我的实现有什么问题。

代码如下:

该程序只是通过从第一个fragmnet单击按钮将输入文本从一个 fragment 发送到另一个 fragment textView区域。我有一个activity_main.xml和两个 fragment 布局(两个单独的xml文件，而不是activity_main中的一部分。 xml)

Frag1.java

public class Frag1 extends Fragment {
public Frag1(){

}

buttonClickListener buttonListener;

@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try {
    buttonListener = (buttonClickListener) getActivity();
} catch (ClassCastException e) {
    throw new ClassCastException(activity.toString() + " must implement OnButtonPressListener");
}
}

View myFragmentView;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    myFragmentView = inflater.inflate(R.layout.frag1, container, false);


    //SetValue Button
    Button setValueButton = (Button) myFragmentView.findViewById(R.id.setValueButton);
    setValueButton.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            buttonListener.onButtonPressed("Message received");
        }
    });


    return myFragmentView;
}

}

Frag2.java

public class Frag2 extends Fragment {

View myFragmentView;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
 myFragmentView = inflater.inflate(R.layout.frag2, container, false);
    return myFragmentView;
}

void setMessage(String msg){
    TextView txt=(TextView)myFragmentView.findViewById(R.id.textView1);
    txt.setText(msg);
}
}

buttonClickListener.java

public interface buttonClickListener {
public void onButtonPressed(String msg);
}

MainActivity.java

public class MainActivity extends FragmentActivity implements
    ActionBar.TabListener, buttonClickListener {
SectionsPagerAdapter mSectionsPagerAdapter;
@Override
public void onButtonPressed(String msg) {
    // TODO Auto-generated method stub
    Frag2 fragmentObj=(Frag2) getSupportFragmentManager().findFragmentById(R.layout.frag2);


    fragmentObj.setMessage(msg);
}

请告诉我哪里出错了？

编辑:我正在使用由 Android Plug-in eclipse IDE 生成的模板创建 fragment 。所以 fragment 是使用 android.support.v4.app.Fragment

创建的

@Override
    public Fragment getItem(int position) {
        Fragment fragment = null;
        switch(position)
        {
        case 0:
            return new Frag1();
        case 1: 
            return new Frag2();
        }
        return fragment;
    }

代码库保留在这里以供引用 https://skydrive.live.com/redir?resid=D37E0F56FEC9B499!259

Answer 1

试试这对我有用How to put Google Maps V2 on a Fragment Using ViewPager

<fragment
            android:id="@+id/map"
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            class="com.google.android.gms.maps.SupportMapFragment" />

GoogleMap mGoogleMap = ((SupportMapFragment) getChildFragmentManager().findFragmentById(R.id.map)).getMap();