我的 Android 应用程序包含三个片段：A、B 和 C。它们被加载到定义的两个容器中MainActivity布局。
当应用程序启动时，它会显示fragmentA 加载到 left_container 中和right_container中的fragmentC.
如果您按下片段A, a FragmentTransaction变化片段C by 片段B.
目前一切正常。但是当我尝试使用以下命令获取对加载的fragmentB的引用时，问题就出现了 findFragmentByTag()，因为它返回null。我已经使用了方法替换FragmentTransaction我已经完成了commit()，但是没有办法调用片段B方法。我的代码：
MainActivity.java：
public class MainActivity extends Activity{
static String fragmentTag = "FRAGMENTB_TAG";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//Adds the left container's fragment
getFragmentManager().beginTransaction().add(R.id.left_container, new FragmentA()).commit(); //Adds the fragment A to the left container
//Adds the right container's fragment
getFragmentManager().beginTransaction().add(R.id.right_container, new FragmentC()).commit(); //Adds the Fragment C to the right container
}
/**
* Called when the button "Activate Fragment B" is pressed
*/
public void buttonListener(View v){
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(R.id.right_container, new FragmentB(),fragmentTag); //Replaces the Fragment C previously in the right_container with a new Fragment B
ft.commit(); //Finishes the transaction
//!!HERE THE APP CRASHES (java.lang.NullPointerException = findFragmentByTag returns null
((FragmentB) getFragmentManager().findFragmentByTag(fragmentTag)).testView();
}
}
片段B.java：
public class FragmentB extends Fragment {
public View onCreateView(LayoutInflater inflater,
ViewGroup container,
Bundle savedInstanceState) {
return inflater.inflate(R.layout.fragment_b, container,false);
}
/**
* Gets a reference to the text_fragment_b TextView and calls its method setText(), changing "It doesn't work" text by "It works!"
*/
public void testView(){
TextView tv = (TextView)getView().findViewById(R.id.text_fragment_b);
tv.setText("It works!");
}
}
活动_main.xml：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal" >
<FrameLayout android:id="@+id/left_container" android:layout_width="0px" android:layout_weight="50" android:layout_height="match_parent"/>
<FrameLayout android:id="@+id/right_container" android:layout_width="0px" android:layout_weight="50" android:layout_height="match_parent"/>
</LinearLayout>
片段_b.xml：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:layout_margin="5sp">
<TextView
android:id="@+id/text_fragment_b"
android:text="It doesn't works!"
android:layout_width="match_parent"
android:layout_height="wrap_content"/>
</LinearLayout>
请帮我！我是Android开发的初学者！
