我有一个MainActivity, 布局包括两个FrameLayout要加载Fragments，一个用于带有垂直菜单的顶部栏，另一个用于内容。
一切正常，除了当我单击设备的主页按钮并再次启动应用程序时。
例如，当我启动应用程序时，单击菜单选项 2，它将内容片段替换为SomeFragment如下所示的实例，然后单击设备的主页按钮，然后再次启动应用程序，它SomeFragment Fragment按预期重新启动，但我的应用程序崩溃在 中TopBarFragment，返回错误，因为getView()为空。
我想我在搞乱Fragment生命周期，但我找不到解决方案。
注意：为了测试我的应用程序，我检查了Don't keep activities我设备的开发者选项中的选项。
有人可以帮忙吗？
编辑
重新启动应用程序时，onResume不会在TopBarFragment实例中调用。为什么？调用的实例
((TopBarFragment)getActivity().getSupportFragmentManager()
.findFragmentByTag("top_bar_fragment")).setSelectedButton(1);
似乎是“空的”……为什么？
编辑2：更多实验
当点击设备的 Home 键时，onDetach被onDestroyView调用TopBarFragment，为什么onCreateView重启应用时没有再次调用？
编辑 3：更多实验
重新启动应用程序时，MainActivity onCreate会调用 ，所以
fragmentTransaction.replace(R.id.content_container, homeFragment, "");
被执行。但是，它加载了SomeFragment，这很好，因为它是最后一次Fragment加载的content_container，但出乎意料的是因为MainActivity onCreate被调用并且它应该加载HomeFragment......
堆栈跟踪
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.my.package/com.my.package.MainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1967)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1992)
at android.app.ActivityThread.access$600(ActivityThread.java:127)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1158)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4511)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:980)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:747)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.my.package.TopBarFragment.setSelectedButton(TopBarFragment.java:345)
at com.my.package.SomeFragment.onCreateView(SomeFragment.java:50)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:1460)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:911)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1088)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1070)
at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:1861)
at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:547)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1136)
at android.app.Activity.performStart(Activity.java:4480)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1940)
... 11 more
MainActivity.java
public class MainActivity extends FragmentActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
Fragment homeFragment = new HomeFragment();
fragmentTransaction.replace(R.id.content_container, homeFragment, "");
Fragment topBarFragment = new TopBarFragment();
fragmentTransaction.replace(R.id.top_bar_container, topBarFragment, "top_bar_fragment");
fragmentTransaction.commit();
}
}
主要的.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<FrameLayout
android:id="@+id/content_container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingTop="44dp" />
<FrameLayout
android:id="@+id/top_bar_container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:clipChildren="false" />
</RelativeLayout>
TopBarFragment.java
public class TopBarFragment extends Fragment{
private int mSelectedMenuOption = 0;
private LinearLayout mVerticalMenu;
private Boolean mMenuIsOpen = true;
private ImageButton btn_01, btn_02; // there are more
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
return inflater.inflate(R.layout.top_bar, container, false);
}
@Override
public void onActivityCreated (Bundle savedInstanceState){
super.onActivityCreated(savedInstanceState);
btn_01 = (ImageButton) getView().findViewById(R.id.btn_01);
btn_02 = (ImageButton) getView().findViewById(R.id.btn_02);
btn_01.setOnClickListener(mButtonClickListener);
btn_02.setOnClickListener(mButtonClickListener);
mVerticalMenu = (LinearLayout) getView().findViewById(R.id.vertical_menu);
toggleMenu(0);
Button btn_menu = (Button) getView().findViewById(R.id.btn_menu);
btn_menu.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// toggle vertical menu
}
});
}
private OnClickListener mButtonClickListener = new OnClickListener()
{
@Override
public void onClick(View v) {
/* ... */
if(!v.isSelected()){
FragmentTransaction fragmentTransaction = getActivity().getSupportFragmentManager().beginTransaction();
switch(v.getId()){
case R.id.btn_01:
Fragment homeFragment = new HomeFragment();
fragmentTransaction.replace(R.id.content_container,homeFragment, "");
fragmentTransaction.commit();
break;
case R.id.btn_02:
Fragment someFragment = new SomeFragment();
fragmentTransaction.replace(R.id.content_container, someFragment, "");
fragmentTransaction.commit();
break;
}
}
}
};
public void setSelectedButton(int i){
// Crashes when starting the app, clicking on btn_02 to load a SomeFragment instance, clicking on the
// device's Home button, and starting the app again: getView() is null
// why?
/* ... */
mSelectedMenuOption = i;
}
}
一些片段
public class SomeFragment extends Fragment{
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
((TopBarFragment)getActivity().getSupportFragmentManager().findFragmentByTag("top_bar_fragment")).setSelectedButton(1);
return inflater.inflate(R.layout.some_fragment, container, false);
}
}