片段setRetainInstace（true）无法按预期工作

Question

设置savedInstanceState!=null后

setRetainInstance(true)这是预期的行为吗？

我有一个带有ParentFragment的MainActivity，而ParentFragment拥有另一个ChildFragment

public class MainActivity extends AppCompatActivity {

@Override
public void onCreate(Bundle savedInstanceState) {
    recLifeCycle_with_savedInstanceState(savedInstanceState);
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (savedInstanceState == null) {
        // if's the first time created

        ParentFragment parentFragment = new ParentFragment();
        FragmentManager supportFragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = supportFragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.container, parentFragment, ParentFragment.TAG);
        fragmentTransaction.commit();
        supportFragmentManager.executePendingTransactions();
    }
}
}

public class ParentFragment extends BaseFragment {

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    recLifeCycle_with_savedInstanceState(savedInstanceState);
    View root = inflater.inflate(R.layout.fragment_parent, container, false);

    if(savedInstanceState==null){
        ChildFragment childFragment = new ChildFragment();
        FragmentManager childFragmentManager = getNestedChildFragmentManager();
        FragmentTransaction fragmentTransaction = childFragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.parent_container, childFragment, ChildFragment.TAG);
        fragmentTransaction.commit();
        childFragmentManager.executePendingTransactions();
    }
    recLifeCycle(sampleString);
    return root;
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    recLifeCycle_with_savedInstanceState(savedInstanceState);
    super.onActivityCreated(savedInstanceState);
    setRetainInstance(true);
}
}

public class ChildFragment extends BaseFragment {

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    recLifeCycle(sampleString);
    recLifeCycle_with_savedInstanceState(savedInstanceState);
    return inflater.inflate(R.layout.fragment_child, container, false);

}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    recLifeCycle_with_savedInstanceState(savedInstanceState);
    super.onActivityCreated(savedInstanceState);
}
}

public class BaseFragment extends Fragment {
private FragmentManager mRetainedChildFragmentManager;

public FragmentManager getNestedChildFragmentManager() {
    if(mRetainedChildFragmentManager == null) {
        mRetainedChildFragmentManager = getChildFragmentManager();
    }
    return mRetainedChildFragmentManager;
}

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

    if (mRetainedChildFragmentManager != null) {
        //restore the last retained child fragment manager to the new
        //created fragment
        try {
            Field childFMField = Fragment.class.getDeclaredField("mChildFragmentManager");
            childFMField.setAccessible(true);
            childFMField.set(this, mRetainedChildFragmentManager);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

}

观察到的生命周期日志如下

I/MYTAG: MainActivity.onCreate / savedInstanceState == null
I/MYTAG: ParentFragment.onAttach
I/MYTAG: ParentFragment.onCreate / savedInstanceState == null
I/MYTAG: MainActivity.onStart
I/MYTAG: ParentFragment.onCreateView / savedInstanceState == null
I/MYTAG: ChildFragment.onAttach
I/MYTAG: ChildFragment.onCreate / savedInstanceState == null
I/MYTAG: ParentFragment.onCreateView / 0.9485654467684422
I/MYTAG: ParentFragment.onActivityCreated / savedInstanceState == null
I/MYTAG: ChildFragment.onCreateView / 0.267993285910222
I/MYTAG: ChildFragment.onCreateView / savedInstanceState == null
I/MYTAG: ChildFragment.onActivityCreated / savedInstanceState == null
I/MYTAG: ParentFragment.onStart
I/MYTAG: ChildFragment.onStart
I/MYTAG: MainActivity.onResume
I/MYTAG: ParentFragment.onResume
I/MYTAG: ChildFragment.onResume

当屏幕旋转时，我发现父片段的savedinstancestate不为null，但是将子片段的实例状态保存为null

I/MYTAG: MainActivity.onPause
I/MYTAG: ChildFragment.onPause
I/MYTAG: ParentFragment.onPause
I/MYTAG: MainActivity.onStop
I/MYTAG: ChildFragment.onStop
I/MYTAG: ParentFragment.onStop
I/MYTAG: MainActivity.onDestroy
I/MYTAG: ChildFragment.onDestroyView
I/MYTAG: ParentFragment.onDestroyView
I/MYTAG: ParentFragment.onDetach
I/MYTAG: MainActivity.onCreate / savedInstanceState != null
I/MYTAG: ParentFragment.onAttach
I/MYTAG: MainActivity.onStart
I/MYTAG: ParentFragment.onCreateView / savedInstanceState != null
I/MYTAG: ParentFragment.onCreateView / 0.9485654467684422
I/MYTAG: ParentFragment.onActivityCreated / savedInstanceState != null
I/MYTAG: ChildFragment.onCreateView / 0.267993285910222
I/MYTAG: ChildFragment.onCreateView / savedInstanceState == null
I/MYTAG: ChildFragment.onActivityCreated / savedInstanceState == null
I/MYTAG: ParentFragment.onStart
I/MYTAG: ChildFragment.onStart
I/MYTAG: MainActivity.onResume
I/MYTAG: ParentFragment.onResume
I/MYTAG: ChildFragment.onResume