android – Proper use of sub sub fragments with (Child)FragmentManager-ThrowExceptions

Exception or error:

How do I properly use Fragments in Fragments?

My (simplified) use case is following, I have an activity with a layout fragment and this fragment theirself contains a sub fragment… all fragments are added manually to their parents…

----------------------------------------------------------
- Activity                                               -
-                                                        -
-                                                        -
-     ---------------------------------------            -
-     - Fragment                            -            -
-     -                                     -            -
-     -    -----------------                -            -
-     -    - SubFragment   -                -            -
-     -    -               -                -            -
-     -    -               -                -            -
-     -    -----------------                -            -
-     ---------------------------------------            -
-                                                        -
----------------------------------------------------------

Now in my activity’s onCreate I do following:

if (savedInstanceState == null)
{
    // I create the fragment
    mMainFragment = new MainFragment();
    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
    transaction.replace(R.id.fragment_main, mMainFragment);
    transaction.commit();
}
else
{
    // I retrieve the fragment
    mMainFragment = (BaseFragment) getSupportFragmentManager().findFragmentById(R.id.fragment_main);
}

And in my fragments onCreate I get/create my SubFragment:

mSubFragment = getChildFragmentManager().findFragmentByTag(SubFragment.class.getName());
if (mSubFragment == null)
{
    mSubFragment = new SubFragment();
    getChildFragmentManager().beginTransaction().add(R.id.fragment_sub, mSubFragment, SubFragment.class.getName()).commit();
}

Problem

After screen rotation, my SubFragment is added twice… If I use the activity’s FragmentManager then it works… But why does it not work with the ChildFragmentManager? Of course, the Fragment is a new fragment, but the activity is a new one as well, so why does it work with the activity’s FragmentManager but not with the parent fragment’s one?

In a fragment, I should use the fragments ChildFragmentManager, shouldn’t I?

How to solve:

You should add SubFragment to Fragment the same way like you add Fragment to Activity. I mean adding Fragment to Activity should look like:

 @Override
 public void onCreate(Bundle savedInstanceState) {
   ....
   if (savedInstanceState == null){
      //add fragment
      mMainFragment = new MainFragment();
      FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
      transaction.replace(R.id.fragment_main, mMainFragment);
      transaction.commit();
   }
 }

Adding SubFragment to MainFragment should look like:

    public class MainFragment extends Fragment{

      @Override
      public View onCreateView(LayoutInflater i, ViewGroup c, Bundle savedInstanceState) {
           ...
        if (savedInstanceState == null){
           mSubFragment = new SubFragment();

           //add child fragment
           getChildFragmentManager()
                   .beginTransaction()
                   .add(R.id.fragment_sub, mSubFragment, "tag")
                   .commit();
        }
      }
    }

or you can add child fragment to Fragment in onCreate method

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