savedInstanceState在片段中始终为null

Question

Below is my code:

public class MyListFragmentActivity extends FragmentActivity{

@Override
    public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    System.out.println("DEBUG : MLFA onCreate");
    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction().replace(fragmentID, new MyListFragment())
            .replace(detailFragmentID, new MyDetailFragment()).commit();
        }
    }



     @Override
    protected void onRestart() {
            FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
            Fragment prevFrag = getSupportFragmentManager().findFragmentById(detailFragmentID);
    if (prevFrag != null) {
        fragmentTransaction.remove(prevFrag);
        getSupportFragmentManager().beginTransaction().replace(detailFragmentID, new MyDetailFragment()).commitAllowingStateLoss();
    } else {
        getSupportFragmentManager().beginTransaction().replace(detailFragmentID, new MyDetailFragment()).commitAllowingStateLoss();
    }
}

MyListFragment

public class MyListFragment extends Fragment{

//When we click on each item in list view call detail fragment to relad its layout
OnItemClickListener onItemClickListener = new OnItemClickListener() {

/** Getting the fragmenttransaction object, which can be used to add, remove or replace a fragment */
        FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction();
        /** Getting the existing detailed fragment object, if it already exists.
        *  The fragment object is retrieved by its tag name
        * */
        Fragment prevFrag = getFragmentManager().findFragmentById(detailFragmentID);

        /** Remove the existing detailed fragment object if it exists */
        if (prevFrag != null) {
            fragmentTransaction.remove(prevFrag);
           MyDetailFragment mydetailFragment = new MyDetailFragment();
            fragmentTransaction.replace(detailFragmentID, mydetailFragment);
            fragmentTransaction.addToBackStack(null);
            fragmentTransaction.show(getFragmentManager().findFragmentById(detailFragmentID));
            fragmentTransaction.commit();
        }
}

MyDetailFragment

public class MyDetailFragment extends Fragment{

@Override
public void onCreate(Bundle savedInstanceState) {
 setRetainInstance(true);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
  if (savedInstanceState != null) {
    // it is not entering the inside here 
}

@Override
    public void onSaveInstanceState(Bundle savedInstanceState) {
    super.onSaveInstanceState(savedInstanceState);
        // saving some values
    }

When i title my device after me setting the setRetainInstance(true); the savedInstanceState is always null , so how can i get my saved values here ?

Why so? What am i doing wrong here and how to fix this ?

Answer 1

I think that you loose your instanceState because you alway create a new Fragment instance in your onRestart() method.

Try it this way:

@Override
protected void onRestart() {
    Fragment prevFrag = getSupportFragmentManager().findFragmentById(detailFragmentID);
    if (prevFrag == null || !(prevFrag instanceof MyDetailFragment)) {
        getSupportFragmentManager().beginTransaction().replace(detailFragmentID, new MyDetailFragment());
    }
}

This way you only attach a new instance of your Fragment only if there's no another valid Fragment of the same type.

Note that setRetainInstance(true); prevents the FragmentManager to destroy your Fragment instance when a configuration change happens. So it has no sense to manually destroy your Fragment (by calling .remove(...)) and then init a new one with .replace(..., new MyDetailFragment()). This is why you always get an empty savedInstanceState: you are in a new instance, so no previous saved states!

Also remember that calling .commitAllowingStateLoss() on a FragmentTransaction allows the Fragment Manager it to avoid saving the savedInstanceState, so you should use it only if you really know what you're doing.

Have a nice day! :)

Answer 2

According to Android:

onSaveInstanceState() will be called by default for a view if and only it has an id.

The default implementation takes care of most of the UI per-instance state for you by calling onSaveInstanceState() on each view in the hierarchy that has an id, and by saving the id of the currently focused view (all of which is restored by the default implementation of onRestoreInstanceState(Bundle)).