Android Saving and Restoring Fragment State


Example

Fragments also have a onSaveInstanceState() method which is called when their state needs to be saved:

public class MySimpleFragment extends Fragment {
    private int someStateValue;
    private final String SOME_VALUE_KEY = "someValueToSave";
   
    // Fires when a configuration change occurs and fragment needs to save state
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        outState.putInt(SOME_VALUE_KEY, someStateValue);
        super.onSaveInstanceState(outState);
    }
}

Then we can pull data out of this saved state in onCreateView:

public class MySimpleFragment extends Fragment {
   // ...

   // Inflate the view for the fragment based on layout XML
   @Override
   public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.my_simple_fragment, container, false);
        if (savedInstanceState != null) {
            someStateValue = savedInstanceState.getInt(SOME_VALUE_KEY);
            // Do something with value if needed
        }
        return view;
   }
}

For the fragment state to be saved properly, we need to be sure that we aren't unnecessarily recreating the fragment on configuration changes. This means being careful not to reinitialize existing fragments when they already exist. Any fragments being initialized in an Activity need to be looked up by tag after a configuration change:

public class ParentActivity extends AppCompatActivity {
    private MySimpleFragment fragmentSimple;
    private final String SIMPLE_FRAGMENT_TAG = "myfragmenttag";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        if (savedInstanceState != null) { // saved instance state, fragment may exist
           // look up the instance that already exists by tag
           fragmentSimple = (MySimpleFragment)  
              getSupportFragmentManager().findFragmentByTag(SIMPLE_FRAGMENT_TAG);
        } else if (fragmentSimple == null) { 
           // only create fragment if they haven't been instantiated already
           fragmentSimple = new MySimpleFragment();
        }
    }
}

This requires us to be careful to include a tag for lookup whenever putting a fragment into the activity within a transaction:

public class ParentActivity extends AppCompatActivity {
    private MySimpleFragment fragmentSimple;
    private final String SIMPLE_FRAGMENT_TAG = "myfragmenttag";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // ... fragment lookup or instantation from above...
        // Always add a tag to a fragment being inserted into container
        if (!fragmentSimple.isInLayout()) {
            getSupportFragmentManager()
                .beginTransaction()
                .replace(R.id.container, fragmentSimple, SIMPLE_FRAGMENT_TAG)
                .commit();
        }
    }
}

With this simple pattern, we can properly re-use fragments and restore their state across configuration changes.