How to correctly save instance state of Fragments in back stack

Question

I have found many instances of a similar question on SO but no answer unfortunately meets my requirements   I have different layouts for portrait and landscape and I am using back stack  which both prevents me from using setRetainState   and tricks using configuration change routines   I show certain information to the user in TextViews  which do not get saved in the default handler  When writing my application solely using Activities  the following worked well   TextView vstup    Override public void onCreate Bundle savedInstanceState        super onCreate savedInstanceState       setContentView R layout whatever       vstup    TextView findViewById R id whatever                       Override public void onSaveInstanceState Bundle state        super onSaveInstanceState state       state putCharSequence App VSTUP  vstup getText         Override public void onRestoreInstanceState Bundle state        super onRestoreInstanceState state       vstup setText state getCharSequence App VSTUP        With Fragments  this works only in very specific situations  Specifically  what breaks horribly is replacing a fragment  putting it in the back stack and then rotating the screen while the new fragment is shown  From what I understood  the old fragment does not receive a call to onSaveInstanceState   when being replaced but stays somehow linked to the Activity and this method is called later when its View does not exist anymore  so looking for any of my TextViews results into a NullPointerException   Also  I found that keeping the reference to my TextViews is not a good idea with Fragments  even if it was OK with Activity s  In that case  onSaveInstanceState   actually saves the state but the problem reappears if I rotate the screen twice when the fragment is hidden  as its onCreateView   does not get called in the new instance   I thought of saving the state in onDestroyView   into some Bundle-type class member element  it s actually more data  not just one TextView  and saving that in onSaveInstanceState   but there are other drawbacks  Primarily  if the fragment is currently shown  the order of calling the two functions is reversed  so I d need to account for two different situations  There must be a cleaner and correct solution

User · Answer

I just want to give the solution that I came up with that handles all cases presented in this post that I derived from Vasek and devconsole. This solution also handles the special case when the phone is rotated more than once while fragments aren't visible.

Here is were I store the bundle for later use since onCreate and onSaveInstanceState are the only calls that are made when the fragment isn't visible

MyObject myObject;
private Bundle savedState = null;
private boolean createdStateInDestroyView;
private static final String SAVED_BUNDLE_TAG = "saved_bundle";

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (savedInstanceState != null) {
        savedState = savedInstanceState.getBundle(SAVED_BUNDLE_TAG);
    }
}

Since destroyView isn't called in the special rotation situation we can be certain that if it creates the state we should use it.

@Override
public void onDestroyView() {
    super.onDestroyView();
    savedState = saveState();
    createdStateInDestroyView = true;
    myObject = null;
}

This part would be the same.

private Bundle saveState() { 
    Bundle state = new Bundle();
    state.putSerializable(SAVED_BUNDLE_TAG, myObject);
    return state;
}

Now here is the tricky part. In my onActivityCreated method I instantiate the "myObject" variable but the rotation happens onActivity and onCreateView don't get called. Therefor, myObject will be null in this situation when the orientation rotates more than once. I get around this by reusing the same bundle that was saved in onCreate as the out going bundle.

    @Override
public void onSaveInstanceState(Bundle outState) {

    if (myObject == null) {
        outState.putBundle(SAVED_BUNDLE_TAG, savedState);
    } else {
        outState.putBundle(SAVED_BUNDLE_TAG, createdStateInDestroyView ? savedState : saveState());
    }
    createdStateInDestroyView = false;
    super.onSaveInstanceState(outState);
}

Now wherever you want to restore the state just use the savedState bundle

  @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    ...
    if(savedState != null) {
        myObject = (MyObject) savedState.getSerializable(SAVED_BUNDLE_TAG);
    }
    ...
}

User · Answer

On the latest support library none of the solutions discussed here are necessary anymore  You can play with your Activity s fragments as you like using the FragmentTransaction  Just make sure that your fragments can be identified either with an id or tag   The fragments will be restored automatically as long as you don t try to recreate them on every call to onCreate    Instead  you should check if savedInstanceState is not null and find the old references to the created fragments in this case   Here is an example    Override protected void onCreate Bundle savedInstanceState        super onCreate savedInstanceState        if  savedInstanceState    null            myFragment   MyFragment newInstance            getSupportFragmentManager                    beginTransaction                    add R id my container  myFragment  MY FRAGMENT TAG                   commit          else           myFragment    MyFragment  getSupportFragmentManager                    findFragmentByTag MY FRAGMENT TAG                 Note however that there is currently a bug when restoring the hidden state of a fragment  If you are hiding fragments in your activity  you will need to restore this state manually in this case

User · Answer

This is the way I am using at this moment    it s very complicated but at least it handles all the possible situations  In case anyone is interested   public final class MyFragment extends Fragment       private TextView vstup      private Bundle savedState   null        Override     public View onCreateView LayoutInflater inflater  ViewGroup container  Bundle savedInstanceState            View v   inflater inflate R layout whatever  null           vstup    TextView v findViewById R id whatever                                    If the Fragment was destroyed inbetween  screen rotation   we need to recover the savedState first               However  if it was not  it stays in the instance from the last onDestroyView   and we don t want to overwrite it            if savedInstanceState    null  amp  amp  savedState    null                savedState   savedInstanceState getBundle App STAV                     if savedState    null                vstup setText savedState getCharSequence App VSTUP                      savedState   null           return v              Override     public void onDestroyView             super onDestroyView            savedState   saveState       vstup defined here for sure            vstup   null             private Bundle saveState        called either from onDestroyView   or onSaveInstanceState              Bundle state   new Bundle            state putCharSequence App VSTUP  vstup getText             return state              Override     public void onSaveInstanceState Bundle outState            super onSaveInstanceState outState              If onDestroyView   is called first  we can use the previously savedState but we can t call saveState   anymore               If onSaveInstanceState   is called first  we don t have savedState  so we need to call saveState                   gt       operator inevitable             outState putBundle App STAV   savedState    null    savedState   saveState                                 Alternatively  it is always a possibility to keep the data displayed in passive Views in variables and using the Views only for displaying them  keeping the two things in sync  I don t consider the last part very clean  though

User · Answer

Thanks to DroidT  I made this   I realize that if the Fragment does not execute onCreateView    its view is not instantiated  So  if the fragment on back stack did not create its views  I save the last stored state  otherwise I build my own bundle with the data I want to save restore   1  Extend this class   import android os Bundle  import android support v4 app Fragment   public abstract class StatefulFragment extends Fragment        private Bundle savedState      private boolean saved      private static final String  FRAGMENT STATE    FRAGMENT STATE         Override     public void onSaveInstanceState Bundle state            if  getView      null                state putBundle  FRAGMENT STATE  savedState             else               Bundle bundle   saved   savedState   getStateToSave                 state putBundle  FRAGMENT STATE  bundle                      saved   false           super onSaveInstanceState state               Override     public void onCreate Bundle state            super onCreate state            if  state    null                savedState   state getBundle  FRAGMENT STATE                         Override     public void onDestroyView             savedState   getStateToSave            saved   true           super onDestroyView               protected Bundle getSavedState             return savedState             protected abstract boolean hasSavedState         protected abstract Bundle getStateToSave         2  In your Fragment  you must have this    Override protected boolean hasSavedState         Bundle state   getSavedState         if  state    null            return false               restore your data here      return true      3  For example  you can call hasSavedState in onActivityCreated    Override public void onActivityCreated Bundle state        super onActivityCreated state        if  hasSavedState              return               your code here

User · Answer

To correctly save the instance state of Fragment you should do the following   1  In the fragment  save instance state by overriding onSaveInstanceState   and restore in onActivityCreated     class MyFragment extends Fragment         Override     public void onActivityCreated Bundle savedInstanceState            super onActivityCreated savedInstanceState                       if  savedInstanceState    null                  Restore the fragment s state here                              Override     public void onSaveInstanceState Bundle outState            super onSaveInstanceState outState              Save the fragment s state here            2  And important point  in the activity  you have to save the fragment s instance in onSaveInstanceState   and restore in onCreate     class MyActivity extends Activity        private MyFragment       public void onCreate Bundle savedInstanceState                        if  savedInstanceState    null                  Restore the fragment s instance             mMyFragment   getSupportFragmentManager   getFragment savedInstanceState   myFragmentName                                                      Override     protected void onSaveInstanceState Bundle outState            super onSaveInstanceState outState              Save the fragment s instance         getSupportFragmentManager   putFragment outState   myFragmentName   mMyFragment              Hope this helps

User · Answer

final FragmentTransaction ft   getFragmentManager   beginTransaction    ft hide currentFragment   ft add R id content frame  newFragment newInstance context    Profile    ft addToBackStack null   ft commit

[android] How to correctly save instance state of Fragments in back stack?

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