Once for all, how to correctly save instance state of Fragments in back stack?
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 Fragment s, 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 TextView s results into a NullPointerException .
Also, I found that keeping the reference to my TextViews is not a good idea with Fragment s, 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!
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() :
@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() .
public void onCreate(Bundle savedInstanceState) {
...
if (savedInstanceState != null) {
//Restore the fragment's instance
mContent = getSupportFragmentManager().getFragment(savedInstanceState, "myFragmentName");
...
}
...
}
@Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
//Save the fragment's instance
getSupportFragmentManager().putFragment(outState, "myFragmentName", mContent);
}
Hope this helps.
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 && 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() */
/* => (?:) operator inevitable! */
outState.putBundle(App.STAV, (savedState != null) ? savedState : saveState());
}
/* (...) */
}
Alternatively , it is always a possibility to keep the data displayed in passive View s in variables and using the View s only for displaying them, keeping the two things in sync. I don't consider the last part very clean, though.
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.
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