Develop Reference

Android-Save state of Fragment with RecyclerView [duplicate]

I've written up a dummy activity that switches between two fragments. When you go from FragmentA to FragmentB, FragmentA gets added to the back stack. However, when I return to FragmentA (by pressing back), a totally new FragmentA is created and the state it was in is lost. I get the feeling I'm after the same thing as this question, but I've included a complete code sample to help root out the issue:
public class FooActivity extends Activity {
#Override public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final FragmentTransaction transaction = getFragmentManager().beginTransaction();
transaction.replace(android.R.id.content, new FragmentA());
transaction.commit();
}
public void nextFragment() {
final FragmentTransaction transaction = getFragmentManager().beginTransaction();
transaction.replace(android.R.id.content, new FragmentB());
transaction.addToBackStack(null);
transaction.commit();
}
public static class FragmentA extends Fragment {
#Override public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final View main = inflater.inflate(R.layout.main, container, false);
main.findViewById(R.id.next_fragment_button).setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
((FooActivity) getActivity()).nextFragment();
}
});
return main;
}
#Override public void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
// Save some state!
}
}
public static class FragmentB extends Fragment {
#Override public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
return inflater.inflate(R.layout.b, container, false);
}
}
}
With some log messages added:
07-05 14:28:59.722 D/OMG ( 1260): FooActivity.onCreate
07-05 14:28:59.742 D/OMG ( 1260): FragmentA.onCreateView
07-05 14:28:59.742 D/OMG ( 1260): FooActivity.onResume
<Tap Button on FragmentA>
07-05 14:29:12.842 D/OMG ( 1260): FooActivity.nextFragment
07-05 14:29:12.852 D/OMG ( 1260): FragmentB.onCreateView
<Tap 'Back'>
07-05 14:29:16.792 D/OMG ( 1260): FragmentA.onCreateView
It's never calling FragmentA.onSaveInstanceState and it creates a new FragmentA when you hit back. However, if I'm on FragmentA and I lock the screen, FragmentA.onSaveInstanceState does get called. So weird...am I wrong in expecting a fragment added to the back stack to not need re-creation? Here's what the docs say:
Whereas, if you do call addToBackStack() when removing a fragment,
then the fragment is stopped and will be resumed if the user navigates
back.

If you return to a fragment from the back stack it does not re-create the fragment but re-uses the same instance and starts with onCreateView() in the fragment lifecycle, see Fragment lifecycle.
So if you want to store state you should use instance variables and not rely on onSaveInstanceState().

Comparing to Apple's UINavigationController and UIViewController, Google does not do well in Android software architecture. And Android's document about Fragment does not help much.
When you enter FragmentB from FragmentA, the existing FragmentA instance is not destroyed. When you press Back in FragmentB and return to FragmentA, we don't create a new FragmentA instance. The existing FragmentA instance's onCreateView() will be called.
The key thing is we should not inflate view again in FragmentA's onCreateView(), because we are using the existing FragmentA's instance. We need to save and reuse the rootView.
The following code works well. It does not only keep fragment state, but also reduces the RAM and CPU load (because we only inflate layout if necessary). I can't believe Google's sample code and document never mention it but always inflate layout.
Version 1(Don't use version 1. Use version 2)
public class FragmentA extends Fragment {
View _rootView;
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
if (_rootView == null) {
// Inflate the layout for this fragment
_rootView = inflater.inflate(R.layout.fragment_a, container, false);
// Find and setup subviews
_listView = (ListView)_rootView.findViewById(R.id.listView);
...
} else {
// Do not inflate the layout again.
// The returned View of onCreateView will be added into the fragment.
// However it is not allowed to be added twice even if the parent is same.
// So we must remove _rootView from the existing parent view group
// (it will be added back).
((ViewGroup)_rootView.getParent()).removeView(_rootView);
}
return _rootView;
}
}
------Update on May 3 2005:-------
As the comments mentioned, sometimes _rootView.getParent() is null in onCreateView, which causes the crash. Version 2 removes _rootView in onDestroyView(), as dell116 suggested. Tested on Android 4.0.3, 4.4.4, 5.1.0.
Version 2
public class FragmentA extends Fragment {
View _rootView;
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
if (_rootView == null) {
// Inflate the layout for this fragment
_rootView = inflater.inflate(R.layout.fragment_a, container, false);
// Find and setup subviews
_listView = (ListView)_rootView.findViewById(R.id.listView);
...
} else {
// Do not inflate the layout again.
// The returned View of onCreateView will be added into the fragment.
// However it is not allowed to be added twice even if the parent is same.
// So we must remove _rootView from the existing parent view group
// in onDestroyView() (it will be added back).
}
return _rootView;
}
#Override
public void onDestroyView() {
if (_rootView.getParent() != null) {
((ViewGroup)_rootView.getParent()).removeView(_rootView);
}
super.onDestroyView();
}
}
WARNING!!!
This is a HACK! Though I am using it in my app, you need to test and read comments carefully.

I guess there is an alternative way to achieve what you are looking for.
I don't say its a complete solution but it served the purpose in my case.
What I did is instead of replacing the fragment I just added target fragment.
So basically you will be going to use add() method instead replace().
What else I did.
I hide my current fragment and also add it to backstack.
Hence it overlaps new fragment over the current fragment without destroying its view.(check that its onDestroyView() method is not being called. Plus adding it to backstate gives me the advantage of resuming the fragment.
Here is the code :
Fragment fragment=new DestinationFragment();
FragmentManager fragmentManager = getFragmentManager();
android.app.FragmentTransaction ft=fragmentManager.beginTransaction();
ft.add(R.id.content_frame, fragment);
ft.hide(SourceFragment.this);
ft.addToBackStack(SourceFragment.class.getName());
ft.commit();
AFAIK System only calls onCreateView() if the view is destroyed or not created.
But here we have saved the view by not removing it from memory. So it will not create a new view.
And when you get back from Destination Fragment it will pop the last FragmentTransaction removing top fragment which will make the topmost(SourceFragment's) view to appear over the screen.
COMMENT: As I said it is not a complete solution as it doesn't remove the view of Source fragment and hence occupying more memory than usual. But still, serve the purpose. Also, we are using a totally different mechanism of hiding view instead of replacing it which is non traditional.
So it's not really for how you maintain the state, but for how you maintain the view.

I would suggest a very simple solution.
Take the View reference variable and set view in OnCreateView. Check if view already exists in this variable, then return same view.
private View fragmentView;
public View onCreateView(LayoutInflater inflater, #Nullable ViewGroup container, #Nullable Bundle savedInstanceState) {
super.onCreateView(inflater, container, savedInstanceState);
if (fragmentView != null) {
return fragmentView;
}
View view = inflater.inflate(R.layout.yourfragment, container, false);
fragmentView = view;
return view;
}

I came across this problem in a Fragment containing a map, which has too many setup details to save/reload.
My solution was to basically keep this Fragment active the whole time (similar to what #kaushal mentioned).
Say you have current Fragment A and wants to display Fragment B.
Summarizing the consequences:
replace() - remove Fragment A and replace it with Fragment B. Fragment A will be recreated once brought to the front again
add() - (create and) add a Fragment B and it overlap Fragment A, which is still active in the background
remove() - can be used to remove Fragment B and return to A. Fragment B will be recreated when called later on
Hence, if you want to keep both Fragments "saved", just toggle them using hide()/show().
Pros: easy and simple method to keep multiple Fragments running
Cons: you use a lot more memory to keep all of them running. May run into problems, e.g. displaying many large bitmaps

onSaveInstanceState() is only called if there is configuration change.
Since changing from one fragment to another there is no configuration change so no call to onSaveInstanceState() is there. What state is not being save? Can you specify?
If you enter some text in EditText it will be saved automatically. Any UI item without any ID is the item whose view state shall not be saved.

first: just use add method instead of replace method of FragmentTransaction class then you have to add secondFragment to stack by addToBackStack method
second :on back click you have to call popBackStackImmediate()
Fragment sourceFragment = new SourceFragment ();
final Fragment secondFragment = new SecondFragment();
final FragmentTransaction ft = getChildFragmentManager().beginTransaction();
ft.add(R.id.child_fragment_container, secondFragment );
ft.hide(sourceFragment );
ft.addToBackStack(NewsShow.class.getName());
ft.commit();
((SecondFragment)secondFragment).backFragmentInstanceClick = new SecondFragment.backFragmentNewsResult()
{
#Override
public void backFragmentNewsResult()
{
getChildFragmentManager().popBackStackImmediate();
}
};

Kotlin and ViewBinding Solution
I am using replace() and backstack() method for FragmentTransaction. The problem is that the backstack() method calls the onCreateView of the Previous Fragment which causes in re-built of Fragment UI. Here is a solution for that:
private lateinit var binding: FragmentAdRelevantDetailsBinding
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,savedInstanceState: Bundle?
): View {
if (!this::binding.isInitialized)
binding = FragmentAdRelevantDetailsBinding.inflate(layoutInflater, container, false)
return binding.root
}

Here, since onSaveInstanceState in fragment does not call when you add fragment into backstack. The fragment lifecycle in backstack when restored start onCreateView and end onDestroyView while onSaveInstanceState is called between onDestroyView and onDestroy. My solution is create instance variable and init in onCreate. Sample code:
private boolean isDataLoading = true;
private ArrayList<String> listData;
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
isDataLoading = false;
// init list at once when create fragment
listData = new ArrayList();
}
And check it in onActivityCreated:
public void onViewCreated(View view, #Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
if(isDataLoading){
fetchData();
}else{
//get saved instance variable listData()
}
}
private void fetchData(){
// do fetch data into listData
}

getSupportFragmentManager().addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener()
{
#Override
public void onBackStackChanged()
{
if (getSupportFragmentManager().getBackStackEntryCount() == 0)
{
//setToolbarTitle("Main Activity");
}
else
{
Log.e("fragment_replace11111", "replace");
}
}
});
YourActivity.java
#Override
public void onBackPressed()
{
Fragment fragment = getSupportFragmentManager().findFragmentById(R.id.Fragment_content);
if (fragment instanceof YourFragmentName)
{
fragmentReplace(new HomeFragment(),"Home Fragment");
txt_toolbar_title.setText("Your Fragment");
}
else{
super.onBackPressed();
}
}
public void fragmentReplace(Fragment fragment, String fragment_name)
{
try
{
fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.replace(R.id.Fragment_content, fragment, fragment_name);
fragmentTransaction.setCustomAnimations(R.anim.enter_from_right, R.anim.exit_to_left, R.anim.enter_from_left, R.anim.exit_to_right);
fragmentTransaction.addToBackStack(fragment_name);
fragmentTransaction.commitAllowingStateLoss();
}
catch (Exception e)
{
e.printStackTrace();
}
}

My problem was similar but I overcame me without keeping the fragment alive. Suppose you have an activity that has 2 fragments - F1 and F2. F1 is started initially and lets say in contains some user info and then upon some condition F2 pops on asking user to fill in additional attribute - their phone number. Next, you want that phone number to pop back to F1 and complete signup but you realize all previous user info is lost and you don't have their previous data. The fragment is recreated from scratch and even if you saved this information in onSaveInstanceState the bundle comes back null in onActivityCreated.
Solution:
Save required information as an instance variable in calling activity. Then pass that instance variable into your fragment.
#Override
public void onActivityCreated(#Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
Bundle args = getArguments();
// this will be null the first time F1 is created.
// it will be populated once you replace fragment and provide bundle data
if (args != null) {
if (args.get("your_info") != null) {
// do what you want with restored information
}
}
}
So following on with my example: before I display F2 I save user data in the instance variable using a callback. Then I start F2, user fills in phone number and presses save. I use another callback in activity, collect this information and replace my fragment F1, this time it has bundle data that I can use.
#Override
public void onPhoneAdded(String phone) {
//replace fragment
F1 f1 = new F1 ();
Bundle args = new Bundle();
yourInfo.setPhone(phone);
args.putSerializable("you_info", yourInfo);
f1.setArguments(args);
getFragmentManager().beginTransaction()
.replace(R.id.fragmentContainer, f1).addToBackStack(null).commit();
}
}
More information about callbacks can be found here: https://developer.android.com/training/basics/fragments/communicating.html

Replace a Fragment using following code:
Fragment fragment = new AddPaymentFragment();
getSupportFragmentManager().beginTransaction().replace(R.id.frame, fragment, "Tag_AddPayment")
.addToBackStack("Tag_AddPayment")
.commit();
Activity's onBackPressed() is :
#Override
public void onBackPressed() {
android.support.v4.app.FragmentManager fm = getSupportFragmentManager();
if (fm.getBackStackEntryCount() > 1) {
fm.popBackStack();
} else {
finish();
}
Log.e("popping BACKSTRACK===> ",""+fm.getBackStackEntryCount());
}

Public void replaceFragment(Fragment mFragment, int id, String tag, boolean addToStack) {
FragmentTransaction mTransaction = getSupportFragmentManager().beginTransaction();
mTransaction.replace(id, mFragment);
hideKeyboard();
if (addToStack) {
mTransaction.addToBackStack(tag);
}
mTransaction.commitAllowingStateLoss();
}
replaceFragment(new Splash_Fragment(), R.id.container, null, false);

Perfect solution that find old fragment in stack and load it if exist in stack.
/**
* replace or add fragment to the container
*
* #param fragment pass android.support.v4.app.Fragment
* #param bundle pass your extra bundle if any
* #param popBackStack if true it will clear back stack
* #param findInStack if true it will load old fragment if found
*/
public void replaceFragment(Fragment fragment, #Nullable Bundle bundle, boolean popBackStack, boolean findInStack) {
FragmentManager fm = getSupportFragmentManager();
FragmentTransaction ft = fm.beginTransaction();
String tag = fragment.getClass().getName();
Fragment parentFragment;
if (findInStack && fm.findFragmentByTag(tag) != null) {
parentFragment = fm.findFragmentByTag(tag);
} else {
parentFragment = fragment;
}
// if user passes the #bundle in not null, then can be added to the fragment
if (bundle != null)
parentFragment.setArguments(bundle);
else parentFragment.setArguments(null);
// this is for the very first fragment not to be added into the back stack.
if (popBackStack) {
fm.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
} else {
ft.addToBackStack(parentFragment.getClass().getName() + "");
}
ft.replace(R.id.contenedor_principal, parentFragment, tag);
ft.commit();
fm.executePendingTransactions();
}
use it like
Fragment f = new YourFragment();
replaceFragment(f, null, boolean true, true);

Calling the Fragment lifecycle methods properly and using onSavedInstanceState() can solve the problem.
i.e Call onCreate(), onCreateView(), onViewCreated() and onSavedInstanceState() properly and save Bundle in onSaveInstanceState() and resotre it in onCreate() method.
I don't know how but it worked for me without any error.
If anyone can explain it will very much appreciated.
public class DiagnosisFragment extends Fragment {
private static final String TITLE = "TITLE";
private String mTitle;
private List mList = null;
private ListAdapter adapter;
public DiagnosisFragment(){}
public DiagnosisFragment(List list, String title){
mList = list;
mTitle = title;
}
#Override
public void onCreate(#Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if(savedInstanceState != null){
mList = savedInstanceState.getParcelableArrayList(HEALTH_ITEMS);
mTitle = savedInstanceState.getString(TITLE);
itemId = savedInstanceState.getInt(ID);
mChoiceMode = savedInstanceState.getInt(CHOICE_MODE);
}
getActivity().setTitle(mTitle);
adapter = (ListAdapter) new HealthAdapter(mList, getContext()).load(itemId);
}
#Nullable
#Override
public View onCreateView(#NonNull LayoutInflater inflater, #Nullable ViewGroup container, #Nullable Bundle savedInstanceState) {
return inflater.inflate(R.layout.diagnosis_fragment, container, false);
}
#Override
public void onViewCreated(#NonNull View view, #Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
ListView lv = view.findViewById(R.id.subLocationsSymptomsList);
lv.setAdapter(adapter);
}
#Override
public void onSaveInstanceState(#NonNull Bundle outState) {
outState.putParcelableArrayList(HEALTH_ITEMS, (ArrayList) mList);
outState.putString(TITLE, mTitle);
}
}

For who has looking for solution :
#Override
public void onDestroyView() {
Bundle savedState=new Bundle();
// put your data in bundle
// if you have object and want to restore you can use gson to convert it
//to sring
if (yourObject!=null){
savedState.putString("your_object_key",new Gson().toJson(yourObject));
}
if (getArguments()==null){
setArguments(new Bundle());
}
getArguments().putBundle("saved_state",savedState);
super.onDestroyView();
}
and in onViewCreated() method :
Bundle savedState=null;
if (getArguments()!=null){
savedState=getArguments().getBundle("saved_state");
}
if (savedState!=null){
// set your restored data to your view
}

How to reuse fragment's view in Android?

I am creating a Fragment Activity with tabs. Now whenever i select a tab, corresponding fragment gets recreated and its
onCreateView()
method is called.
But what i want is to reuse the view of the fragment so that whenever a tab is selected system does not call
onCreateView()
of fragment instead it shows the previous view of the fragment if it exists.
Please reply soon.

You may want to show and hide the fragments instead of adding and removing them when selecting a tab.
For example:
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
if (frag.isAdded()) {
transaction.show(R.id.layout, frag);
} else {
transaction.add(R.id.layout, frag);
}
transaction.commit();

setOffScreenPageLimit() method can be used to set how many fragments you want to keep alive even if they are not visible to the user.

Try this, it works for screen rotation and should work for your situation as well:
1) When you add fragment for the first time, create it with name parameter:
getSupportFragmentManager()
.beginTransaction()
.replace(<containerID>, <fragment>, YOUR_FRAGMENT_NAME_CONST)
.commit();
2) When you need to find it:
YourFragmentType fragment;
fragment = (YourFragmentType) getSupportFragmentManager()
.findFragmentByTag(YOUR_FRAGMENT_NAME_CONST);
if (fragment != null){
//TODO set data to the existing fragment
}
else{
//TODO create and initialize your fragment
}
Edit:
You should differentiate Fragment object creation and onCreateView() being called.
It's right that you should avoid unnecessary object creation and so reuse fragments.On the other hand (as for my experience) it's better to adhere Android's onCreateView() politics to guarantee consistent user experience.
If you really want to save CPU time and avoid re-inflating complicated view (and settle all issues yourselves) - you may just check it is null like this:
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
// View
if (mView == null) {
mView = inflater.inflate(R.layout.your_fragment, container, false);
...
}
...
}

Android FragmentTab host and Fragments inside Fragments

I have an app with hierarchy like this:
FragmentTabHost (Main Activity)
- Fragment (tab 1 content - splitter view)
- Fragment (lhs, list)
- Framment (rhs, content view)
- Fragment (tab 2 content)
- Fragment (tab 2 content)
All fragment views are being inflated from resources.
When the app starts everything appears and looks fine. When I switch from the first tab to another tab and back again I get inflate exceptions trying to recreate tab 1's views.
Digging a little deeper, this is what's happening:
On the first load, inflating the splitter view causes its two child fragments to be added to the fragment manager.
On switching away from the first tab, it's view is destroyed but it's child fragments are left in the fragment manager
On switching back to the first tab, the view is re-inflated and since the old child fragments are still in the fragment manager an exception is thrown when the new child fragments are instantiated (by inflation)
I've worked around this by removing the child fragments from the fragment manager (I'm using Mono) and now I can switch tabs without the exception.
public override void OnDestroyView()
{
var ft = FragmentManager.BeginTransaction();
ft.Remove(FragmentManager.FindFragmentById(Resource.Id.ListFragment));
ft.Remove(FragmentManager.FindFragmentById(Resource.Id.ContentFragment));
ft.Commit();
base.OnDestroyView();
}
So I have a few questions:
Is the above the correct way to do this?
If not, how should I be doing it?
Either way, how does saving instance state tie into all of this so that I don't lose view state when switching tabs?

I'm not sure how to do this in Mono, but to add child fragments to another fragment, you can't use the FragmentManager of the Activity. Instead, you have to use the ChildFragmentManager of the hosting Fragment:
http://developer.android.com/reference/android/app/Fragment.html#getChildFragmentManager()
http://developer.android.com/reference/android/support/v4/app/Fragment.html#getChildFragmentManager()
The main FragmentManager of the Activity handles your tabs.
The ChildFragmentManager of tab1 handles the split views.

OK, I finally figured this out:
As suggested above, first I changed the fragment creation to be done programatically and had them added to the child fragment manager, like so:
public override View OnCreateView(LayoutInflater inflater, ViewGroup viewGroup, Bundle savedInstance)
{
var view = inflater.Inflate(Resource.Layout.MyView, viewGroup, false);
// Add fragments to the child fragment manager
// DONT DO THIS, SEE BELOW
var tx = ChildFragmentManager.BeginTransaction();
tx.Add(Resource.Id.lhs_fragment_frame, new LhsFragment());
tx.Add(Resource.Id.rhs_fragment_frame, new RhsFragment());
tx.Commit();
return view;
}
As expected, each time I switch tabs, an extra instance of Lhs/RhsFragment would be created, but I noticed that the old Lhs/RhsFragment's OnCreateView would also get called. So after each tab switch, there would be one more call to OnCreateView. Switch tabs 10 times = 11 calls to OnCreateView. This is obviously wrong.
Looking at the source code for FragmentTabHost, I can see that it simply detaches and re-attaches the tab's content fragment when switching tabs. It seems the parent Fragment's ChildFragmentManager is keeping the child fragments around and automatically recreating their views when the parent fragment is re-attached.
So, I moved the creation of fragments to OnCreate, and only if we're not loading from saved state:
public override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
if (savedInstanceState == null)
{
var tx = ChildFragmentManager.BeginTransaction();
tx.Add(Resource.Id.lhs_fragment_frame, new LhsFragment());
tx.Add(Resource.Id.rhs_fragment_frame, new RhsFragment());
tx.Commit();
}
}
public override View OnCreateView(LayoutInflater inflater, ViewGroup viewGroup, Bundle savedInstance)
{
// Don't instatiate child fragments here
return inflater.Inflate(Resource.Layout.MyView, viewGroup, false);
}
This fixed the creation of the additional views and switching tab's basically worked now.
The next question was saving and restoring view state. In the child fragments I need to save and restore the currently selected item. Originally I had something like this (this is the child fragment's OnCreateView)
public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstance)
{
var view = inflater.Inflate(Resource.Layout.CentresList, container, false);
// ... other code ommitted ...
// DONT DO THIS, SEE BELOW
if (savedInstance != null)
{
// Restore selection
_selection = savedInstance.GetString(KEY_SELECTION);
}
else
{
// Select first item
_selection =_items[0];
}
return view;
}
The problem with this is that the tab host doesn't call OnSaveInstanceState when switching tabs. Rather the child fragment is kept alive and it's _selection variable can be just left alone.
So I moved the code to manage selection to OnCreate:
public override void OnCreate(Bundle savedInstance)
{
base.OnCreate(savedInstance);
if (savedInstance != null)
{
// Restore Selection
_selection = savedInstance.GetString(BK_SELECTION);
}
else
{
// Select first item
_selection = _items[0];
}
}
public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstance)
{
// Don't restore/init _selection here
return inflater.Inflate(Resource.Layout.CentresList, container, false);
}
Now it all seems to be working perfectly, both when switching tabs and changing orientation.

Detaching a Fragment not triggering onSaveInstanceState()

My Android application has an ActionBar that changes which Fragment occupies a certain FrameLayout. I am trying to use onSaveInstanceState to save the state of a Fragment when the tab is changed, so that it can be recovered in onCreateView.
The problem is, onSaveInstanceState is never called. The Fragment's onDestroyView and onCreateView methods are called, but the Bundle supplied to onCreateView remains null.
Can someone please explain to me when onSaveInstanceState is actually called, how I can make sure it gets called when switching tabs, or the best practice for saving and restoring the state of a Fragment when it is detached and re-attached?
Fragment:
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.event_log, container, false);
// Retrieve saved state
if (savedInstanceState != null){
System.out.println("log retrieved");
} else {
System.out.println("log null");
}
return view;
}
#Override
public void onSaveInstanceState(Bundle outState) {
System.out.println("log saved");
super.onSaveInstanceState(outState);
// more code
}
Activity:
/**
* Detach the current Fragment, because another one is being attached.
*/
#Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
if (tab.getText().equals(getString(R.string.tab_events))){
if (frEventLog != null) {
ft.detach(frEventLog);
}
}

Fragment#onSaveInstanceState is only called when the Activity hosting the Fragment is destroyed AND there is a chance that you can come back to the same activity AND the fragment is still added to the FragmentManager. The most common case would be screen rotation.
I think your Fragment will also need to do setRetainInstance(true) in onCreate for example. Not exactly sure about that point though.
You should also see this method being called when you press the home button for example. That will destroy the activity but you can go back to it by using the task list for example.
If you just detach() the fragment all you need to do to get it back is to ask the FragmentManager for it.
There are two examples you should have a look at:
ActionBar FragmentTabs and TabHost FragmentTabs
The TabHost example uses
ft.add(containerId, fragment, tag);
// later
fragment = mActivity.getSupportFragmentManager().findFragmentByTag(tag);
to find the instances of previously added Fragments, works until you remove() a Fragment
Regarding onCreateView / onDestroyView: That is called once a fragment gets detached because the next time you attach it needs to create a new View. Note that Fragment#onDetached() is not called when you detach() the fragment because it is still attached to the Activity. It is only detached from the view-hierarchy.
There is another nice example on how to retain fragment state / how to use fragments to retain state in Android Training - Caching Bitmaps.
That example is missing a critical line though:
public static RetainFragment findOrCreateRetainFragment(FragmentManager fm) {
RetainFragment fragment = (RetainFragment) fm.findFragmentByTag(TAG);
if (fragment == null) {
fragment = new RetainFragment();
fm.beginTransaction().add(fragment, TAG).commit(); // << add this
}
return fragment;
}

Fragment gets initialized twice when reloading activity with tabs when orientation changes

I have a problem reloading an activity with tabs and fragments when I change the orientation of my device.
Here's the situation:
I have an activity which has 3 tabs in the action bar. Each tab loads a different fragment in a FrameLayout in main view. Everything works fine if I don't change the orientation of the device. But when I do that Android tries to initialize the currently selected fragment twice which produce the following error:
E/AndroidRuntime(2022): Caused by: android.view.InflateException: Binary XML file line #39: Error inflating class fragment
Here's the sequence of steps that produce the error:
I load the activity, select tab nr 2. and change the orientation of the device.
Android destroys the activity and the instance of the fragment loaded by tab nr 2 (from now on, 'Fragment 2'). Then it proceeds to create new instances of the activity and the fragment.
Inside Activity.onCreate() I add the first tab to the action bar. When I do that, this tab gets automatically selected. It may represent a problem in the future, but I don't mind about that now. onTabSelected gets called and a new instance of the first fragment is created and loaded (see code below).
I add all the other tabs without any event being triggered, which is fine.
I call ActionBar.selectTab(myTab) to select Tab nr 2.
onTabUnselected() gets called for the first tab, and then onTabSelected() for the second tab. This sequence replaces the current fragment for an instance of Fragment 2 (see code below).
Next, Fragment.onCreateView() is called on Fragment 2 instance and the fragment layout gets inflated.
Here is the problem. Android Calls onCreate() and then onCreateView() on the fragment instance ONCE AGAIN, which produces the exception when I try to inflate (a second time) the layout.
Obviously the problem is Android is initializing the fragment twice, but I don't know why.
I tried NOT selecting the second tab when I reaload the activity but the second fragment gets initialized anyway and it is not shown (since I didn't select its tab).
I found this question: Android Fragments recreated on orientation change
The user asks basically the same I do, but I don't like the chosen answer (it's only a workaroud). There must be some way to get this working without the android:configChanges trick.
In case it's not clear, what I want to know how whether to prevent the recreation of the fragment or to avoid the double initialization of it. It would be nice to know why is this happening also. :P
Here is the relevant code:
public class MyActivity extends Activity implements ActionBar.TabListener {
private static final String TAG_FRAGMENT_1 = "frag1";
private static final String TAG_FRAGMENT_2 = "frag2";
private static final String TAG_FRAGMENT_3 = "frag3";
Fragment frag1;
Fragment frag2;
Fragment frag3;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// my_layout contains a FragmentLayout inside
setContentView(R.layout.my_layout);
// Get a reference to the fragments created automatically by Android
// when reloading the activity
FragmentManager fm = getFragmentManager();
this.frag1 = fm.findFragmentByTag(MyActivity.TAG_FRAGMENT_1);
this.frag2 = fm.findFragmentByTag(MyActivity.TAG_FRAGMENT_2);
this.frag3 = fm.findFragmentByTag(MyActivity.TAG_FRAGMENT_3)
ActionBar actionBar = getActionBar();
// snip...
// This triggers onTabSelected for the first tab
actionBar.addTab(actionBar.newTab()
.setText("Tab1").setTabListener(this)
.setTag(MyActivity.TAG_FRAGMENT_1));
actionBar.addTab(actionBar.newTab()
.setText("Tab2").setTabListener(this)
.setTag(MyActivity.TAG_FRAGMENT_2));
actionBar.addTab(actionBar.newTab()
.setText("Tab3").setTabListener(this)
.setTag(MyActivity.TAG_FRAGMENT_3));
Tab t = null;
// here I get a reference to the tab that must be selected
// snip...
// This triggers onTabUnselected/onTabSelected
ab.selectTab(t);
}
#Override
protected void onDestroy() {
// Not sure if this is necessary
this.frag1 = null;
this.frag2 = null;
this.frag3 = null;
super.onDestroy();
}
#Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
Fragment curFrag = getFragmentInstanceForTag(tab.getTag().toString());
if (curFrag == null) {
curFrag = createFragmentInstanceForTag(tab.getTag().toString());
if(curFrag == null) {
// snip...
return;
}
}
ft.replace(R.id.fragment_container, curFrag, tab.getTag().toString());
}
#Override
public void onTabUnselected(Tab tab, FragmentTransaction ft)
{
Fragment curFrag = getFragmentInstanceForTag(tab.getTag().toString());
if (curFrag == null) {
// snip...
return;
}
ft.remove(curFrag);
}
private Fragment getFragmentInstanceForTag(String tag)
{
// Returns this.frag1, this.frag2 or this.frag3
// depending on which tag was passed as parameter
}
private Fragment createFragmentInstanceForTag(String tag)
{
// Returns a new instance of the fragment requested by tag
// and assigns it to this.frag1, this.frag2 or this.frag3
}
}
The code for the Fragment is irrelevant, it just returns an inflated view on onCreateView() method override.

I got a simple answer for that:
Just add setRetainInstance(true); to the Fragment's onAttach(Activity activity) or onActivityCreated(Bundle savedInstanceState).
These two are call-backs in the Fragment Class.
So basically, what setRetainInstance(true) does is:
It maintains the state of your fragment as it is, when it goes through:
onPause();
onStop();
It maintains the instance of the Fragment no matter what the Activity goes through.
The problem with it could be, if there are too many Fragments, it may put a strain on the System.
Hope it helps.
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
setRetainInstance(true);
}
Open for Correction as always. Regards, Edward Quixote.

It seems that, when the screen is rotated and the app restarted, it is recreating each Fragment by calling the default constructor for the Fragment's class.

I have encountered the same issue and used the following workaround:
in the fragment's onCreateView begining of:
if (mView != null) {
// Log.w(TAG, "Fragment initialized again");
((ViewGroup) mView.getParent()).removeView(mView);
return mView;
}
// normal onCreateView
mView = inflater.inflate(R.layout...)

I think this is a fool proof way to avoid re-inflating of the root view of the fragment:
private WeakReference<View> mRootView;
private LayoutInflater mInflater;
/**
* inflate the fragment layout , or use a previous one if already stored <br/>
* WARNING: do not use in any function other than onCreateView
* */
private View inflateRootView() {
View rootView = mRootView == null ? null : mRootView.get();
if (rootView != null) {
final ViewParent parent = rootView.getParent();
if (parent != null && parent instanceof ViewGroup)
((ViewGroup) parent).removeView(rootView);
return rootView;
}
rootView = mFadingHelper.createView(mInflater);
mRootView = new WeakReference<View>(rootView);
return rootView;
}
#Override
public View onCreateView(final LayoutInflater inflater, final ViewGroup container, final Bundle savedInstanceState) {
mInflater=inflater!=null?inflater:LayoutInflater.from(getActivity());
final View view = inflateRootView();
... //update your data on the views if needed
}

add
android:configChanges="orientation|screenSize"
in the manifest file

To protect activity recreate try to add configChanges in your Activity tag (in manifest), like:
android:configChanges="keyboardHidden|orientation|screenSize"

My code was a little different, but I believe our problem is the same.
In the onTabSelected I didn't use replace, I use add when is the first time creating the fragment and attach if isn't. In the onTabUnselected I use detach.
The problem is that when the view is destroyed, my Fragment was attached to the FragmentManager and never destroyed. To solve that I implemented on the onSaveInstanceBundle to detach the fragment from the FragmentManager.
The code was something like that:
FragmentTransition ft = getSupportFragmentManager().begin();
ft.detach(myFragment);
ft.commit();
In the first try I put that code in the onDestroy, but I get a exception telling me that I couldn't do it after the onSaveInstanceBundle, so I moved the code to the onSaveInstanceBundle and everything worked.
Sorry but the place where I work don't allow me to put the code here on StackOverflow. This is what I remember from the code. Feel free to edit the answer to add the code.

I think you are facing what I faced. I had a thread downloader for json which starts in onCreate() , each time I changed the orientation the thread is called and download is fired. I fixed this using onSaveInstance() and onRestoreInstance() to pass the json response in a list, in combination of checking if the list is not empty, so the extra download is not needed.
I hope this gives you a hint.

I solved this problem by using below code.
private void loadFragment(){
LogUtil.l(TAG,"loadFragment",true);
fm = getSupportFragmentManager();
Fragment hf = fm.findFragmentByTag("HOME");
Fragment sf = fm.findFragmentByTag("SETTING");
if(hf==null) {
homeFragment = getHomeFragment();// new HomeFragment();
settingsFragment = getSettingsFragment();// new Fragment();
fm.beginTransaction().add(R.id.fm_place, settingsFragment, "SETTING").hide(settingsFragment).commit();
fm.beginTransaction().add(R.id.fm_place, homeFragment, "HOME").commit();
activeFragment = homeFragment;
}else{
homeFragment = hf;
settingsFragment = sf;
activeFragment = sf;
}
}
Initiate this method in OnCreate();