How can I maintain fragment state when added to the back stack

Question

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  lt Tap Button on FragmentA gt  07-05 14 29 12 842 D OMG       1260   FooActivity nextFragment 07-05 14 29 12 852 D OMG       1260   FragmentB onCreateView  lt Tap  Back  gt  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt String gt  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

User · Answer

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

User · Answer

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

User · Answer

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  amp  amp  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

User · Answer

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

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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

User · Answer

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

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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    gt  1             fm popBackStack          else             finish               Log e  popping BACKSTRACK    gt       fm getBackStackEntryCount

[android] How can I maintain fragment state when added to the back stack?

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