ViewPager and fragments what s the right way to store fragment s state

Question

Fragments seem to be very nice for separation of UI logic into some modules  But along with ViewPager its lifecycle is still misty to me  So Guru thoughts are badly needed    Edit  See dumb solution below  -   Scope  Main activity has a ViewPager with fragments  Those fragments could implement a little bit different logic for other  submain  activities  so the fragments  data is filled via a callback interface inside the activity  And everything works fine on first launch  but      Problem  When the activity gets recreated  e g  on orientation change  so do the ViewPager s fragments  The code  you ll find below  says that every time the activity is created I try to create a new ViewPager fragments adapter the same as fragments  maybe this is the problem  but FragmentManager already has all these fragments stored somewhere  where   and starts the recreation mechanism for those  So the recreation mechanism calls the  old  fragment s onAttach  onCreateView  etc  with my callback interface call for initiating data via the Activity s implemented method  But this method points to the newly created fragment which is created via the Activity s onCreate method   Issue  Maybe I m using wrong patterns but even Android 3 Pro book doesn t have much about it  So  please  give me one-two punch and point out how to do it the right way  Many thanks   Code  Main Activity  public class DashboardActivity extends BasePagerActivity implements OnMessageListActionListener    private MessagesFragment mMessagesFragment    Override protected void onCreate Bundle savedInstanceState        Logger d  Dash onCreate        super onCreate savedInstanceState        setContentView R layout viewpager container       new DefaultToolbar this           create fragments to use     mMessagesFragment   new MessagesFragment        mStreamsFragment   new StreamsFragment            set titles and fragments for view pager     Map lt String  Fragment gt  screens   new LinkedHashMap lt String  Fragment gt         screens put getApplicationContext   getString R string dashboard title dumb   new DumbFragment         screens put getApplicationContext   getString R string dashboard title messages   mMessagesFragment           instantiate view pager via adapter     mPager    ViewPager  findViewById R id viewpager pager       mPagerAdapter   new BasePagerAdapter screens  getSupportFragmentManager         mPager setAdapter mPagerAdapter           set title indicator     TitlePageIndicator indicator    TitlePageIndicator  findViewById R id viewpager titles       indicator setViewPager mPager  1          set of fragments callback interface implementations      Override public void onMessageInitialisation          Logger d  Dash onMessageInitialisation        if  mMessagesFragment    null          mMessagesFragment loadLastMessages        Override public void onMessageSelected Message selectedMessage         Intent intent   new Intent this  StreamActivity class       intent putExtra Message class getName    selectedMessage       startActivity intent       BasePagerActivity aka helper  public class BasePagerActivity extends FragmentActivity    BasePagerAdapter mPagerAdapter  ViewPager mPager      Adapter  public class BasePagerAdapter extends FragmentPagerAdapter implements TitleProvider    private Map lt String  Fragment gt  mScreens   public BasePagerAdapter Map lt String  Fragment gt  screenMap  FragmentManager fm         super fm       this mScreens   screenMap      Override public Fragment getItem int position         return mScreens values   toArray new Fragment mScreens size     position       Override public int getCount          return mScreens size        Override public String getTitle int position         return mScreens keySet   toArray new String mScreens size     position         hack  we don t want to destroy our fragments and re-initiate them after  Override public void destroyItem View container  int position  Object object            TODO Auto-generated method stub        Fragment  public class MessagesFragment extends ListFragment    private boolean mIsLastMessages   private List lt Message gt  mMessagesList  private MessageArrayAdapter mAdapter   private LoadMessagesTask mLoadMessagesTask  private OnMessageListActionListener mListener      define callback interface public interface OnMessageListActionListener       public void onMessageInitialisation        public void onMessageSelected Message selectedMessage       Override public void onAttach Activity activity        super onAttach activity          setting callback     mListener    OnMessageListActionListener  activity      mIsLastMessages   activity instanceof DashboardActivity       Override public View onCreateView LayoutInflater inflater  ViewGroup container  Bundle savedInstanceState        inflater inflate R layout fragment listview  container       mProgressView   inflater inflate R layout listrow progress  null       mEmptyView   inflater inflate R layout fragment nodata  null       return super onCreateView inflater  container  savedInstanceState       Override public void onActivityCreated Bundle savedInstanceState        super onActivityCreated savedInstanceState           instantiate loading task     mLoadMessagesTask   new LoadMessagesTask            instantiate list of messages     mMessagesList   new ArrayList lt Message gt         mAdapter   new MessageArrayAdapter getActivity    mMessagesList       setListAdapter mAdapter       Override public void onResume         mListener onMessageInitialisation        super onResume       public void onListItemClick ListView l  View v  int position  long id        Message selectedMessage    Message  getListAdapter   getItem position       mListener onMessageSelected selectedMessage       super onListItemClick l  v  position  id         public methods to load messages from host acitivity  etc           Solution  The dumb solution is to save the fragments inside onSaveInstanceState  of host Activity  with putFragment and get them inside onCreate via getFragment  But I still have a strange feeling that things shouldn t work like that    See code below        Override protected void onSaveInstanceState Bundle outState         super onSaveInstanceState outState       getSupportFragmentManager                putFragment outState  MessagesFragment class getName    mMessagesFragment      protected void onCreate Bundle savedInstanceState        Logger d  Dash onCreate        super onCreate savedInstanceState                   create fragments to use     if  savedInstanceState    null            mMessagesFragment    MessagesFragment  getSupportFragmentManager   getFragment                  savedInstanceState  MessagesFragment class getName                     StreamsFragment class getName               if  mMessagesFragment    null          mMessagesFragment   new MessagesFragment

User · Answer

add       SuppressLint  ValidFragment     before your class   it it doesn  t work do something like this     SuppressLint    ValidFragment    HandlerLeak

User · Answer

I found another relatively easy solution for your question   As you can see from the FragmentPagerAdapter source code  the fragments managed by FragmentPagerAdapter store in the FragmentManager under the tag generated using   String tag  android switcher     viewId         index    The viewId is the container getId    the container is your ViewPager instance  The index is the position of the fragment  Hence you can save the object id to the outState    Override protected void onSaveInstanceState Bundle outState        super onSaveInstanceState outState       outState putInt  viewpagerid    mViewPager getId          Override     protected void onCreate Bundle savedInstanceState        setContentView R layout activity main       if  savedInstanceState    null          viewpagerid savedInstanceState getInt  viewpagerid   -1           MyFragmentPagerAdapter titleAdapter   new MyFragmentPagerAdapter  getSupportFragmentManager     this               mViewPager    ViewPager  findViewById R id pager       if  viewpagerid    -1            mViewPager setId viewpagerid        else          viewpagerid mViewPager getId              mViewPager setAdapter titleAdapter     If you want to communicate with this fragment  you can get if from FragmentManager  such as   getSupportFragmentManager   findFragmentByTag  android switcher     viewpagerid     0

User · Answer

To get the fragments after orientation change you have to use the  getTag         getSupportFragmentManager   findFragmentByTag  android switcher     viewPagerId         positionOfItemInViewPager    For a bit more handling i wrote my own ArrayList for my PageAdapter to get the fragment by viewPagerId and the FragmentClass at any Position   public class MyPageAdapter extends FragmentPagerAdapter implements Serializable   private final String logTAG   MyPageAdapter class getName           private ArrayList lt MyPageBuilder gt  fragmentPages   public MyPageAdapter FragmentManager fm  ArrayList lt MyPageBuilder gt  fragments        super fm       fragmentPages   fragments      Override public Fragment getItem int position        return this fragmentPages get position  getFragment        Override public CharSequence getPageTitle int position        return this fragmentPages get position  getPageTitle        Override public int getCount         return this fragmentPages size        public int getItemPosition Object object          ben  tigt  damit bei notifyDataSetChanged alle Fragemnts refrehsed werden      Log d logTAG  object getClass   getName         return POSITION NONE     public Fragment getFragment int position        return getItem position      public String getTag int position  int viewPagerId          getSupportFragmentManager   findFragmentByTag  android switcher     R id shares detail activity viewpager         myViewPager getCurrentItem         return  android switcher     viewPagerId         position     public MyPageBuilder getPageBuilder String pageTitle  int icon  int selectedIcon  Fragment frag        return new MyPageBuilder pageTitle  icon  selectedIcon  frag       public static class MyPageBuilder        private Fragment fragment       public Fragment getFragment             return fragment             public void setFragment Fragment fragment            this fragment   fragment             private String pageTitle       public String getPageTitle             return pageTitle             public void setPageTitle String pageTitle            this pageTitle   pageTitle             private int icon       public int getIconUnselected             return icon             public void setIconUnselected int iconUnselected            this icon   iconUnselected             private int iconSelected       public int getIconSelected             return iconSelected             public void setIconSelected int iconSelected            this iconSelected   iconSelected             public MyPageBuilder String pageTitle  int icon  int selectedIcon  Fragment frag            this pageTitle   pageTitle          this icon   icon          this iconSelected   selectedIcon          this fragment   frag           public static class MyPageArrayList extends ArrayList lt MyPageBuilder gt        private final String logTAG   MyPageArrayList class getName               public MyPageBuilder get Class cls               Fragment   ber FragmentClass holen         for  MyPageBuilder item   this                if  item fragment getClass   getName   equalsIgnoreCase cls getName                       return super get indexOf item                                    return null             public String getTag int viewPagerId  Class cls               Tag des Fragment unabh  ngig vom State z B  nach bei Orientation change         for  MyPageBuilder item   this                if  item fragment getClass   getName   equalsIgnoreCase cls getName                       return  android switcher     viewPagerId         indexOf item                                   return null            So just create a MyPageArrayList with the fragments       myFragPages   new MyPageAdapter MyPageArrayList         myFragPages add new MyPageAdapter MyPageBuilder              getString R string widget config data frag               R drawable ic sd storage 24dp              R drawable ic sd storage selected 24dp              new WidgetDataFrag           myFragPages add new MyPageAdapter MyPageBuilder              getString R string widget config color frag               R drawable ic color 24dp              R drawable ic color selected 24dp              new WidgetColorFrag           myFragPages add new MyPageAdapter MyPageBuilder              getString R string widget config textsize frag               R drawable ic settings widget 24dp              R drawable ic settings selected 24dp              new WidgetTextSizeFrag        and add them to the viewPager       mAdapter   new MyPageAdapter getSupportFragmentManager    myFragPages       myViewPager setAdapter mAdapter     after this you can get after orientation change the correct fragment by using its class           WidgetDataFrag dataFragment    WidgetDataFrag  getSupportFragmentManager                findFragmentByTag myFragPages getTag myViewPager getId    WidgetDataFrag class

User · Answer

If anyone is having issues with their FragmentStatePagerAdapter not properly restoring the state of its fragments   ie   new Fragments are being created by the FragmentStatePagerAdapter instead of it restoring them from state     Make sure you call ViewPager setOffscreenPageLimit   BEFORE you call ViewPager setAdapter fragmentStatePagerAdapter   Upon calling ViewPager setOffscreenPageLimit     the ViewPager will immediately look to its adapter and try to get its fragments   This could happen before the ViewPager has a chance to restore the Fragments from savedInstanceState thus creating new Fragments that can t be re-initialized from SavedInstanceState because they re new

User · Answer

My solution is very rude but works  being my fragments dynamically created from retained data  I simply remove all fragment from the PageAdapter before calling super onSaveInstanceState   and then recreate them on activity creation    Override protected void onSaveInstanceState Bundle outState        outState putInt  viewpagerpos   mViewPager getCurrentItem          mSectionsPagerAdapter removeAllfragments        super onSaveInstanceState outState       You can t remove them in onDestroy    otherwise you get this exception   java lang IllegalStateException  Can not perform this action after onSaveInstanceState  Here the code in the page adapter   public void removeAllfragments         if   mFragmentList    null             for   Fragment fragment   mFragmentList                 mFm beginTransaction   remove fragment  commit                      mFragmentList clear            notifyDataSetChanged              I only save the current page and restore it in onCreate    after the fragments have been created   if  savedInstanceState    null      mViewPager setCurrentItem  savedInstanceState getInt  viewpagerpos   0

User · Answer

What is that BasePagerAdapter   You should use one of the standard pager adapters -- either FragmentPagerAdapter or FragmentStatePagerAdapter  depending on whether you want Fragments that are no longer needed by the ViewPager to either be kept around  the former  or have their state saved  the latter  and re-created if needed again   Sample code for using ViewPager can be found here  It is true that the management of fragments in a view pager across activity instances is a little complicated  because the FragmentManager in the framework takes care of saving the state and restoring any active fragments that the pager has made   All this really means is that the adapter when initializing needs to make sure it re-connects with whatever restored fragments there are   You can look at the code for FragmentPagerAdapter or FragmentStatePagerAdapter to see how this is done

User · Answer

When the FragmentPagerAdapter adds a fragment to the FragmentManager  it uses a special tag based on the particular position that the fragment will be placed  FragmentPagerAdapter getItem int position  is only called when a fragment for that position does not exist  After rotating  Android will notice that it already created saved a fragment for this particular position and so it simply tries to reconnect with it with FragmentManager findFragmentByTag    instead of creating a new one  All of this comes free when using the FragmentPagerAdapter and is why it is usual to have your fragment initialisation code inside the getItem int  method   Even if we were not using a FragmentPagerAdapter  it is not a good idea to create a new fragment every single time in Activity onCreate Bundle   As you have noticed  when a fragment is added to the FragmentManager  it will be recreated for you after rotating and there is no need to add it again  Doing so is a common cause of errors when working with fragments   A usual approach when working with fragments is this   protected void onCreate Bundle savedInstanceState        super onCreate savedInstanceState                 CustomFragment fragment      if  savedInstanceState    null            fragment    CustomFragment  getSupportFragmentManager   findFragmentByTag  customtag          else           fragment   new CustomFragment            getSupportFragmentManager   beginTransaction   add R id container  fragment   customtag   commit                         When using a FragmentPagerAdapter  we relinquish fragment management to the adapter  and do not have to perform the above steps  By default  it will only preload one Fragment in front and behind the current position  although it does not destroy them unless you are using FragmentStatePagerAdapter   This is controlled by ViewPager setOffscreenPageLimit int   Because of this  directly calling methods on the fragments outside of the adapter is not guaranteed to be valid  because they may not even be alive    To cut a long story short  your solution to use putFragment to be able to get a reference afterwards is not so crazy  and not so unlike the normal way to use fragments anyway  above   It is difficult to obtain a reference otherwise because the fragment is added by the adapter  and not you personally  Just make sure that the offscreenPageLimit is high enough to load your desired fragments at all times  since you rely on it being present  This bypasses lazy loading capabilities of the ViewPager  but seems to be what you desire for your application   Another approach is to override FragmentPageAdapter instantiateItem View  int  and save a reference to the fragment returned from the super call before returning it  it has the logic to find the fragment  if already present    For a fuller picture  have a look at some of the source of FragmentPagerAdapter  short  and ViewPager  long

User · Answer

I want to offer a solution that expands on antonyt s wonderful answer and mention of overriding FragmentPageAdapter instantiateItem View  int  to save references to created Fragments so you can do work on them later  This should also work with FragmentStatePagerAdapter  see notes for details     Here s a simple example of how to get a reference to the Fragments returned by FragmentPagerAdapter that doesn t rely on the internal tags set on the Fragments  The key is to override instantiateItem   and save references in there instead of in getItem     public class SomeActivity extends Activity       private FragmentA m1stFragment      private FragmentB m2ndFragment          other code in your Activity         private class CustomPagerAdapter extends FragmentPagerAdapter              other code in your custom FragmentPagerAdapter             public CustomPagerAdapter FragmentManager fm                super fm                       Override         public Fragment getItem int position                   Do NOT try to save references to the Fragments in getItem                   because getItem   is not always called  If the Fragment                was already created then it will be retrieved from the FragmentManger                and not here  i e  getItem   won t be called again               switch  position                    case 0                      return new FragmentA                    case 1                      return new FragmentB                    default                         This should never happen  Always account for each position above                     return null                                      Here we can finally safely save a reference to the created            Fragment  no matter where it came from  either getItem   or            FragmentManger   Simply save the returned Fragment from            super instantiateItem   into an appropriate reference depending            on the ViewPager position           Override         public Object instantiateItem ViewGroup container  int position                Fragment createdFragment    Fragment  super instantiateItem container  position                  save the appropriate reference depending on position             switch  position                    case 0                      m1stFragment    FragmentA  createdFragment                      break                  case 1                      m2ndFragment    FragmentB  createdFragment                      break                            return createdFragment                       public void someMethod                do work on the referenced Fragments  but first check if they            even exist yet  otherwise you ll get an NPE           if  m1stFragment    null                   m1stFragment doWork                       if  m2ndFragment    null                   m2ndFragment doSomeWorkToo                        or if you prefer to work with tags instead of class member variables references to the Fragments you can also grab the tags set by FragmentPagerAdapter in the same manner  NOTE  this doesn t apply to FragmentStatePagerAdapter since it doesn t set tags when creating its Fragments    Override public Object instantiateItem ViewGroup container  int position        Fragment createdFragment    Fragment  super instantiateItem container  position          get the tags set by FragmentPagerAdapter     switch  position            case 0              String firstTag   createdFragment getTag                break          case 1              String secondTag   createdFragment getTag                break                   save the tags somewhere so you can reference them later     return createdFragment      Note that this method does NOT rely on mimicking the internal tag set by FragmentPagerAdapter and instead uses proper APIs for retrieving them  This way even if the tag changes in future versions of the SupportLibrary you ll still be safe     Don t forget that depending on the design of your Activity  the Fragments you re trying to work on may or may not exist yet  so you have to account for that by doing null checks before using your references   Also  if instead you re working with FragmentStatePagerAdapter  then you don t want to keep hard references to your Fragments because you might have many of them and hard references would unnecessarily keep them in memory  Instead save the Fragment references in WeakReference variables instead of standard ones  Like this   WeakReference lt Fragment gt  m1stFragment   new WeakReference lt Fragment gt  createdFragment         and access them like so Fragment firstFragment   m1stFragment get    if  firstFragment    null           reference hasn t been cleared yet  do work

User · Answer

A bit different opinion instead of storing the Fragments yourself just leave it to the FragmentManager and when you need to do something with the fragments look for them in the FragmentManager    make sure you have the right FragmentManager    getSupportFragmentManager or getChildFragmentManager depending on what you are using to manage this stack of fragments List lt Fragment gt  fragments   fragmentManager getFragments    if fragments    null       int count   fragments size       for  int x   0  x  lt  count  x             Fragment fragment   fragments get x            check if this is the fragment we want            it may be some other inspection  tag etc         if  fragment instanceof MyFragment                 do whatever we need to do with it                  If you have a lot of Fragments and the cost of instanceof check may be not what you want  but it is good thing to have in mind that the FragmentManager already keeps account of Fragments

User · Answer

I came up with this simple and elegant solution  It assumes that the activity is responsible for creating the Fragments  and the Adapter just serves them   This is the adapter s code  nothing weird here  except for the fact that mFragments is a list of fragments maintained by the Activity   class MyFragmentPagerAdapter extends FragmentStatePagerAdapter        public MyFragmentPagerAdapter FragmentManager fm            super fm               Override     public Fragment getItem int position            return mFragments get position               Override     public int getCount             return mFragments size                Override     public int getItemPosition Object object            return POSITION NONE              Override     public CharSequence getPageTitle int position            TabFragment fragment    TabFragment mFragments get position           return fragment getTitle               The whole problem of this thread is getting a reference of the  old  fragments  so I use this code in the Activity s onCreate       if  savedInstanceState  null            if  getSupportFragmentManager   getFragments    null                for  Fragment fragment   getSupportFragmentManager   getFragments                      mFragments add fragment                                   Of course you can further fine tune this code if needed  for example making sure the fragments are instances of a particular class

User · Answer

I want to offer an alternate solution for perhaps a slightly different case  since many of my searches for answers kept leading me to this thread   My case - I m creating adding pages dynamically and sliding them into a ViewPager  but when rotated  onConfigurationChange  I end up with a new page because of course OnCreate is called again   But I want to keep reference to all the pages that were created prior to the rotation   Problem - I don t have unique identifiers for each fragment I create  so the only way to reference was to somehow store references in an Array to be restored after the rotation configuration change   Workaround - The key concept was to have the Activity  which displays the Fragments  also manage the array of references to existing Fragments  since this activity can utilize Bundles in onSaveInstanceState  public class MainActivity extends FragmentActivity   So within this Activity  I declare a private member to track the open pages  private List lt Fragment gt  retainedPages   new ArrayList lt Fragment gt       This is updated everytime onSaveInstanceState is called and restored in onCreate   Override protected void onSaveInstanceState Bundle outState        retainedPages    adapter exportList        outState putSerializable  retainedPages    Serializable  retainedPages       super onSaveInstanceState outState          so once it s stored  it can be retrieved      Override public void onCreate Bundle savedInstanceState        super onCreate savedInstanceState       setContentView R layout activity main        if  savedInstanceState    null            retainedPages    List lt Fragment gt   savedInstanceState getSerializable  retainedPages               mViewPager    CustomViewPager  findViewById R id viewPager        adapter   new ViewPagerAdapter getApplicationContext    getSupportFragmentManager         if  retainedPages size    gt  0             adapter importList retainedPages              mViewPager setAdapter  adapter        mViewPager setCurrentItem  adapter getCount  -1       These were the necessary changes to the main activity  and so I needed the members and methods within my FragmentPagerAdapter for this to work  so within  public class ViewPagerAdapter extends FragmentPagerAdapter   an identical construct  as shown above in MainActivity    private List lt Fragment gt   pages   new ArrayList lt Fragment gt       and this syncing  as used above in onSaveInstanceState  is supported specifically by the methods  public List lt Fragment gt  exportList         return  pages     public void importList List lt Fragment gt  savedPages         pages   savedPages      And then finally  in the fragment class   public class CustomFragment extends Fragment   in order for all this to work  there were two changes  first  public class CustomFragment extends Fragment implements Serializable   and then adding this to onCreate so Fragments aren t destroyed  setRetainInstance true     I m still in the process of wrapping my head around Fragments and Android life cycle  so caveat here is there may be redundancies inefficiencies in this method   But it works for me and I hope might be helpful for others with cases similar to mine

[android] ViewPager and fragments — what's the right way to store fragment's state?

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