support FragmentPagerAdapter holds reference to old fragments

Question

LATEST INFO   i have narrowed my problem down to being a problem with the fragmentManager retaining instances of old fragments and my viewpager being out of sync with my FragmentManager  See this issue    http   code google com p android issues detail id 19211 makechanges   I still have no clue how to solve this  Any suggestions     I have tried to debug this for a long time and any help would be greatly appreciated  I am using a FragmentPagerAdapter which accepts a list of fragments like so   List lt Fragment gt  fragments   new Vector lt Fragment gt     fragments add Fragment instantiate this  Fragment1 class getName           new PagerAdapter getSupportFragmentManager    fragments     The implementation is standard  I am using ActionBarSherlock and v4 computability library for Fragments   My problem is that after leaving the app and opening several other applications and coming back  the fragments lose their reference back to the FragmentActivity  ie  getActivity      null   I can not figure out why this is happening  I tried to manually set setRetainInstance true   but this does not help  I figured that this happens when my FragmentActivity gets destroyed  however this still happens if I open the app before I get the log message  Are there any ideas    Override protected void onDestroy        Log w TAG   DESTROYDESTROYDESTROYDESTROYDESTROYDESTROYDESTROY        super onDestroy        The adapter   public class PagerAdapter extends FragmentPagerAdapter       private List lt Fragment gt  fragments       public PagerAdapter FragmentManager fm  List lt Fragment gt  fragments            super fm            this fragments   fragments               Override     public Fragment getItem int position             return this fragments get position                Override     public int getCount              return this fragments size                One of my Fragments stripped but i commetned everyhting out thats stripped and still doesnt work     public class MyFragment extends Fragment implements MyFragmentInterface  OnScrollListener         Override public void onCreate Bundle savedInstanceState       super onCreate savedInstanceState       handler   new Handler            setHasOptionsMenu true       Override public void onAttach Activity activity        super onAttach activity       Log w TAG  ATTACHATTACHATTACHATTACHATTACH        context   activity      if context   null           Log e  IS NULL    NULLNULLNULLNULLNULLNULLNULLNULLNULLNULLNULL         else          Log d  IS NOT NULL    NOTNOTNOTNOTNOTNOTNOTNOT               Override public void onActivityCreated Bundle savedState        super onActivityCreated savedState       Override public View onCreateView LayoutInflater inflater  ViewGroup container  Bundle savedInstanceState        View v   inflater inflate R layout my fragment container  false        return v       Override public void onResume        super onResume       private void callService           do not call another service is already running     if startLoad     canSet  return         set flag     startLoad   true      canSet   false         show the bottom spinner     addFooter        Intent intent   new Intent context  MyService class       intent putExtra MyService STATUS RECEIVER  resultReceiver       context startService intent      private ResultReceiver resultReceiver   new ResultReceiver null         Override     protected void onReceiveResult int resultCode  final Bundle resultData            boolean isSet   false          if resultData  null          if resultData containsKey MyService STATUS FINISHED GET                if resultData getBoolean MyService STATUS FINISHED GET                    removeFooter                    startLoad   false                  isSet   true                                   switch resultCode           case MyService STATUS FINISHED               stopSpinning                break          case SyncService STATUS RUNNING              break          case SyncService STATUS ERROR              break                      public void onCreateOptionsMenu Menu menu  MenuInflater inflater        menu clear        inflater inflate R menu activity  menu       Override public void onPause        super onPause       public void onScroll AbsListView arg0  int firstVisible  int visibleCount  int totalCount        boolean loadMore      maybe add a padding            firstVisible   visibleCount  gt   totalCount       boolean away   firstVisible  visibleCount  lt   totalCount - visibleCount       if away              startLoad can now be set again         canSet   true             if loadMore       public void onScrollStateChanged AbsListView arg0  int state        switch state       case OnScrollListener SCROLL STATE FLING           adapter setLoad false            lastState   OnScrollListener SCROLL STATE FLING          break      case OnScrollListener SCROLL STATE IDLE           adapter setLoad true           if lastState    SCROLL STATE FLING                  load the images on screen                    lastState   OnScrollListener SCROLL STATE IDLE          break      case OnScrollListener SCROLL STATE TOUCH SCROLL          adapter setLoad true           if lastState    SCROLL STATE FLING                  load the images on screen                    lastState   OnScrollListener SCROLL STATE TOUCH SCROLL          break            Override public void onDetach        super onDetach        if this adapter  null          this adapter clearContext         Log w TAG   DETACHEDDETACHEDDETACHEDDETACHEDDETACHEDDETACHED       public void update final int id  String name        if name  null           getActivity   getSupportActionBar   setTitle name                 The update method is called when a user interacts with a different fragment and the getActivity is returning null  Here is the method the other fragment is calling       MyFragment  pagerAdapter getItem 1   update id  name     I believe that when the app is destroyed then created again instead of just starting the app up to the default fragment the app starts up and then viewpager navigates to the last known page  This seems strange  shouldn t the app just load to the default fragment

User · Answer

I faced the same issue but my ViewPager was inside a TopFragment which created and set an adapter using setAdapter(new FragmentPagerAdapter(getChildFragmentManager())).

I fixed this issue by overriding onAttachFragment(Fragment childFragment) in the TopFragment like this:

@Override
public void onAttachFragment(Fragment childFragment) {
    if (childFragment instanceof OnboardingDiamondsFragment) {
        mChildFragment = (ChildFragment) childFragment;
    }

    super.onAttachFragment(childFragment);
}

As known already (see answers above), when the childFragmentManager recreate itself, it also create the fragments which were inside the viewPager.
The important part is that after that, he calls onAttachFragment and now we have a reference to the new recreated fragment!

Hope this will help anyone getting this old Q like me :)

User · Answer

Since people don t tend to read comments  here is an answer that mostly duplicates what I wrote here   the root cause of the issue is the fact that android system does not call getItem to obtain fragments that are actually displayed  but instantiateItem  This method first tries to lookup and reuse a fragment instance for a given tab in FragmentManager  Only if this lookup fails  which happens only the first time when FragmentManager is newly created  then getItem is called  It is for obvious reasons not to recreate fragments  that may be heavy  for example each time a user rotates his device  To solve this  instead of creating fragments with Fragment instantiate in your activity  you should do it with pagerAdapter instantiateItem and all these calls should be surrounded by startUpdate finishUpdate method calls that start commit fragment transaction respectively  getItem should be the place where fragments are really created using their respective constructors   List lt Fragment gt  fragments   new Vector lt Fragment gt       Override protected void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout myLayout           ViewPager viewPager    ViewPager  findViewById R id myViewPager           MyPagerAdapter adapter   new MyPagerAdapter getSupportFragmentManager             viewPager setAdapter adapter             TabLayout  findViewById R id tabs   setupWithViewPager viewPager            adapter startUpdate viewPager           fragments add adapter instantiateItem viewPager  0            fragments add adapter instantiateItem viewPager  1               and so on if you have more tabs            adapter finishUpdate viewPager      class MyPagerAdapter extends FragmentPagerAdapter            public MyPagerAdapter FragmentManager manager   super manager              Override public int getCount    return 2             Override public Fragment getItem int position                if  position    0  return new Fragment0                if  position    1  return new Fragment1                return null      or throw some exception                     Override public CharSequence getPageTitle int position                if  position    0  return getString R string tab0               if  position    1  return getString R string tab1               return null      or throw some exception

User · Answer

You are running into a problem because you are instantiating and keeping references to your fragments outside of PagerAdapter getItem  and are trying to use those references independently of the ViewPager  As Seraph says  you do have guarantees that a fragment has been instantiated added in a ViewPager at a particular time - this should be considered an implementation detail  A ViewPager does lazy loading of its pages  by default it only loads the current page  and the one to the left and right   If you put your app into the background  the fragments that have been added to the fragment manager are saved automatically  Even if your app is killed  this information is restored when you relaunch your app   Now consider that you have viewed a few pages  Fragments A  B and C  You know that these have been added to the fragment manager  Because you are using FragmentPagerAdapter and not FragmentStatePagerAdapter  these fragments will still be added  but potentially detached  when you scroll to other pages   Consider that you then background your application  and then it gets killed  When you come back  Android will remember that you used to have Fragments A  B and C in the fragment manager and so it recreates them for you and then adds them  However  the ones that are added to the fragment manager now are NOT the ones you have in your fragments list in your Activity   The FragmentPagerAdapter will not try to call getPosition if there is already a fragment added for that particular page position  In fact  since the fragment recreated by Android will never be removed  you have no hope of replacing it with a call to getPosition  Getting a handle on it is also pretty difficult to obtain a reference to it because it was added with a tag that is unknown to you  This is by design  you are discouraged from messing with the fragments that the view pager is managing  You should be performing all your actions within a fragment  communicating with the activity  and requesting to switch to a particular page  if necessary   Now  back to your problem with the missing activity  Calling pagerAdapter getItem 1   update id  name  after all of this has happened returns you the fragment in your list  which has yet to be added to the fragment manager  and so it will not have an Activity reference  I would that suggest your update method should modify some shared data structure  possibly managed by the activity   and then when you move to a particular page it can draw itself based on this updated data

User · Answer

I solved the problem by saving the fragments in SparceArray   public abstract class SaveFragmentsPagerAdapter extends FragmentPagerAdapter        SparseArray lt Fragment gt  fragments   new SparseArray lt  gt          public SaveFragmentsPagerAdapter FragmentManager fm            super fm               Override     public Object instantiateItem ViewGroup container  int position            Fragment fragment    Fragment  super instantiateItem container  position           fragments append position  fragment           return fragment              Nullable     public Fragment getFragmentByPosition int position           return fragments get position

User · Answer

Do not try to interact between fragments in ViewPager  You cannot guarantee that other fragment attached or even exists  Istead of changing actionbar title from fragment  you can do it from your activity  Use standart interface pattern for this   public interface UpdateCallback       void update String name      public class MyActivity extends FragmentActivity implements UpdateCallback        Override     public void update String name                getSupportActionBar   setTitle name             public class MyFragment extends Fragment       private UpdateCallback callback        Override     public void onAttach SupportActivity activity                super onAttach activity           callback    UpdateCallback  activity              Override     public void onDetach                 super onDetach            callback   null             public void updateActionbar String name                if callback    null              callback update name

User · Answer

You can remove the fragments when destroy the viewpager  in my case  I removed them on onDestroyView   of my fragment    Override public void onDestroyView          if  getChildFragmentManager   getFragments      null            for  Fragment fragment   getChildFragmentManager   getFragments                  getChildFragmentManager   beginTransaction   remove fragment  commitAllowingStateLoss                         super onDestroyView

User · Answer

Global working tested solution   getSupportFragmentManager   keeps the null reference some times and View pager does not create new since it find reference to same fragment  So to over come this use getChildFragmentManager   solves problem in simple way   Don t do this    new PagerAdapter getSupportFragmentManager    fragments    Do this   new PagerAdapter getChildFragmentManager     fragments

User · Answer

My solution  I set almost every View as static  Now my app interacts perfect  Being able to call the static methods from everywhere is maybe not a good style  but why to play around with code that doesn t work  I read a lot of questions and their answers here on SO and no solution brought success  for me     I know it can leak the memory  and waste heap  and my code will not be fit on other projects  but I don t feel scared about this - I tested the app on different devices and conditions  no problems at all  the Android Platform seems to be able handle this  The UI gets refreshed every second and even on a S2 ICS  4 0 3  device the app is able to handle thousands of geo-markers

User · Answer

Just so you know     Adding to the litany of woes with these classes  there is a rather interesting bug that s worth sharing    I m using a ViewPager to navigate a tree of items  select an item and the view pager animates scrolling to the right  and the next branch appears  navigate back  and the ViewPager scrolls in the opposite direction to return to the previous node    The problem arises when I push and pop fragments off the end of the FragmentStatePagerAdapter  It s smart enough to notice that the items change  and smart enough to create and replace a fragment when the item has changed  But not smart enough to discard the fragment state  or smart enough to trim the internally saved fragment states when the adapter size changes  So when you pop an item  and push a new one onto the end  the fragment for the new item gets the saved state of the fragment for the old item  which caused absolute havoc in my code  My fragments carry data that may require a lot of work to refetch from the internet  so not saving state really wasn t an option   I don t have a clean workaround  I used something like this     public void onSaveInstanceState Bundle outState        IFragmentListener listener    IFragmentListener getActivity        if  listener   null                if   listener isStillInTheAdapter this getAdapterItem                           return     return empty state                       super onSaveInstanceState outState           normal saving of state for flips and         paging out of the activity follows                An imperfect solution because the new fragment instance still gets a savedState Bundle  but at least it doesn t carry stale data

User · Answer

I solved this issue by accessing my fragments directly through the FragmentManager instead of via the FragmentPagerAdapter like so  First I need to figure out the tag of the fragment auto generated by the FragmentPagerAdapter     private String getFragmentTag int pos       return  android switcher   R id viewpager     pos      Then I simply get a reference to that fragment and do what I need like so     Fragment f   this getSupportFragmentManager   findFragmentByTag getFragmentTag 1      MyFragmentInterface  f  update id  name   viewPager setCurrentItem 1  true     Inside my fragments I set the setRetainInstance false   so that I can manually add values to the savedInstanceState bundle     Override public void onSaveInstanceState Bundle outState        if this my   null          outState putInt  myId   this my getId          super onSaveInstanceState outState       and then in the OnCreate i grab that key and restore the state of the fragment as necessary    An easy solution which was hard  for me at least  to figure out

User · Answer

After a few hours of looking for a similar issue I think a have another solution  This one at least it worked for me and I only have to changed a couple of lines   This is the problem I had  I have an activity with a view pager that uses a FragmentStatePagerAdapter with two Fragments  Everything works fine until I force the activity to get destroyed  developer options  or I rotate the screen  I do keep a reference to the two fragments after they get created inside the method getItem   At that point the activity will be created again and everything works fine at this point but I have lost the reference to my fragmetns as getItem doesn t  get called again    This is how I fixed that problem  inside the FragmentStatePagerAdapter        Override     public Object instantiateItem ViewGroup container  int position            Object aux   super instantiateItem container  position              Update the references to the Fragments we have on the view pager         if position  0               fragTabOne    FragOffersList aux                    else              fragTabTwo    FragOffersList  aux                     return aux          You won t get a call on getItem again if the adapter already has a reference to it internally  and you shouldn t change that  Instead you can get the fragment it s being used by looking at this other method instantiateItem   which will be called for each of your fragments   Hope that helps anyone

User · Answer

Since the FragmentManager will take care of restoring your Fragments for you as soon as the onResume   method is called I have the fragment call out to the activity and add itself to a list   In my instance I am storing all of this in my PagerAdapter implementation   Each fragment knows it s position because it is added to the fragment arguments on creation   Now whenever I need to manipulate a fragment at a specific index all I have to do is use the list from my adapter   The following is an example of an Adapter for a custom ViewPager that will grow the fragment as it moves into focus  and scale it down as it moves out of focus   Besides the Adapter and Fragment classes I have here all you need is for the parent activity to be able to reference the adapter variable and you are set   Adapter  public class GrowPagerAdapter extends FragmentPagerAdapter implements OnPageChangeListener  OnScrollChangedListener    public final String TAG   this getClass   getSimpleName     private final int COUNT   4   public static final float BASE SIZE   0 8f  public static final float BASE ALPHA   0 8f   private int mCurrentPage   0  private boolean mScrollingLeft   private List lt SummaryTabletFragment gt  mFragments   public int getCurrentPage         return mCurrentPage     public void addFragment SummaryTabletFragment fragment        mFragments add fragment getPosition    fragment      public GrowPagerAdapter FragmentManager fm        super fm        mFragments   new ArrayList lt SummaryTabletFragment gt         Override public int getCount         return COUNT      Override public Fragment getItem int position        return SummaryTabletFragment newInstance position       Override public void onPageScrollStateChanged int state       Override public void onPageScrolled int position  float positionOffset  int positionOffsetPixels         adjustSize position  positionOffset       Override public void onPageSelected int position        mCurrentPage   position            Used to adjust the size of each view in the viewpager as the user    scrolls   This provides the effect of children scaling down as they    are moved out and back to full size as they come into focus          param position     param percent     private void adjustSize int position  float percent         position     mScrollingLeft   1   0       int secondary   position    mScrollingLeft   -1   1       int tertiary   position    mScrollingLeft   1   -1        float scaleUp   mScrollingLeft   percent   1 0f - percent      float scaleDown   mScrollingLeft   1 0f - percent   percent       float percentOut   scaleUp  gt  BASE ALPHA   BASE ALPHA   scaleUp      float percentIn   scaleDown  gt  BASE ALPHA   BASE ALPHA   scaleDown       if  scaleUp  lt  BASE SIZE          scaleUp   BASE SIZE       if  scaleDown  lt  BASE SIZE          scaleDown   BASE SIZE          Adjust the fragments that are  or will be  on screen     SummaryTabletFragment current    position  lt  mFragments size      mFragments get position    null      SummaryTabletFragment next    secondary  lt  mFragments size    amp  amp  secondary  gt  -1    mFragments get secondary    null      SummaryTabletFragment afterNext    tertiary  lt  mFragments size    amp  amp  tertiary  gt  -1    mFragments get tertiary    null       if  current    null  amp  amp  next    null                Apply the adjustments to each fragment         current transitionFragment percentIn  scaleUp           next transitionFragment percentOut  scaleDown            if  afterNext    null                afterNext transitionFragment BASE ALPHA  BASE SIZE                       Override public void onScrollChanged int l  int t  int oldl  int oldt            Keep track of which direction we are scrolling     mScrollingLeft    oldl - l   lt  0        Fragment  public class SummaryTabletFragment extends BaseTabletFragment    public final String TAG   this getClass   getSimpleName     private final float SCALE SIZE   0 8f   private RelativeLayout mBackground  mCover  private TextView mTitle  private VerticalTextView mLeft  mRight   private String mTitleText  private Integer mColor   private boolean mInit   false  private Float mScale  mPercent   private GrowPagerAdapter mAdapter  private int mCurrentPosition   0   public String getTitleText         return mTitleText     public void setTitleText String titleText        this mTitleText   titleText     public static SummaryTabletFragment newInstance int position         SummaryTabletFragment fragment   new SummaryTabletFragment        fragment setRetainInstance true        Bundle args   new Bundle        args putInt  position   position       fragment setArguments args        return fragment      Override public View onCreateView LayoutInflater inflater  ViewGroup container  Bundle savedInstanceState        super onCreateView inflater  container  savedInstanceState        mRoot   inflater inflate R layout tablet dummy view  null        setupViews        configureView         return mRoot      Override public void onViewStateRestored Bundle savedInstanceState        super onViewStateRestored savedInstanceState        if  savedInstanceState    null            mColor   savedInstanceState getInt  color   Color BLACK              configureView        Override public void onSaveInstanceState Bundle outState          outState putInt  color   mColor        super onSaveInstanceState outState       Override public int getPosition         return getArguments   getInt  position   -1       Override public void setPosition int position        getArguments   putInt  position   position      public void onResume         super onResume         mAdapter   mActivity getPagerAdapter        mAdapter addFragment this       mCurrentPosition   mAdapter getCurrentPage         if   getPosition       mCurrentPosition   1     getPosition       mCurrentPosition - 1    amp  amp   mInit            mInit   true          transitionFragment GrowPagerAdapter BASE ALPHA  GrowPagerAdapter BASE SIZE           return             if  getPosition      mCurrentPosition  amp  amp   mInit            mInit   true          transitionFragment 0 00f  1 0f            private void setupViews          mCover    RelativeLayout  mRoot findViewById R id cover       mLeft    VerticalTextView  mRoot findViewById R id title left       mRight    VerticalTextView  mRoot findViewById R id title right       mBackground    RelativeLayout  mRoot findViewById R id root       mTitle    TextView  mRoot findViewById R id title      private void configureView          Fonts applyPrimaryBoldFont mLeft  15       Fonts applyPrimaryBoldFont mRight  15        float   size   UiUtils getScreenMeasurements mActivity       int width    int   size 0    SCALE SIZE       int height    int   size 1    SCALE SIZE        RelativeLayout LayoutParams params   new RelativeLayout LayoutParams width  height       mBackground setLayoutParams params        if  mScale    null          transitionFragment mPercent  mScale        setRandomBackground         setTitleText  Fragment     getPosition          mTitle setText getTitleText   toUpperCase         mLeft setText getTitleText   toUpperCase         mRight setText getTitleText   toUpperCase          mLeft setOnClickListener new OnClickListener               Override         public void onClick View v                 mActivity showNextPage                           mRight setOnClickListener new OnClickListener               Override         public void onClick View v                 mActivity showPrevPage                         private void setRandomBackground          if  mColor    null            Random r   new Random            mColor   Color rgb r nextInt 255   r nextInt 255   r nextInt 255               mBackground setBackgroundColor mColor      public void transitionFragment float percent  float scale         this mScale   scale      this mPercent   percent       if  getView      null  amp  amp  mCover    null             getView   setScaleX scale           getView   setScaleY scale            mCover setAlpha percent           mCover setVisibility  percent  lt   0 05f    View GONE   View VISIBLE             Override public String getFragmentTitle         return null

User · Answer

I found simple solution which worked for me   Make your fragment adapter extends FragmentStatePagerAdapter instead of FragmentPagerAdapter and override method onSave to return null   Override public Parcelable saveState         return null      This prevent android from recreating fragment    One day later I found another and better solution   Call setRetainInstance true  for all your fragments and save references to them somewhere  I did that in static variable in my activity  because it s declared as singleTask and fragments can stay the same all the time   This way android not recreate fragments but use same instances

[android] support FragmentPagerAdapter holds reference to old fragments

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