support FragmentPagerAdapter holds reference to old fragments

Solution 1:

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.

Solution 2:

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.

Solution 3:

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.