What is the correct way to restore a fragment?

Question

First of all I'm sorry if this explanation seems unclear, I'm new to Android. I have a ViewPager in main activity showing fragments added dynamically by user. Fragments are created initially on activity start up and are added to ViewPager via Adapter i.e. adapter simply returns proper fragment and as I understand correctly fragment's content is created at this time when ViewPager 'retrieves' a fragment first time.

The problem is when main activity gets restored after orientation changing all fragments are resurrected as well and when Adapter tries to return newly created by user Fragment method createView() is no longer called and it fails with NullPointerException. It seems ViewPager retains fragments attached to it initially and doesn't call createView() for newly added ones for the same position.

I have a feeling I'm missing vital point on the Fragment lifecycle. I wouldn't like to change the design. My main question is what the correct way is to return a Fragment added to ViewPage after activity is restored? Is there any way to locate recently attached fragments?

Answer 1

If the fragment already exists, it will be re-used. However the state of the fragment will not. You should take a look at http://developer.android.com/training/basics/activity-lifecycle/recreating.html for more information.

In particular you should look at overriding onSavedInstanceState and onRestoreInstanceState as a method to repopulate the field that is generating that nullpointerexception

To determine if an instance of a fragment has already been created you can use 'findFragmentByTag' like so:

String fragmentTag = MyFramgment.getClass().getName();
MyFragment frag = getFragmentManager().findFragmentByTag(fragmentTag);

if(frag == null)
   frag = new MyFragment();

Whatever ends up being referenced in 'frag', show this to the user.