adamsp / fragmentstatepagerissueexample Goto Github PK

In instantiateItem, we need to add additional check to make sure the fragment retrieved from mFragments is correct.

String fragmentTag = getTag(position); 
if (mFragments.size() > position) {
    Fragment f = mFragments.get(position);
    if (f != null && TextUtils.equals(f.getTag(), fragmentTag)) {
        return f;
    }
}

This is because calling notifyDataSetChanged(), eventually calling getItemPosition(), does not remove the fragment at the end of list.

In my case, A(0), B(1), C(2) are initiated. When deleting B(1), I animate viewpager to move current item to C(2) first. This makes mFragments to be A-destroyed(0), B(1), C-currentItem(2), D-initiated(3).

I then call notifyDataSetChanged() and this return POSITION_NONE for B(1). This updates mFragments to be A-recalled(0), C-currentItem(1), D-reinserted(2), D-not deleted(3). So when I swipe right, a new fragment E(3) was not created. Fragment D-not deleted was recalled and it gave a black screen in viewpager.

Hi I found your blog from Google, but I didn't really like the idea of maintaining a separate FragmentStatePagerAdapter. A simpler solution is to use FragmentPagerAdapter instead, and override the getItemId() and getItemPosition() methods.

See here for some source code

Hi,

I found your blog post while searching for my own bug with FragmentStatePagerAdapter.

So I'll share my experience...

You can test this bug on support library 23.1.1:

• Put a ViewPager on your activity and use a FragmentStatePagerAdapter as adapter, which provides a list of fragments.
• Each fragment saves its own index within the adapter in its arguments (you provide that index).
• On each fragment, put a button. When a button is clicked, you get the fragment manager, and call findFragmentById() with the current fragment ID (via getId()).
• Take the result fragment, get the index back to compare it to current fragment index.

The bug is: findFragmentById() returns random fragment. The index number is wrong in most case.

So be careful :-)