IllegalStateException: is not currently in the FragmentManager

Question

I know it sounds like a duplicate of FragmentStatePagerAdapter IllegalStateException: is not currently in the FragmentManager but his solution isn\'t relevan

Answer 1

I've encountered the exact exception.

In my case, I have several fragments managed by FragmentPagerStateAdapter, but sometimes the fragment will get switched in position.

I override getItemPosition() and return the new position and call notifyDataSetChanged() to refresh ViewPager. Everything works fine but sometimes when I leave the ViewPager the crash occurs.

After several hours digging into it, I found there's a bug in FragmentPagerStateAdapter.

The adapter not only caches states, but also caches the fragments that it consider 'active'

private ArrayList mSavedState = new ArrayList();
private ArrayList mFragments = new ArrayList();

But it does not check if the fragments' position has invalidated or moved.

That's how it get this exception:

1.I have A,B,C three Fragments in ViewPager and Adapter.

2.I switched the position to B,A,C in adapter and called notifyDataSetChanged().

3.ViewPager re-layout the fragment by new order. But the mFragments cache is still {A,B,C}

4.Swipe few pages, ViewPager will ask adapter to destroy the first Fragment. then the fragment B, instead of A, is set to null in the cache. Now the mFragments is {null,A,C}.

5.Leave the ViewPager, and onSaveInstanceState is triggered. all fragments in mFragments is considered active and putFragment() is called until FragmentManagerImpl found A not in it and throw a Exception.

So here's my not so gentle solution:

1.Copy the entire FragmentPagerStateAdapter source code.

2.Override the notifyDataSetChanged() method to rearrange the caches as below.

@Override
public void notifyDataSetChanged() {
  List oldFragments = new ArrayList<>(mFragments);
  List oldStates = new ArrayList<>(mSavedState);
  for (int i = 0; i < getCount(); i++) {
    if (i < mFragments.size()) {
      Fragment f = mFragments.get(i);
      if (f != null) {
        int newPosition = getItemPosition(f);
        if (newPosition == POSITION_UNCHANGED || newPosition == i) {
        } else if (newPosition == POSITION_NONE) {
          if (i < mSavedState.size()) {
            mSavedState.set(i, null);
          }
          mFragments.set(i, null);
        } else {
          while (i >= mFragments.size()) {
            mFragments.add(null);
          }
          if (oldStates.size() > i) {
            mSavedState.set(newPosition, oldStates.get(i));
          }
          mFragments.set(newPosition, oldFragments.get(i));
        }
      } else {
        /*
         * No Fragment but that's possible there's savedState and position has
         * changed.
         */
      }
    }
  }
  super.notifyDataSetChanged();
}

Hope that works for some of you!