FragmentStatePagerAdapter (support package, revision 10) doesn't function correctly when overriding getItemPosition() with the purpose of reordering the pages.

Attached is a simple example adapter (TestPagerAdapter.java) extending FragmentStatePagerAdapter. In the initial state, the order of the pages is {A, B, C}. Upon calling toggleState(), the order of the pages changes to {A, C, B}. By overriding getItemPosition(Object object), the adapter should ensure that the current page being viewed (A, B, or C) does not change after a toggle.

The example does not work. At least two types of incorrect behaviour may be observed.

1. An IndexOutOfBoundsException may occur. This can be reproduced by immediately calling toggleState() (while viewing page A, before swiping to any other page).

2. Empty pages may occur. This can be reproduced by swiping to B, swiping back to A, calling toggleState(), and swiping right to C. Instead of fragment C, we get an empty page.

Looking at the source of FragmentStatePagerAdapter, I figured out exactly what is going wrong. The FragmentStatePagerAdapter caches the fragments and saved states in ArrayLists: mFragments and mSavedState. But when the fragments are reordered, there's no mechanism for reordering the elements of mFragments and mSavedState. Therefore, the adapter will provide the wrong fragments to the pager.

I've attached a fixed implementation (NewFragmentStatePagerAdapter.java). I added a getItemId() function to FragmentStatePagerAdapter. (This mirrors the reordering implementation in FragmentPagerAdapter.) An array of the itemIds by adapter position is stored at all times. Then, in notifyDataSetChanged(), the adapter checks if the itemIds array has changed. If it has, then mFragments and mSavedState are reordered accordingly. Further modifications can be found in destroyItem(), saveState() and restoreState().

To use this class, getItemPosition() and getItemId() must be implemented consistently with getItem().

I don't claim that my NewFragmentStatePagerAdapter is a nice implementation. It's simply an implementation which reuses as much of the existing code as possible. In a more elegant implementation, I don't think that the fragments and saved states should be indexed by position.

Issue 36956111

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