Question

We’re suffering from a very strange issue with ViewPager here. We embed lists on each ViewPager page, and trigger notifyDataSetChanged both on the list adapter and the view pager adapter when updating list data.

What we observe is that sometimes, the page does not update its view tree, i.e. remains blank, or sometimes even disappears when paging to it. When paging back and forth a few times, the content will suddenly reappear. It seems as if Android is missing a view update here. I also noticed that when debugging with hierarchy viewer, selecting a view will always make it reappear, apparently because hierarchy viewer forces the selected view to redraw itself.

I could not make this work programmatically though; invalidating the list view, or the entire view pager even, had no effect.

This is with the compatibility-v4_r7 library. I also tried to use the latest revision, since it claims to fix many issues related to view pager, but it made matters even worse (for instance, gestures were broken so that it wouldn’t let me page through all pages anymore sometimes.)

Is anyone else running into these issues, too, or do you have an idea of what could be causing this?

Answer 1

We finally managed to find a solution. Apparently our implementation suffered of two issues:

1. our adapter did not remove the view in destroyItem().
2. we were caching views so that we’d have to inflate our layout just once, and, since we were not removing the view in destroyItem(), we were not adding it in instantiateItem() but just returning the cached view corresponding to the current position.

I haven’t looked too deeply in the source code of the ViewPager – and it’s not exactly explicit that you have to do that – but the docs says :

destroyItem()
Remove a page for the given position. The adapter is responsible for removing the view from its container, although it only must ensure this is done by the time it returns from finishUpdate(ViewGroup).

and:

A very simple PagerAdapter may choose to use the page Views themselves as key objects, returning them from instantiateItem(ViewGroup, int) after creation and adding them to the parent ViewGroup. A matching destroyItem(ViewGroup, int, Object) implementation would remove the View from the parent ViewGroup and isViewFromObject(View, Object) could be implemented as return view == object;.

So my conclusion is that ViewPager relies on its underlying adapter to explicitly add/remove its children in instantiateItem()/destroyItem(). That is, if your adapter is a subclass of PagerAdapter, your subclass must implement this logic.

Side note: be aware of this if you use lists inside ViewPager.