UICollectionReusableView child resize animation bug
This repo contains an iOS app to showcase a bug where the child view of a UICollectionReusableView is animated in its resizing but that child's children do not animate. Think of it like this:
UICollectionView
-> UICollectionReusableView
-> UIViewController view (resize is animated)
-> UIView/UITableView/etc (resize is immediate, not animated)
The resize of the UICollectionReusableView comes from a UICollectionViewFlowLayout subclass that knows about the need to make room in the collection for the UICollectionReusableView height.
Using the demo
Clone or download the repo and run it in in Xcode 7 or 8 beta 1, targeting iOS 9.3 or 10. Earlier of iOS may be susceptible but have not been tested.
All relevant logic is in CollectionController.
Tapping the green cell will present the supplementary view and add a basic UIViewController's child done in selectCell. Tapping the supplementary view will toggle the collection to resize its size between 300 and 350 pts, seen in supplementaryTapped. This resizing is seen to smoothly animate
Tapping the red cell will present a UIViewController that has a UITableView and UIView as child views.
Thanks
A sincere thanks to every Apple engineer that spent time with me in the 2016 WWDC labs to diagnose, debug, and attempt to solve this issue.