Cells disappearing during layout animation

[enarity]

Hi! First of all, thank you for this wonderfully crafted library.

Now, I have an issue that has been haunting me for a long time both in my custom layouts and this ChatLayout. The issue is that during layout animations that would change the viewport of the collection view, the cells that would be out-of-screen by the end of the animation disappear immediately when animation begins. You can see it in the example app if you open the keyboard: cells at the top disappear as soon as the animation begins.

From what I could gather, this happens because the animations that are triggered by the layoutIfNeeded() call (used in restoreContentOffset method) are run out-of-process on the RenderServer and collection view has already done its layout pass by the time the animation begins, thus removing the cells.
There are some workarounds I thought of, like expanding the collection view frame beyond the top edge of the superview, or moving it with a transform, but these seem very hacky.
Have you ever encountered this issue and do you have any ideas on how to fix this?

Simulator.Screen.Recording.-.iPhone.14.-.2023-02-16.at.17.00.00.mp4

[ekazaev]

@enarity Hi, Thank you.

Yep. I see the same issue and Ive seen it before. And your explanation is they way how I understand the problem. I couldn't find good enough solution for that issue and it looks not so noticeable on the device with the normal animation speed so I considered it to be a necessary evil and dropped fighting it. It was quite some time ago, but as far as I remember changing the content offset itself also causes extra layoutIfNeeded() call so it wasn't a way to go either.

If you will come across a good solution for that issue - please share and Ill add it to the library.

[ekazaev]

@enarity

Forgot to mention - you can always achieve such result using setContentOffset animated

It is a demo code below but you got the idea. Just replace it in the Example app.

            // Blocks possible updates when keyboard is being hidden interactively
            currentInterfaceActions.options.insert(.changingContentInsets)
            UIView.animate(withDuration: info.animationDuration, animations: {
                self.collectionView.performBatchUpdates({
                    self.collectionView.contentInset.bottom = newBottomInset
                    self.collectionView.scrollIndicatorInsets.bottom = newBottomInset
                }, completion: nil)

                self.collectionView.setContentOffset(CGPoint(x: 0, y: self.collectionView.contentOffset.y + newBottomInset), animated: true)

//                if let positionSnapshot, !self.isUserInitiatedScrolling {
//                    self.chatLayout.restoreContentOffset(with: positionSnapshot)
//                }
                if #available(iOS 13.0, *) {
                } else {
                    // When contentInset is changed programmatically IOs 13 calls invalidate context automatically.
                    // this does not happen in ios 12 so we do it manually
                    self.collectionView.collectionViewLayout.invalidateLayout()
                }

[enarity]

@ekazaev
Thanks for your reply. This approach is suboptimal because the animation parameters in this case do not match the keyboard animation parameters, and the calculations must be more involved to exactly match the content offset like your restoreContentOffset approach does (see how the bottom edge of the visible content changes after the keyboard appears in your example gif).
I'm still very actively looking for an optimal solution and surely will share if I ever find one.

[ekazaev]

@enarity The thing that comes to my head is to try something related to CADisplayLink related solution. But it would lay outside of the layout itself. The problem is that set setContentOffset(_ contentOffset: CGPoint, animated: Bool) as soon as you say animated: false it will also call layoutIfNeeded and give you the same effect.

[ekazaev]

@enarity Ill close this issue as it is not exactly related to the layout itself. But please contact me if youll find a good solution