Cell loaded in cellForItemAt, but not in visibleCells - swift

Question

I have a custom UICollectionView and the cells are loaded in cellForItemAt but when I try to get all the visible cells by using visibleCells I'm not getting all the cells.
For example, in cellForItemAt, I'm setting the alpha of the labels in the cells to 0. When panned, I want the alpha of those labels change to 1:
func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
handleLabel(scrollView, active: true)
}
func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
if pickerIsActive { handleLabel(scrollView, active: false) }
}
private func handleLabel(_ scrollView: UIScrollView, active: Bool) {
guard let pickerView = scrollView as? UICollectionView else { return }
let cells = pickerView.visibleCells.flatMap { $0 as? CustomCell }
panningIsActive = active
UIView.animate(duration: 0.3) {
cells.forEach { $0.label.alpha = $0.isSelected || active ? 1 : 0 }
}
}
And cellForItemAt:
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! CustomCell
cell.label.alpha = 0
return cell
}
What can I do to change all the "loaded" cells instead of just the "visible" cells?

Answer 1

The visibleCells are only the on screen cells. This used to be everything initialized in cellForItem:at: but as of iOS 10 UICollectionView now prefetches to improve scrolling performance (see WWD 2016 video) which maybe why you are having this problem. Anyways it sounds like all you want to do is animate the cells to fade in when they come on screen. You can either move your animation logic to willDisplayCell or subclass UICollectionViewCell. UIColectionViewCell inherits from UIView, so you can override didMoveToSuperView in your UICollectionViewCell and call your animation method there, which will cause the cell to animate as it appears.

Answer 2

I am using Xcode 11.4 and Swift 5, and I had the exactly the same issue: .visibleCells is not giving me all the loaded cells.
By reading #Josh Homann's answer and the comments below, I figured out 2 solutions.
The first solution is same as the solution you reached at: customize cell appearance in collectionView(_:willDisplay:_:) after it's loaded but before it's displayed on the screen.
Another quick and dirty solution is to simply uncheck UICollectionView's 'Prefetch' option in attributes inspector.
This fixes the issue because by disabling prefetching, UICollectionView will stop pre-loading cells that are not displayed on the screen, so .visibleCells are now all the loaded cells. This solution will work fine if you're simply loading static or small local data in the cells. If you're prefetching large data (e.g. images) from network for upcoming cells, you probably need Prefetching Enabled, then solution 1 is your go-to option.

Answer 3

It sounds like you might want to try using layoutAttributesForElements(in:).
You'll need to implement your own collection view layout subclass (rather than using the delegate methods) but I think it will be worth it in the long term.
Rather than manually managing the animations (via UIView.animateWithDuration) you use this method to tell the collection view what properties cells should have at different positions, and as people pan the collection view, the correct properties are automatically applied.
I tried to find a good Swift reference for this, but I could't, but here's a post in Objective-C that you can follow if you want to try this approach:
https://bradbambara.wordpress.com/2014/05/24/getting-started-with-custom-uicollectionview-layouts/