How to animate UICollectionview cells as i scroll - ios

Question

How can i animate Horizontal Collectionview as it is scrolled i changed alpha to 0 in the cell and in cellForItemAt i animate the alpha back to 1 but that only happens when the Collectionview is scrolled through the first time here is the code i have tried
UIView.animate(withDuration: 0.8) {
cell.imageView.alpha = 1
cell.onboardLabel.alpha = 1
}
I also tried to do this in scrollViewDidEndDecelerating but still not working
let index = Int(scrollView.contentOffset.x) / Int(scrollView.frame.width)
let indexPath = IndexPath(item: index, section: 0)
let cell = collectionView.cellForItem(at: indexPath) as? OnboardingCell
UIView.animate(withDuration: 0.8) {
cell?.imageView.alpha = 1
cell?.onboardLabel.alpha = 1
}

Answer 1

Swift 4:
Use this function from UICollectionViewDelegate:
override func collectionView(_ collectionView: UICollectionView,
willDisplay cell: UICollectionViewCell,
forItemAt indexPath: IndexPath) {
cell.alpha = 0
UIView.animate(withDuration: 0.8) {
cell.alpha = 1
}
}

Answer 2

Firstly you need to know which cells are visible so set this variable at the top of the file.
var visibleIndexPath: IndexPath? = nil
In scrollViewDidEndDecelerating use this code to set the visibleIndexPath:
func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
var visibleRect = CGRect()
visibleRect.origin = collectionView.contentOffset
visibleRect.size = collectionView.bounds.size
let visiblePoint = CGPoint(x: visibleRect.midX, y: visibleRect.midY)
if let visibleIndexPath = collectionView.indexPathForItem(at: visiblePoint) {
self.visibleIndexPath = visibleIndexPath
}
}
Now that you have a visibleIndexPath you can animate the cell in the willDisplay cell function.
func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
if let visibleIndexPath = self.visibleIndexPath {
// This conditional makes sure you only animate cells from the bottom and not the top, your choice to remove.
if indexPath.row > visibleIndexPath.row {
cell.contentView.alpha = 0.3
cell.layer.transform = CATransform3DMakeScale(0.5, 0.5, 0.5)
// Simple Animation
UIView.animate(withDuration: 0.5) {
cell.contentView.alpha = 1
cell.layer.transform = CATransform3DScale(CATransform3DIdentity, 1, 1, 1)
}
}
}
}