Make UICollectionViewFlowLayout look like cells are hiding to the background - ios

Question

I came across this demo and really want to learn how to do this:
So far, I've created a subclass to UICollectionViewFlowLayout that fades out the start and end of the cells. Taken from https://gist.github.com/vinhnx/bb1354b247ebfe3790563173ac72baa9
This is the part that fades out the start and end cells
for attrs in attributes {
if attrs.frame.intersects(rect) {
let distance = visibleRect.midY - attrs.center.y
let normalizedDistance = abs(distance) / (visibleRect.height * fadeFactor)
let fade = 1 - normalizedDistance
attrs.alpha = fade
}
}
How do I make the cells that is at the top stand still and not continue to slide up?
Full code of the function handling the fade:
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
let attributesSuper: [UICollectionViewLayoutAttributes] = super.layoutAttributesForElements(in: rect) as [UICollectionViewLayoutAttributes]!
if let attributes = NSArray(array: attributesSuper, copyItems: true) as? [UICollectionViewLayoutAttributes]{
var visibleRect = CGRect()
visibleRect.origin = collectionView!.contentOffset
visibleRect.size = collectionView!.bounds.size
for attrs in attributes {
if attrs.frame.intersects(rect) {
let distance = visibleRect.midY - attrs.center.y
let normalizedDistance = abs(distance) / (visibleRect.height * fadeFactor)
let fade = 1 - normalizedDistance
attrs.alpha = fade
print(fade)
}
}
return attributes
}else{
return nil
}
}
The look of my app right now: