In UICollectionView not obeying zIndex of UICollectionViewLayoutAttributes

Question

I'm trying to make the top cell of the collection be behind all the other cells by the zIndex factor

┌──────────┐ 
│          │ 
│  Cell 0  │ 
│┌─────────┴┐
└┤          │
 │  Cell 4  │
 │          │
 └──────────┘
 ┌──────────┐
 │          │
 │  Cell 5  │
 │          │
 └──────────┘
 ┌──────────┐
 │          │
 │  Cell 6  │
 │          │
 └──────────┘

In custom layout in method prepare I add Zindex as follows

override func prepare() {
  super.prepare()
  /// Some code
  
  let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
  attributes.zIndex = zIndex
  
  /// Some code
}

Next, I add the activation of this zIndex in UICollectionViewCell

override func apply(_ layoutAttributes: UICollectionViewLayoutAttributes) {
    super.apply(layoutAttributes)
    layer.zPosition = CGFloat(layoutAttributes.zIndex)
}

When reusing a cell. The topmost cell, which should be in the background, becomes in the foreground

Tell me who faced this problem. What am I doing wrong 🙏

Answer 1

You should create your own UICollectionViewFlowLayout subclass and use the following method to set attributes:

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    let attributes = super.layoutAttributesForElements(in: rect)
    let backMostCellIndex = 0
    
    guard  let attributes = attributes else {
        return nil
    }

    for attribute in attributes {
        if attribute.indexPath.item == backMostCellIndex {
            attribute.zIndex = 0
        } else {
            attribute.zIndex = 1
        }
    }
    return attributes
}