UICollectionView Cell Overlap

Question

I have a horizontal UICollectionView. I want to make the cells overlap each other by a certain amount of 60 pixels so that the second cells overlaps the first by 60 pixels a

Answer 1

Based on Maik Fruhner's answer. To use:

let someCollectionView = UICollectionView(frame: ,
                            collectionViewLayout: CollectionViewOverlappingLayout())

Code for the custom layout:

class CollectionViewOverlappingLayout: UICollectionViewFlowLayout {

  var overlap: CGFloat = 14


  override init() {
    super.init()
    self.sharedInit()
  }
  required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
    self.sharedInit()
  }
  func sharedInit(){
    self.scrollDirection = .horizontal
    self.minimumInteritemSpacing = 0
  }

  override var collectionViewContentSize: CGSize{
    let xSize = CGFloat(self.collectionView!.numberOfItems(inSection: 0)) * self.itemSize.width
    let ySize = CGFloat(self.collectionView!.numberOfSections) * self.itemSize.height

    var contentSize = CGSize(width: xSize, height: ySize)

    if self.collectionView!.bounds.size.width > contentSize.width {
      contentSize.width = self.collectionView!.bounds.size.width
    }

    if self.collectionView!.bounds.size.height > contentSize.height {
      contentSize.height = self.collectionView!.bounds.size.height
    }

    return contentSize
  }

  override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

    let attributesArray = super.layoutAttributesForElements(in: rect)
    let numberOfItems = self.collectionView!.numberOfItems(inSection: 0)

    for attributes in attributesArray! {
      var xPosition = attributes.center.x
      let yPosition = attributes.center.y

      if attributes.indexPath.row == 0 {
        attributes.zIndex = Int(INT_MAX) // Put the first cell on top of the stack
      } else {
        xPosition -= self.overlap * CGFloat(attributes.indexPath.row)
        attributes.zIndex = numberOfItems - attributes.indexPath.row //Other cells below the first one
      }

      attributes.center = CGPoint(x: xPosition, y: yPosition)
    }

    return attributesArray
  }

  override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
    return UICollectionViewLayoutAttributes(forCellWith: indexPath)
  }
}

Works with swift 5 (and probably 4.x).