var frame: CGRect                // The frame rectangle of the item.

var bounds: CGRect               // The bounds of the item.

var center: CGPoint              // The center point of the item.

var size: CGSize                 // The size of the item.

var transform3D: CATransform3D   // The 3D transform of the item.

var transform: CGAffineTransform // The affine transform of the item.

var alpha: CGFloat               // The transparency of the item.

var zIndex: Int                  // Specifies the item’s position on the z axis.

var isHidden: Bool               // Determines whether the item is currently displayed.

var contentSize: CGSize = .zero
var cachedAttributes = [IndexPath: UICollectionViewLayoutAttributes]()

override func prepare() {
  
  contentSize = .zero // Clear out previous results
  cachedAttributes = [IndexPath: UICollectionViewLayoutAttributes]()
  
  for section in 0 ..< collectionView.numberOfSections {
    for item in 0 ..< collectionView.numberOfItems(inSection: section) {
      let itemFrame = // ...Compute the frame of your cell...

      // Create the layout attributes and set the frame
      let indexPath = IndexPath(item: item, section: section)
      let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
      attributes.frame = itemFrame

      // Store the results
      cachedAttributes[indexPath] = attributes
    }
  }
  
  contentSize = // Store computed content size
}

private var contentSize: CGSize = .zero

override var collectionViewContentSize: CGSize {
  return contentSize
}

var cachedAttributes = [IndexPath: UICollectionViewLayoutAttributes]()

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

  var attributeList = [UICollectionViewLayoutAttributes]()

  for (_, attributes) in cachedAttributes {
    if attributes.frame.intersects(rect) {
      attributeList.append(attributes)
    }
  }

  return attributeList
}

var cachedAttributes = [IndexPath: UICollectionViewLayoutAttributes]()

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