Get UICollectionViewCell viewport percentage

UICollectionViewExtension.swift

import UIKit

extension UICollectionView {
  var cellOffsetForVisibleItems: [IndexPath: CGFloat] {
    let result = self.indexPathsForVisibleItems
      .map { indexPath in
        [indexPath: UICollectionView.cellOffset(with: self, at: indexPath)]
      }
      .reduce([IndexPath: CGFloat]()) { result, map in
        var newResult = result
        newResult += map
        return newResult
    }
    return result
  }
  
  static func cellOffset(with collectionView: UICollectionView, at indexPath: IndexPath) -> CGFloat {
    if let cell = collectionView.cellForItem(at: indexPath) {
      return cellOffset(with: collectionView, for: cell)
    }
    return 0.0
  }
  
  static func cellOffset(with collectionView: UICollectionView, for cell: UICollectionViewCell) -> CGFloat {
    let cellFrame = cell.frame
    let cellFrameInTable = collectionView.convert(cellFrame, to: collectionView.superview)
    let cellOffset = cellFrameInTable.origin.y + cellFrameInTable.size.height
    let tableHeight = collectionView.bounds.size.height + cellFrameInTable.size.height
    let cellOffsetFactor = cellOffset / tableHeight
    return cellOffsetFactor
  }
}