CollectionViewLayout.swift

import Foundation
import UIKit

@objc class CollectionViewLayout: UICollectionViewLayout {
    
    var contentSize: CGSize = .zero
    var cachedAttributes = [IndexPath: UICollectionViewLayoutAttributes]()
    
    override func prepare() {
        
        // Reset previous
        contentSize = .zero
        cachedAttributes = [IndexPath: UICollectionViewLayoutAttributes]()
        
        guard let collectionView = collectionView else {
            return
        }
        
        // Iterate
        for section in 0 ..< collectionView.numberOfSections {
            for item in 0 ..< collectionView.numberOfItems(inSection: section) {
                
                // ...Determine the frame of your cell...
                let itemFrame: CGRect = .zero
                
                // Set the frame
                let indexPath = IndexPath(item: item, section: section)
                let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
                attributes.frame = itemFrame
                
                // Cache
                cachedAttributes[indexPath] = attributes
            }
        }
        
        // Store computed content size
        contentSize = .zero
    }
    
    override var collectionViewContentSize: CGSize {
        return contentSize
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        var attributeList = [UICollectionViewLayoutAttributes]()
        
        for (_, attributes) in cachedAttributes {
            if attributes.frame.intersects(rect) {
                attributeList.append(attributes)
            }
        }
        
        return attributeList
    }
    
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return cachedAttributes[indexPath]
    }
}