Testing UICollectionView in Swift Playground iPad

uicollectionview-playground.playyground

import UIKit
import PlaygroundSupport

let cellSize = CGSize(width: 100, height: 100)

class Cell: UICollectionViewCell {
    var imageView: UIImageView?
    var identifier: String?
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        
        self.initSubview(frame: CGRect(origin: .zero, size: cellSize))
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        self.initSubview(frame: CGRect(origin: .zero, size: cellSize))
    }
    
    override func prepareForReuse() {
        self.identifier = nil
        self.imageView?.image = nil
    }
    
    func initSubview(frame: CGRect) {
        self.backgroundColor = .yellow
        if self.imageView == nil {
            self.contentView.backgroundColor = .gray
            
            self.contentView.frame = CGRect(origin: .zero, size: cellSize)
            self.imageView = UIImageView(frame: CGRect(origin: CGPoint.zero, size: frame.size))
            self.contentView.addSubview(self.imageView!)
            self.imageView?.clipsToBounds = true
            self.imageView?.contentMode = .scaleAspectFill
            self.imageView?.backgroundColor = .gray
            self.imageView?.isOpaque = true
        }
    }
    
}

class MyViewController: UICollectionViewController {
    var allAssets: [String] = []
    override init(collectionViewLayout layout: UICollectionViewLayout) {
        super.init(collectionViewLayout: layout)
        
        self.collectionView?.backgroundColor  = .white
        self.collectionView?.register(Cell.self, forCellWithReuseIdentifier: "cell")
        
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.title = "Loading photos ..."
        
        DispatchQueue.main.asyncAfter(deadline: .now() + 5) {
            for i in 0...900 {
                self.allAssets.append("\(i)")
            }
            self.collectionView?.reloadData()
            self.title = "\(self.allAssets.count) items"
            
        }
    }
    
    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return (self.allAssets.count) ?? 0
    }
    
    override func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }
    
    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! Cell
        
        let asset = self.allAssets[indexPath.row]
        cell.identifier = asset
        
        return cell
    }
}

let layout = UICollectionViewFlowLayout()
layout.minimumInteritemSpacing = 10
layout.minimumLineSpacing = 10
layout.itemSize = cellSize
layout.sectionInset = UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)

PlaygroundPage.current.liveView = UINavigationController(rootViewController: MyViewController(collectionViewLayout: layout))