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// basic cell with centered label
class QuickCell: UICollectionViewCell {
    let label = UILabel()
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    func commonInit() {
        label.backgroundColor = UIColor(white: 0.95, alpha: 1.0)
        label.translatesAutoresizingMaskIntoConstraints = false
        contentView.addSubview(label)
        NSLayoutConstraint.activate([
            label.centerXAnchor.constraint(equalTo: contentView.centerXAnchor),
            label.centerYAnchor.constraint(equalTo: contentView.centerYAnchor),
        ])
    }
}
 
// tap anywhere outside the collection view to add 2-to-7 new items
// tap any cell to reset to Zero items
class QuickTestVC: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {
    
    var photos: [Int] = []
    
    var collectionView: UICollectionView!
 
    let colors: [UIColor] = [
        .red, .green, .blue, .cyan, .yellow, .magenta,
    ]
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let fl = UICollectionViewFlowLayout()
        fl.itemSize = .init(width: 80.0, height: 40.0)
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: fl)
        collectionView.backgroundColor = .systemYellow
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(collectionView)
        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([
            collectionView.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            collectionView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            collectionView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            collectionView.heightAnchor.constraint(equalToConstant: 300.0),
        ])
        
        collectionView.register(QuickCell.self, forCellWithReuseIdentifier: "c")
        collectionView.dataSource = self
        collectionView.delegate = self
    }
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        let startVal: Int = self.photos.count
        let newPhotos: [Int] = Array(startVal..<startVal+3)
        add(photos: newPhotos)
    }
    func add(photos: [Int]) {
        let previousCount = self.photos.count
        self.photos.append(contentsOf: photos)
        
        let indexPaths = (0..<photos.count).map { IndexPath(item: previousCount +  $0, section: 0) }
        collectionView.insertItems(at: indexPaths)
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        print("numberOfItemsInSection:", self.photos.count)
        return self.photos.count
    }
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let c = collectionView.dequeueReusableCell(withReuseIdentifier: "c", for: indexPath) as! QuickCell
        c.label.text = "\(self.photos[indexPath.item])"
        c.contentView.backgroundColor = colors[indexPath.item % colors.count]
        return c
    }
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        self.photos = []
        collectionView.reloadData()
    }
    
}