// 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()
	}
	
}