class MyCellView: UICollectionViewCell {
	
	let theLabel = UILabel()
	
	override init(frame: CGRect) {
		super.init(frame: frame)
		commonInit()
	}
	required init?(coder: NSCoder) {
		super.init(coder: coder)
		commonInit()
	}
	func commonInit() -> Void {
		theLabel.translatesAutoresizingMaskIntoConstraints = false
		theLabel.textAlignment = .center
		theLabel.backgroundColor = UIColor(white: 0.9, alpha: 1.0)
		contentView.addSubview(theLabel)
		let g = contentView.layoutMarginsGuide
		NSLayoutConstraint.activate([
			theLabel.centerXAnchor.constraint(equalTo: g.centerXAnchor),
			theLabel.centerYAnchor.constraint(equalTo: g.centerYAnchor),
		])
	}
	
}

class CollViewVC: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
	
	let cellId: String = "c"
	
	let numItems: Int = 100
	
	let colors: [UIColor] = [
		.systemRed, .systemBlue, .systemGreen,
		.systemPink, .systemYellow, .systemTeal
	]
	
	lazy var navToLabel: UILabel = {
		let v = UILabel()
		v.numberOfLines = 0
		v.backgroundColor = .green
		v.text = "Nav To Label: \(numItems) Items (zero-based)"
		v.translatesAutoresizingMaskIntoConstraints = false
		return v
	}()
	
	let scrollView: UIScrollView = {
		let v = UIScrollView()
		v.translatesAutoresizingMaskIntoConstraints = false
		return v
	}()
	
	public lazy var myCollection : UICollectionView = {
		let routine = UICollectionView(frame: CGRect.zero, collectionViewLayout: UICollectionViewFlowLayout());
		let routineLayout = UICollectionViewFlowLayout()
		routineLayout.scrollDirection = .horizontal
		routine.collectionViewLayout = routineLayout
		routine.decelerationRate = UIScrollView.DecelerationRate.fast
		routine.translatesAutoresizingMaskIntoConstraints = false
		routine.delegate = self
		routine.dataSource = self
		routine.backgroundColor = .clear
		routine.register(MyCellView.self, forCellWithReuseIdentifier: cellId)
		routine.isPagingEnabled = true
		routine.showsHorizontalScrollIndicator = false
		routine.showsVerticalScrollIndicator = false
		routine.layer.cornerRadius = 20
		routine.layer.masksToBounds = true
		return routine
	}()
	
	override func viewDidLoad() {
		super.viewDidLoad()
		
		view.addSubview(myCollection)
		
		view.addSubview(scrollView)
		scrollView.addSubview(navToLabel)
		
		let cg = scrollView.contentLayoutGuide
		let fg = scrollView.frameLayoutGuide
		
		NSLayoutConstraint.activate([
			
			myCollection.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
			myCollection.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor),
			myCollection.widthAnchor.constraint(equalTo: view.safeAreaLayoutGuide.widthAnchor, multiplier: 0.9),
			myCollection.heightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.heightAnchor, multiplier: 0.40),

			scrollView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 40.0),
			scrollView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: -40.0),
			scrollView.topAnchor.constraint(equalTo: myCollection.bottomAnchor, constant: 40.0),
			scrollView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: -20.0),

			navToLabel.topAnchor.constraint(equalTo: cg.topAnchor, constant: 0.0),
			navToLabel.leadingAnchor.constraint(equalTo: cg.leadingAnchor, constant: 0.0),
			navToLabel.trailingAnchor.constraint(equalTo: cg.trailingAnchor, constant: 0.0),
			navToLabel.bottomAnchor.constraint(equalTo: cg.bottomAnchor, constant: 0.0),
			
			navToLabel.widthAnchor.constraint(equalTo: fg.widthAnchor, constant: 0.0),
			
		])
	}
	
	func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
		return 0
	}
	
	func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
		return CGSize(width: collectionView.frame.width, height: collectionView.frame.height)
	}
	
	func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
		let navigatedTo =  Int(targetContentOffset.pointee.x) / Int(myCollection.frame.width)
		guard let t = navToLabel.text else {
			return
		}
		let str = t + "\n\(targetContentOffset.pointee.x) / \(navigatedTo)"
		navToLabel.text = str
	}
	
	func numberOfSections(in collectionView: UICollectionView) -> Int {
		return 1
	}
	func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
		return numItems
	}
	func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
		let c = collectionView.dequeueReusableCell(withReuseIdentifier: cellId, for: indexPath) as! MyCellView
		c.contentView.backgroundColor = colors[indexPath.item % colors.count]
		c.theLabel.text = "\(indexPath.item)"
		return c
	}
	
}