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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
    }
    
}