class MoveImageScrollVC: UIViewController, UIScrollViewDelegate {

    var exampleConstraint: NSLayoutConstraint!

    override func viewDidLoad() {
        super.viewDidLoad()

        let scrollView = UIScrollView()
        scrollView.backgroundColor = .systemYellow
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(scrollView)

        let g = view.safeAreaLayoutGuide
        let cg = scrollView.contentLayoutGuide
        let fg = scrollView.frameLayoutGuide

        NSLayoutConstraint.activate([
            // constrain scroll view to safe area, inset by 20-points
            scrollView.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            scrollView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            scrollView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -20.0),
            scrollView.widthAnchor.constraint(equalToConstant: 100.0),
        ])

        let tallLabel = UILabel()
        tallLabel.backgroundColor = .cyan
        tallLabel.numberOfLines = 0
        tallLabel.textAlignment = .center
        tallLabel.font = .systemFont(ofSize: 24.0, weight: .light)

        var str: String = "1"
        for i in 2...50 {
            str += "\n\(i)"
        }
        tallLabel.text = str

        tallLabel.translatesAutoresizingMaskIntoConstraints = false
        scrollView.addSubview(tallLabel)

        NSLayoutConstraint.activate([
            tallLabel.topAnchor.constraint(equalTo: cg.topAnchor, constant: 8.0),
            tallLabel.leadingAnchor.constraint(equalTo: cg.leadingAnchor, constant: 8.0),
            tallLabel.trailingAnchor.constraint(equalTo: cg.trailingAnchor, constant: 0.0),
            tallLabel.bottomAnchor.constraint(equalTo: cg.bottomAnchor, constant: -8.0),
            tallLabel.widthAnchor.constraint(equalTo: fg.widthAnchor, constant: -16.0),
        ])

        let anchorView = UILabel()
        anchorView.backgroundColor = .yellow
        anchorView.textAlignment = .center
        anchorView.text = "Anchor View"

        let constrainedView = UIImageView()
        if let img = UIImage(named: "sampleImage") {
            constrainedView.image = img
        }

        anchorView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(anchorView)
        constrainedView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(constrainedView)

        exampleConstraint = constrainedView.bottomAnchor.constraint(equalTo: anchorView.topAnchor, constant: 0.0)
        exampleConstraint.priority = .required - 1

        NSLayoutConstraint.activate([

            anchorView.leadingAnchor.constraint(equalTo: scrollView.trailingAnchor, constant: 20.0),
            anchorView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            anchorView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor, constant: 0.0),
            anchorView.heightAnchor.constraint(equalToConstant: 40.0),

            exampleConstraint,
            constrainedView.topAnchor.constraint(greaterThanOrEqualTo: scrollView.topAnchor),
            constrainedView.leadingAnchor.constraint(equalTo: anchorView.leadingAnchor, constant: 0.0),
            constrainedView.trailingAnchor.constraint(equalTo: anchorView.trailingAnchor, constant: 0.0),
            constrainedView.heightAnchor.constraint(equalToConstant: 120.0),
        ])

        scrollView.delegate = self
    }

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let up = scrollView.contentOffset.y
        print("up \(up)")
        exampleConstraint.constant = -up
    }

}