import UIKit

class Test: UIView {

    var vStackView: UIStackView = {
        let sV = UIStackView()
        sV.axis = .vertical
        sV.translatesAutoresizingMaskIntoConstraints = false
        sV.alignment = .center
        sV.distribution = .fillEqually
        return sV
    }()

    var rStackViews: [UIStackView] = (1...4).map { _ in UIStackView() }

    var views: [UIView] = (1...16).map { _ in UIView() }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    init() {

        super.init(frame: .zero)

        self.translatesAutoresizingMaskIntoConstraints = false

       for r in rStackViews {
            r.axis = .horizontal
            r.translatesAutoresizingMaskIntoConstraints = false
            r.alignment = .center
            r.distribution = .fillEqually
            vStackView.addArrangedSubview(r)
        }

        rStackViews.forEach({
            $0.widthAnchor.constraint(equalTo:vStackView.widthAnchor).isActive = true
        })

        for (i, v) in views.enumerated() {
            v.backgroundColor = .darkGray
            v.translatesAutoresizingMaskIntoConstraints = false
            v.heightAnchor.constraint(equalToConstant: 100).isActive = true
            v.widthAnchor.constraint(equalToConstant: 100).isActive = true
            v.layer.borderColor = UIColor.green.cgColor
            v.layer.borderWidth = 1
            rStackViews[i/4].addArrangedSubview(v)
        }



        self.addSubview(vStackView)

        let panGesture = UIPanGestureRecognizer(target: self, action: #selector(panDetected))
        self.gestureRecognizers = [panGesture]

        // debug
        self.layer.borderColor = UIColor.red.cgColor
        self.layer.borderWidth = 1

    }

    @objc func panDetected(_ recoginzer: UIPanGestureRecognizer) {
        let transl = recoginzer.translation(in: self.superview)

        for v in views {
            if v.point(inside: transl.self, with: nil) {
                v.backgroundColor = .red
            } else {
                v.backgroundColor = .darkGray
            }
        }
    }

}