class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let g = view.safeAreaLayoutGuide

        let gv = Gradient()
        gv.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(gv)

        NSLayoutConstraint.activate([
            gv.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            gv.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            gv.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            gv.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -20.0),
        ])

        //gv.diagonalMode = true
        //gv.horizontalMode = true

        view.backgroundColor = .systemYellow
    }

}

class Gradient: UIView {
    var startColor:   UIColor = .white { didSet { updateColors() }}
    var endColor:     UIColor = .red { didSet { updateColors() }}
    var horizontalMode:  Bool =  false { didSet { updatePoints() }}
    var diagonalMode:    Bool =  false { didSet { updatePoints() }}

    override class var layerClass: AnyClass { CAGradientLayer.self }

    var gradientLayer: CAGradientLayer { layer as! CAGradientLayer }

    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    private func commonInit() {
        updatePoints()
        updateLocations()
        updateColors()
    }

    func updatePoints() {
        if horizontalMode {
            gradientLayer.startPoint = diagonalMode ? .init(x: 1, y: 0) : .init(x: 0, y: 0.5)
            gradientLayer.endPoint   = diagonalMode ? .init(x: 0, y: 1) : .init(x: 1, y: 0.5)
        } else {
            gradientLayer.startPoint = diagonalMode ? .init(x: 0, y: 0) : .init(x: 0.5, y: 0)
            gradientLayer.endPoint   = diagonalMode ? .init(x: 1, y: 1) : .init(x: 0.5, y: 1)
        }
    }
    func updateLocations() {
        gradientLayer.locations = [0, 0.5, 0.5, 1.0]
    }
    func updateColors() {
        gradientLayer.colors = [startColor.cgColor, startColor.cgColor, endColor.cgColor, endColor.cgColor]
    }

}