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]
	}
	
}