import UIKit
final class LinearGradientLayer: CALayer {
    enum Direction {
        case vertical
        case horizontal
        case custom(start: CGPoint, end: CGPoint)
        var points: (start: CGPoint, end: CGPoint) {
            switch self {
            case .vertical:
                return (CGPoint(x: 0.5, y: 0.0), CGPoint(x: 0.5, y: 1.0))
            case .horizontal:
                return (CGPoint(x: 0.0, y: 0.5), CGPoint(x: 1.0, y: 0.5))
            case let .custom(start, end):
                return (start, end)
            }
        }
    }
    var direction: Direction = .vertical
    var colorSpace = CGColorSpaceCreateDeviceRGB()
    var colors: [CGColor]?
    var locations: [CGFloat]?
    var options: CGGradientDrawingOptions = CGGradientDrawingOptions(rawValue: 0)
    required override init() {
        super.init()
        masksToBounds = true
        needsDisplayOnBoundsChange = true
    }
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
    required override init(layer: Any) {
        super.init(layer: layer)
    }
    override func draw(in ctx: CGContext) {
        ctx.saveGState()
        guard let colors = colors, let gradient = CGGradient(colorsSpace: colorSpace,
                                                             colors: colors as CFArray, locations: locations) else { return }
        let points = direction.points
        ctx.drawLinearGradient(
            gradient,
            start: transform(points.start),
            end: transform(points.end),
            options: options
        )
    }
    private func transform(_ point: CGPoint) -> CGPoint {
        return CGPoint(x: bounds.width * point.x, y: bounds.height * point.y)
    }
}
final class GradientView: UIView {
    lazy var gradientLayer: LinearGradientLayer = {
        let gradientLayer = LinearGradientLayer()
        gradientLayer.colors = [UIColor(red: 81/255, green: 194/255, blue: 138/255, alpha: 1).cgColor, UIColor(red: 96/255, green: 227/255, blue: 161/255, alpha: 1).cgColor]
        return gradientLayer
    }()
    override init(frame: CGRect) {
        super.init(frame: frame)
        layer.insertSublayer(gradientLayer, at: 0)
    }
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        layer.insertSublayer(gradientLayer, at: 0)
    }
    override func layoutSubviews() {
        super.layoutSubviews()
        gradientLayer.frame = bounds
    }
}