class GradientButton: UIButton {
    let gradient: CAGradientLayer = CAGradientLayer()
    internal override init(frame: CGRect) {
        super.init(frame: frame)
    }
    internal required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
    required init(_ colors: [UIColor], locations: [NSNumber]?) {
        super.init(frame: .zero)
        applyGradient(colors,locations:locations)
    }
    func applyGradient(_ colors: [UIColor], locations: [NSNumber]?) {
        gradient.colors = colors
        gradient.locations = locations
        gradient.startPoint = CGPoint(x: 0.0, y: 1.0)
        gradient.endPoint = CGPoint(x: 1.0, y: 1.0)
        layer.insertSublayer(gradient, at: 0)
    }
    override func layoutSublayers(of layer: CALayer) {
        super.layoutSublayers(of: layer)
        gradient.frame = self.bounds
    }
}

let button = GradientButton([.red,.blue], locations: nil)