private var shadowLayer: CAShapeLayer!
private var cornerRadius: CGFloat = 25.0
private var fillColor: UIColor = .blue // the color applied to the shadowLayer, rather than the view's backgroundColor

override func layoutSubviews() {
    super.layoutSubviews()

    if shadowLayer == nil {
        shadowLayer = CAShapeLayer()

        shadowLayer.path = UIBezierPath(roundedRect: bounds, cornerRadius: cornerRadius).cgPath
        shadowLayer.fillColor = fillColor.cgColor

        shadowLayer.shadowColor = UIColor.black.cgColor
        shadowLayer.shadowPath = shadowLayer.path
        shadowLayer.shadowOffset = CGSize(width: 0.0, height: 1.0)
        shadowLayer.shadowOpacity = 0.2
        shadowLayer.shadowRadius = 3

        layer.insertSublayer(shadowLayer, at: 0)
    }
}