UIView extension to programmatically round specific corners of an UIView

UIView+.swift

extension UIView {
    /**
     Rounds the given set of corners to the specified radius
     
     - parameter corners: Corners to round
     - parameter radius:  Radius to round to
     */
    func roundCorners(corners: UIRectCorner, radius: CGFloat) {
        _roundCorners(corners, radius: radius)
    }
    
    /**
     Rounds the given set of corners to the specified radius with a border
     
     - parameter corners:     Corners to round
     - parameter radius:      Radius to round to
     - parameter borderColor: The border color
     - parameter borderWidth: The border width
     */
    func roundCorners(corners: UIRectCorner, radius: CGFloat, borderColor: UIColor, borderWidth: CGFloat) {
        let mask = _roundCorners(corners, radius: radius)
        addBorder(mask, borderColor: borderColor, borderWidth: borderWidth)
    }
}

private extension UIView {
    func _roundCorners(corners: UIRectCorner, radius: CGFloat) -> CAShapeLayer {
        let path = UIBezierPath(roundedRect: bounds, byRoundingCorners: corners, cornerRadii: CGSize(width: radius, height: radius))
        let mask = CAShapeLayer()
        mask.path = path.CGPath
        self.layer.mask = mask
        return mask
    }
    
    func addBorder(mask: CAShapeLayer, borderColor: UIColor, borderWidth: CGFloat) {
        let borderLayer = CAShapeLayer()
        borderLayer.path = mask.path
        borderLayer.fillColor = UIColor.clearColor().CGColor
        borderLayer.strokeColor = borderColor.CGColor
        borderLayer.lineWidth = borderWidth
        borderLayer.frame = bounds
        layer.addSublayer(borderLayer)
    }
}