UIView нижняя граница?

Быстро 5.1. Используйте с двумя расширениями, возвратом метода CALayer, таким образом, Вы снова использовали бы его для обновления кадров.

enum Border: Int {
    case top = 0
    case bottom
    case right
    case left
}

extension UIView {
    func addBorder(for side: Border, withColor color: UIColor, borderWidth: CGFloat) -> CALayer {
       let borderLayer = CALayer()
       borderLayer.backgroundColor = color.cgColor

       let xOrigin: CGFloat = (side == .right ? frame.width - borderWidth : 0)
       let yOrigin: CGFloat = (side == .bottom ? frame.height - borderWidth : 0)

       let width: CGFloat = (side == .right || side == .left) ? borderWidth : frame.width
       let height: CGFloat = (side == .top || side == .bottom) ? borderWidth : frame.height

       borderLayer.frame = CGRect(x: xOrigin, y: yOrigin, width: width, height: height)
       layer.addSublayer(borderLayer)
       return borderLayer
    }
}

extension CALayer {
    func updateBorderLayer(for side: Border, withViewFrame viewFrame: CGRect) {
        let xOrigin: CGFloat = (side == .right ? viewFrame.width - frame.width : 0)
        let yOrigin: CGFloat = (side == .bottom ? viewFrame.height - frame.height : 0)

        let width: CGFloat = (side == .right || side == .left) ? frame.width : viewFrame.width
        let height: CGFloat = (side == .top || side == .bottom) ? frame.height : viewFrame.height

        frame = CGRect(x: xOrigin, y: yOrigin, width: width, height: height)
    }
}