How to underline an UITextField

enum LINE_POSITION {
    case top
    case bottom
}

extension UIView {
    func addLine(position: LINE_POSITION = .bottom, color: UIColor = UIColor.lightGray, width: Double = 1.0) {
        let lineView = UIView()
        lineView.backgroundColor = color
        lineView.translatesAutoresizingMaskIntoConstraints = false
        self.addSubview(lineView)
        
        let metrics = ["width" : NSNumber(value: width)]
        let views = ["lineView" : lineView]
        self.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|[lineView]|", options:NSLayoutConstraint.FormatOptions(rawValue: 0), metrics:metrics, views:views))
        
        switch position {
        case .top:
            self.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|[lineView(width)]", options:NSLayoutConstraint.FormatOptions(rawValue: 0), metrics:metrics, views:views))
            break
        case .bottom:
            self.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:[lineView(width)]|", options:NSLayoutConstraint.FormatOptions(rawValue: 0), metrics:metrics, views:views))
            break
        }
    }
}