// simpan diluar class
extension UITextField{

  func underline(){
    let border = CALayer()
    let width = CGFloat(2.0)
    border.borderColor = UIColor.lightGray.cgColor
    border.frame = CGRect(x: 0, y: self.frame.size.height - width, width:  self.frame.size.width, height: self.frame.size.height)
    border.borderWidth = width
    borderStyle = UITextBorderStyle.none;
    self.layer.addSublayer(border)
    self.layer.masksToBounds = true
  }

  func paddingLeft(size: CGFloat){
    let width = CGFloat(size)
    let paddingView = UIView(frame: CGRect(x: 0, y: 0, width:  width, height: self.frame.size.height))
    leftView = paddingView
    leftViewMode = UITextFieldViewMode.always
  }

}

// implementasi di ViewController
@IBOutlet weak var txtUsername: UITextField!
@IBOutlet weak var txtPassword: UITextField!

override func viewDidLoad() {
  super.viewDidLoad()

  // configure uitextfield
  txtUsername.underline()
  txtUsername.paddingLeft(size: 0)
  txtPassword.underline()
  txtPassword.paddingLeft(size: 0)
  txtPassword.isSecureTextEntry = true
}