Underline button text in Swift

I have UIButton. In interface builder I set its title to be 'Attributed'. How can I make its title to be underlined from code in Swift?

@IBOutlet weak var myBtn: UIButton!

I created a function called on the touchUpInside event of this button:

var attributedString = NSMutableAttributedString(string:"new text")
    var attrs = [
        NSFontAttributeName : UIFont.systemFontOfSize(19.0),
        NSForegroundColorAttributeName : UIColor.redColor()
    ]
    var gString = NSMutableAttributedString(string:"g", attributes:attrs)
    attributedString.appendAttributedString(gString)

    myBtn.titleLabel?.attributedText = attributedString;

But still no result. Also I need to know how to access the underline attribute. Text, size and color stay the same.

Swift 5 / Xcode 12/13

  @IBOutlet weak var myButton: UIButton!
   
  let yourAttributes: [NSAttributedString.Key: Any] = [
      .font: UIFont.systemFont(ofSize: 14),
      .foregroundColor: UIColor.blue,
      .underlineStyle: NSUnderlineStyle.single.rawValue
  ] // .double.rawValue, .thick.rawValue
         
  
  override func viewDidLoad() {
     super.viewDidLoad()
    
     let attributeString = NSMutableAttributedString(
        string: "Your button text",
        attributes: yourAttributes
     )
     myButton.setAttributedTitle(attributeString, for: .normal)
  }

Swift 4 / Xcode 9

  @IBOutlet weak var myButton: UIButton!
   
  let yourAttributes: [NSAttributedStringKey: Any] = [
      NSAttributedStringKey.font: UIFont.systemFont(ofSize: 14),
      NSAttributedStringKey.foregroundColor: UIColor.blue,
      NSAttributedStringKey.underlineStyle: NSUnderlineStyle.styleSingle.rawValue
  ] // .styleDouble.rawValue, .styleThick.rawValue, .styleNone.rawValue             
  
  override func viewDidLoad() {
    super.viewDidLoad()
    
    let attributeString = NSMutableAttributedString(
      string: "Your button text",
      attributes: yourAttributes
    )
    myButton.setAttributedTitle(attributeString, for: .normal)
  }

Swift 3 / Xcode 8

  @IBOutlet weak var myButton: UIButton!

  let yourAttributes: [String: Any] = [
      NSFontAttributeName: UIFont.systemFont(ofSize: 14),
      NSForegroundColorAttributeName: UIColor.white,
      NSUnderlineStyleAttributeName: NSUnderlineStyle.styleSingle.rawValue
  ] // .styleDouble.rawValue, .styleThick.rawValue, .styleNone.rawValue             

   override func viewDidLoad() {
      super.viewDidLoad()

      let attributeString = NSMutableAttributedString(
        string: "Your button text",
        attributes: yourAttributes
      )        
      myButton.setAttributedTitle(attributeString, for: .normal) 
    }

Here you go, just tested it. (works in xCode 7 Beta at least)

@IBOutlet weak var yourButton: UIButton!

var attrs = [
NSFontAttributeName : UIFont.systemFontOfSize(19.0),
NSForegroundColorAttributeName : UIColor.redColor(),
NSUnderlineStyleAttributeName : 1]

var attributedString = NSMutableAttributedString(string:"")

override func viewDidLoad() {
  super.viewDidLoad()

  let buttonTitleStr = NSMutableAttributedString(string:"My Button", attributes:attrs)
  attributedString.appendAttributedString(buttonTitleStr)
  yourButton.setAttributedTitle(attributedString, forState: .Normal)
}