The answer above is causing an error in my build environment.
This doesn't work in Swift 4.0:
attributedText.addAttribute(NSUnderlineStyleAttributeName,
value: NSUnderlineStyle.styleSingle.rawValue,
range: textRange)
Try this instead:
attributedText.addAttribute(NSAttributedStringKey.underlineStyle,
value: NSUnderlineStyle.styleSingle.rawValue,
range: textRange)
hope this helps someone.