String to UIColor conversion in Swift

String+asUIColor.swift

public extension String {
  var asUIColor: UIColor? {
    var cString = self.trimmingCharacters(in: .whitespacesAndNewlines).uppercased()

    if cString.hasPrefix("#") {
      cString.remove(at: cString.startIndex)
    }

    let allowedCharacters = CharacterSet(charactersIn: "0123456789ABCDEF")
    if cString.rangeOfCharacter(from: allowedCharacters.inverted) != nil {
      return nil
    }

    switch cString.count {
    case 6:
      var rgbValue: UInt64 = 0
      Scanner(string: cString).scanHexInt64(&rgbValue)

      return UIColor(
        red: CGFloat((rgbValue & 0xFF0000) >> 16) / 255.0,
        green: CGFloat((rgbValue & 0x00FF00) >> 8) / 255.0,
        blue: CGFloat(rgbValue & 0x0000FF) / 255.0,
        alpha: CGFloat(1.0)
      )

    case 8:
      var argbValue: UInt64 = 0
      Scanner(string: cString).scanHexInt64(&argbValue)

      return UIColor(
        red: CGFloat((argbValue & 0xFF0000) >> 16) / 255.0,
        green: CGFloat((argbValue & 0x00FF00) >> 8) / 255.0,
        blue: CGFloat(argbValue & 0x0000FF) / 255.0,
        alpha: CGFloat((argbValue & 0xFF000000) >> 24) / 255.0
      )

    default:
      return nil
    }
  }
}