Converta NSCFString em Dicionário Swift

do { 
  let dd =  userInfo["extraData"] as! String  
  let con = try JSONSerialization.jsonObject(with: dd.data(using: .utf8)!, options: []) as! [String:Any]
  print(con["message_id"]) 
catch {
   print(error)
}
Dead Dormouse