Problems with prepareForSegue method passing data

I'm having a problem guys, I'm new to swift, and I have a method that takes a result from a JSON and I wanted to store it in a local variable and then pass that variable to a viewcontroller next, but the variable when step in the prepareForStack method it picks up the variable as null or when I started it:

To explain better

1 - I instantiated a variable at the beginning of my view class "A"

var id = ""

2 - I did the NSURLSession and got the json from one rest per post and put it inside the dispatch to store in the variable that I instantiated.

func postQueEstouFazendo(v1: NSString, v2: NSString, v3: NSString)
{
    //url para aonde vou mandar o post
    let myUrl = NSURL(string: "http://minhapiqueestoupegando");   

    //inicia a variavel que vai fazer o request
    let request = NSMutableURLRequest(URL:myUrl!);

    //define o metodo do request
    request.HTTPMethod = "POST";// Compose a query string

    //coloca os dados em uma string de dados com um titulo
    let postString = "variavel=\(v1)&variavel2=\(v2)&variavel3=\(v3)";

    //define o encoding do que vai ser passado
    request.HTTPBody = postString.dataUsingEncoding(NSUTF8StringEncoding);

    //inicia o envio dos dados
    let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {
        data, response, error in           

        //se o erro de envio existir mostra um print
        if error != nil
        {
            print("error ao fazer o recolhimento =\(error)")

            return
        }           

        //aqui é a resposta do envio
        // You can print out response object
        print("response = \(response)")

        //aqui é uma outra forma mais completa de visualizar o retorno do envio
        // Print out response body
        let responseString = NSString(data: data!, encoding: NSUTF8StringEncoding)

        print("responseString = \(responseString)")

        //teste
        //Let's convert response sent from a server side script to a NSDictionary object:
        do
        {
            let myJSON =  try NSJSONSerialization.JSONObjectWithData(data!, options: .MutableContainers) as? NSDictionary

            if let parseJSON = myJSON
            {
                // Now we can access value of First Name by its key
                let idretorno = parseJSON["retorno"] as? String

                //faz a chamada da view voltando para a main thread!
                dispatch_async(dispatch_get_main_queue(), {
                      self. id = idretorno!
                })
            }
        }
        catch
        {
            print(error)
        }
    }

    task.resume() // fim do envio e volta pra thread principal ja que a task é uma background thread
}

3 - I made the method PrepareForSegue and its instantiated the destination and put the value of the instantiated variable in view A for the variable that I want to pass to view B

//Segues
override func shouldPerformSegueWithIdentifier(identifier: String, sender: AnyObject?) -> Bool {
  return true
}

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?){       
        if segue.identifier == "meuidentifier"
        {
          var viewA: viewB = segue.destinationViewController as! viewB
          viewA.idb = self.id
          viewA.outravariavel= 0
        }
    }

THIS LINE: viewA.idb = self.id

When it is in this method prepareForSegue it will not what I'm getting it in the JSON it will "" like when I instantiate it.

Can anyone help me?

Thank you!