UIKeyboardTaskQueue may only be called from the main thread

You should call your UIAlertController on the main thread because you're dealing with the ui.

Swift 2.X

dispatch_async(dispatch_get_main_queue(),{ 
               var alert = UIAlertController(title: "Chargement", message: "Envoi des informations...", preferredStyle: UIAlertControllerStyle.Alert)
                viewController.presentViewController(alert, animated: true, completion: nil)

                answer = NSString(data: data!, encoding: NSUTF8StringEncoding)!
                print(answer)
             var complete = false
    alert.dismissViewControllerAnimated(true, completion: { () -> Void in
        complete = true
    })
    while(!complete)
    {

    }
}

Swift 4.2

DispatchQueue.main.async {
    var alert = UIAlertController(title: "Chargement", message: "Envoi des informations...", preferredStyle: UIAlertControllerStyle.Alert)
    viewController.presentViewController(alert, animated: true, completion: nil)

    let answer = String(data: data!, encoding: .utf8)!
    print(answer)
    var complete = false
    alert.dismissViewControllerAnimated(true, completion: { () -> Void in
        complete = true
    })
    while(!complete)
    {

    }
}

For swift 3.x and 4.x

DispatchQueue.main.async(execute: {
// work Needs to be done
 })