函数只能在主线程中调用，为什么？

在下面的代码中，我调用函数 displayMyAlertMessage() 两次。我调用的第一个函数工作完美，但第二个函数给我一个错误，指出该函数只能从主线程调用。在我的情况下我该如何做到这一点？

我正在使用这段代码：

@IBAction func loginButtonTapped(sender: AnyObject) {
    result2Value = ""
    let userEmail = userEmailTextField.text
    let userPassword = userPasswordTextField.text

    if userPassword.isEmpty || userPassword.isEmpty {
        displayMyAlertMessage("All fields are required")
return;
    }
    //send user data to server side
    let myUrl = NSURL(string: "http://www.avanta.com/extra/test-userLogin.php")
    let request = NSMutableURLRequest(URL:myUrl!)
    request.HTTPMethod = "POST"

    let postString = "email=\(userEmail)&password=\(userPassword)"
    request.HTTPBody = postString.dataUsingEncoding(NSUTF8StringEncoding)

    let task = NSURLSession.sharedSession().dataTaskWithRequest(request){
        data, response, error in
        if error != nil{
            println("error\(error)")
            return
        }
        var err:NSError?
        var json = NSJSONSerialization.JSONObjectWithData(data, options: .MutableContainers, error: &err) as? NSDictionary
        if let parseJSON = json { 
            var resultValue:String = parseJSON["status"] as! String!
            println("result: \(resultValue)")

            if (resultValue == "Success") {
            //login succesful
                NSUserDefaults.standardUserDefaults().setBool(true, forKey: "isUserLogin")
                NSUserDefaults.standardUserDefaults().synchronize()

                self.dismissViewControllerAnimated(true, completion: nil)
               // return
            }
            else{
              self.displayMyAlertMessage("Wrong name or password")
            }
        }
    }
    task.resume()
}

调用该函数：

func displayMyAlertMessage(userMessage:String){
    var myAlert = UIAlertController(title:"Alert", message:userMessage, preferredStyle: UIAlertControllerStyle.Alert)
    let okAction = UIAlertAction(title:"Ok", style:UIAlertActionStyle.Default, handler:nil)
    myAlert.addAction(okAction)
    self.presentViewController(myAlert, animated:true, completion:nil)
}

我收到的错误：

2015-06-20 14:44:41.646 UserLoginAndRegistration[6064:750474] *** Assertion        failure in -[UIKeyboardTaskQueue waitUntilAllTasksAreFinished],    /SourceCache/UIKit/UIKit-3318.16.14/Keyboard/UIKeyboardTaskQueue.m:374
2015-06-20 14:44:41.657 UserLoginAndRegistration[6064:750474] *** Terminating   app due to uncaught exception 'NSInternalInconsistencyException', reason: '- [UIKeyboardTaskQueue waitUntilAllTasksAreFinished] may only be called from the  main thread.'
*** First throw call stack:
(0x186551e48 0x196c4c0e4 0x186551d08 0x1873d5554 0x18ad2b0a8 0x18ad2b734  0x18ad24a1c 0x18b251564 0x18b0156a8 0x18b0169e4 0x18b0188dc 0x18adee0b4 0x100028f9c 0x10002b500 0x10002b5f4 0x10002b624 0x10002b68c 0x185ee6540 0x187407508 0x187358c94 0x18734861c 0x18740a26c 0x1005a4df0 0x1005af854 0x1005a8120 0x1005b175c 0x1005b2f18 0x19746d2e4 0x19746cfa8)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) 

您必须始终从主队列更新 UI 元素，有时在您不这样做时可以工作，但大多数时候您的应用程序中有奇怪的行为，幸运的是编译器会警告您，有时需要几天的时间才能发现这样的错误不工作

使用下面的代码来调用和关闭您的警报视图

dispatch_async(dispatch_get_main_queue()) {
    self.dismissViewControllerAnimated(true, completion: nil)
} 

dispatch_async(dispatch_get_main_queue()) {
    self.displayMyAlertMessage("Wrong name or password")
}

您可以在以下位置查看更多详细信息：Grand Central Dispatch (GCD) 参考 https://developer.apple.com/library/mac/documentation/Performance/Reference/GCD_libdispatch_Ref/index.html