Function can only be called in the main thread, why?

Question

In this code below I am calling the function displayMyAlertMessage() twice. the first I am calling works perfect, but the second one gives me an error saying that the function may only be called from the main thread. How can I do this in my situation?

I am using this code:

@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()
}

To call this function:

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)
}

The error I am getting:

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) 

Answer 1

You must always update UI elements from the main queue, sometimes works when you don't but most the time you have weird behaviours in your app, you luck that the compiler is warning you, sometimes take days to find mistakes like that when things just don't work

Use the code below to call and dismiss your alert views

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

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

You can see more detail about it at Grand Central Dispatch (GCD) Reference

Answer 2

You should put your process in the queue to get it execute.

You can refer to below example how I was able to resolve it in my scenario when I was having same issue.

var storyBoard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
var vc: UINavigationController = storyBoard.instantiateViewControllerWithIdentifier("AppViewController") as! UINavigationController

NSOperationQueue.mainQueue().addOperationWithBlock {
    self.presentViewController(vc, animated: true, completion: nil)
    //Above line was generating same exception in my scenario
    // I got it resolved by using NSOperationQueue.mainQueue().addOperationWithBlock {
}

Answer 3

You need to dismiss your alert views first. Use the code below to do so.

Works with Swift 3

DispatchQueue.main.async {
    your code goes here
}