waitUntilAllTasksAreFinished error Swift

Question

I have this call in my loginViewController when Submit button is pressed:

let http = HTTPHelper()
    http.post("http://someUrl.com/Login/userEmail/\(username.text)/Pswd/\(userPass.text)", postCompleted: self.checkLogin)

while the checkLogin function I send is only performing:

func checkLogin(succeed: Bool, msg: String){
    if (succeed){
        self.performSegueWithIdentifier("logInTrue", sender: self)
    }
}

the post function is HTTPHelper class is :

func post(url : String, postCompleted : (succeeded: Bool, msg: String) -> ()) {
    var request = NSMutableURLRequest(URL: NSURL(string: url)!)
    var session = NSURLSession.sharedSession()
    request.HTTPMethod = "POST"
    var err: NSError?
     self.task = session.dataTaskWithURL(NSURL(string: url)!)  {(data, response, error) in
        var strData = NSString(data: data, encoding: NSUTF8StringEncoding)
        var err: NSError?
        var json = NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments, error: &err) as? NSDictionary
        var msg = "No message"
        // Did the JSONObjectWithData constructor return an error? If so, log the error to the console
        if(err != nil) {
            let jsonStr = NSString(data: data, encoding: NSUTF8StringEncoding)
            postCompleted(succeeded: false, msg: "Error")
        }
        else {
            // The JSONObjectWithData constructor didn't return an error. But, we should still
            // check and make sure that json has a value using optional binding.
            if let parseJSON = json {
                // Okay, the parsedJSON is here, let's get the value for 'success' out of it
                if let success = parseJSON["result"] as? Bool {
                    postCompleted(succeeded: success, msg: "Logged in.")
                }
                return
            }
            else {
                // Woa, okay the json object was nil, something went worng. Maybe the server isn't running?
                let jsonStr = NSString(data: data, encoding: NSUTF8StringEncoding)
                postCompleted(succeeded: false, msg: "Error")
            }
        }
    }

    self.task!.resume()
}

when the checkLogin function is called with success: true it fails to performSegueWithIdentified function .. the error looks like:

Assertion failure in -[UIKeyboardTaskQueue waitUntilAllTasksAreFinished], /SourceCache/UIKit_Sim/UIKit-3318.16.14/Keyboard/UIKeyboardTaskQueue.m:374 2014-11-15 17:41:29.540 Wavesss[8462:846477] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[UIKeyboardTaskQueue waitUntilAllTasksAreFinished] may only be called from the main thread.'

please help me though I am struggling very hard to find a solution for this, it seems like I can't pass between view controllers while the url task is still being performed on other thread. thanks in advance guys!

Answer 1

I was having this issue when trying to change the contents of a text box from inside an Async Task.

The solution was using the DispatchQueue (Xcode 8.0 and Swift 3.0):

    DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
       self.textBox.text = "Some Value"
       }

Answer 2

Your checkLogin function is being called on another thread, so you need to switch back to the main thread before you can call self.performSegueWithIdentifier. I prefer to use NSOperationQueue:

func checkLogin(succeed: Bool, msg: String) {
    if (succeed) {
        NSOperationQueue.mainQueue().addOperationWithBlock {
            self.performSegueWithIdentifier("logInTrue", sender: self)
        }        
    }
}

Alternate: xCode 10.1 1/2019

func checkLogin(succeed: Bool, msg: String) {
    if (succeed) {
        OperationQueue.main.addOperation {
            self.performSegue(withIdentifier: "logInTrue", sender: self)
           }        
      }
 }

Answer 3

I was also having same issue, got resolved by taking reference from above answer. Thank you @Nate

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)
}

Answer 4

With Xcode 8.0 and Swift 3, this has been modified to the following construct:

OperationQueue.main.addOperation{
    <your segue or function call>
}