Why navigation controller does not navigate in callback using Swift?

Question

I have created a navigation controller and assigned it to my View Controller in Swift.

I created the following method:

@IBAction func btnLoginPressed(sender: AnyObject) {
    let userManager = UserManager()
        userManager.login(txtBoxLogin.text, password: txtBoxPassword.text, operationCompleteHandler: {
            (token: String?) -> Void in
                if let token = token {
                    ApplicationState.ApiToken = token
                    var mainView = self.storyboard?.instantiateViewControllerWithIdentifier("MenuView") as! MenuViewController
                    self.navigationController!.pushViewController(mainView, animated: true)
                }
        })
}

The problem is that it does not work in this configuration. However if I put

self.storyboard?.instantiateViewControllerWithIdentifier("MenuView") as! MenuViewController
                    self.navigationController!.pushViewController(mainView, animated: true)

outside the operationCompleteHandler it works flawlessly.

What am I doing wrong and how should I fix this?

Answer 1

Finally, I've found the reason for such weird behavior: The callback is running on a thread which is separate from the UI thread.

In order to allow a fragment of code to do UI-related things you have to use dispatch_async() method. Here is my updated code with working navigation using mentioned method:

@IBAction func btnLoginPressed(sender: AnyObject) {
    let userManager = UserManager()
        userManager.login(txtBoxLogin.text, password: txtBoxPassword.text, operationCompleteHandler: {
            (token: String?) -> Void in
                if let token = token {
                    ApplicationState.ApiToken = token
                    dispatch_async(dispatch_get_main_queue()) {
                        var mainView = self.storyboard?.instantiateViewControllerWithIdentifier("MenuView") as! MenuViewController
                        self.navigationController!.pushViewController(mainView, animated: true)
                    }
                }
        })
}