Swift：在SplitViewController之前有一个登录屏幕

Question

I understand that it is not allowed and UISplitViewController should always be root controller.

How can I have a login screen before splitViewController? It seems to be a very common problem.
I am new to iOS so any example will be much appreciated.
Tried updating viewDidLoad() of MasterViewController:

let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        if (!appDelegate.loggedIn) {
            // display the login form
            let storyboard = UIStoryboard(name: "Main", bundle: nil)
            let login = storyboard.instantiateViewControllerWithIdentifier("LoginVC") as UIViewController
            self.presentViewController(login, animated: false, completion: { () -> Void in
                // user logged in and is valid now
                //self.updateDisplay()
            })
        } else {
            //updateDisplay()
        }

But it threw me a warning: arning: Attempt to present <LoginViewController: 0x7f8ef8f12c80> on <SideBarViewController: 0x7f8ef8d9faf0> whose view is not in the window hierarchy!

Answer 1

Make your initial view controller the split view controller. In its viewDidLoad or viewDidAppear methods or whatever makes sense, check that the user is logged in. If the user isn't logged in, segue modally WITHOUT animation to your login view controller. This will all happen without the user noticing and will allow the storyboard setup you want.