How to present UIAlertController when not in a view controller?

Question

Scenario: The user taps on a button on a view controller. The view controller is the topmost (obviously) in the navigation stack. The tap invokes a utility class method call

Answer 1

The following solution did not work even though it looked quite promising with all the versions. This solution is generating WARNING.

Warning: Attempt to present on whose view is not in the window hierarchy!

https://stackoverflow.com/a/34487871/2369867 => This is looked promising then. But it was not in Swift 3. So I am answering this in Swift 3 and this is not template example.

This is rather fully functional code by itself once you paste inside any function.

Quick Swift 3 self-contained code

let alertController = UIAlertController(title: "", message: "", preferredStyle: UIAlertControllerStyle.alert)
alertController.addAction(UIAlertAction(title: "Close", style: UIAlertActionStyle.cancel, handler: nil))

let alertWindow = UIWindow(frame: UIScreen.main.bounds)
alertWindow.rootViewController = UIViewController()
alertWindow.windowLevel = UIWindowLevelAlert + 1;
alertWindow.makeKeyAndVisible()
alertWindow.rootViewController?.present(alertController, animated: true, completion: nil)

This is tested and working code in Swift 3.