Observer for monitoring when view controllers'

Question

Summary of question

UINavigationControllerDelegate:didShowViewContoller makes it possible to get notified whenever any view controller has been displayed (as opposed to being loaded), provided its within the context of a navigation stack. I want to know if such observation is possible for all view controllers if there isn't a navigation stack.

More background

I have an app where view controllers can suddenly appear based upon timers and local notifications firing, thus their appearance is effectively random.

If one VC triggers and gets displayed at the same time as another was in the process of getting displayed then there can be an issue (if you're experienced with iOS you'll be aware if one VC pushes another from within its viewDidLoad, rather than its viewDidAppear you will get an "attempting to present X on Y whose view is not in the window hierarchy" error)

How I solve this is I have a list of VCs to display and they get displayed by a view controller co-ordinator which implements UINavigationControllerDelegate's didShowViewContoller and doesn't display the new VC until didShowViewController has been invoked.

This works perfectly.

But now my problem is I want to do a similar thing for an app that isn't using a navigation controller, and thus I can't use UINavigationControllerDelegate:didShowViewController to observe globally when a view controller has been displayed. Does anybody know of another elegant mechanism for doing so?