title: Trigger an event / method when UIWindow RootViewController changes SWIFT
body: <p>I'm working on an iOS framework. I have a requirement to log events when user enters or exits a particular View Controller. For that I was thinking if somehow I could be able to register a notification to trigger a custom method when the root view controller changes. Or perhaps use KVO. But I don't understand how to do this from an implementation point of view since I cannot find any such notification.</p>
  
  <p>Any help in this regard would be highly appreciated. Thanks.</p>
  
  <p>Please note that this is a framework project. So the framework is built and then added/embedded into another app. I don't have any information about the view controllers in that app. The only thing I can access is UIWindow's root view controller. So, I need to know when a change occurs in it.</p>