当UIWindow RootViewController更改SWIFT时触发一个事件/方法

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.I'm working on an iOS framework. I have a requi