问题
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.
Any help in this regard would be highly appreciated. Thanks.
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.
回答1:
Got it working. The answer was method swizzling. Not recommended as a first solution to the problem. But if used carefully and you know exactly what you are doing that it is the way to go.
Found a very useful article here: Method Swizzling
回答2:
You can override UIViewController's viewWillAppear and viewWillDisappear to know when a view controller is about to be presented/dismissed.
Alternatively you can use viewDidAppear and viewDidDisappear.
If you want to do the logging on users behalf then you really have two options:
1. provide a base UIViewController sub-class for them to override, which implements a required logic in viewWillAppear/viewWillDisappear methods.
2. implement convenience methods (like logAppearEvent, logDisappearEvent) for them to call manually on their own in their UIViewController subclasses.
来源：https://stackoverflow.com/questions/36124902/trigger-an-event-method-when-uiwindow-rootviewcontroller-changes-swift