Sometimes the simplest way is to use a "cool" language feature.

For instance: say you have an app with 30 different ViewControllers, and you need them to respond to the same notification. Sure, you could go back and refactor every one to subclass from another class that inherits from UIViewController - but then you need to make that for UITableViewControllers and UICollectionViewControllers, and potentially any other new view controllers that come along if your codebase ends up lasting for years.

Or, you could make one class category that method swizzles the viewDidAppear method to add the notification handling in immediately. Every class that inherits from UIViewController will now respond to that notification.