The cocoa framework provided a useful mechanism called Notification, it's really useful when you want to write some event driven app. But when we want to know what kind of system notifications are available, we are doomed. Apple does not provide a list of all notifications resident in each framework, so, I have to do it myself.
Here is my summary of all the system Notifications from UIKit framework. About other frameworks, I will do it later.
To use it, find the Class header file, then refer to the documentation for when it will be fired.