Changing the Standard Event Handling
Go Up to Implementing the Standard Events
If you want to change the way your component responds to a certain kind of event, you might be tempted to write some code and assign it to the event. As an application developer, that is exactly what you would do. But when you are creating a component, you must keep the event available for developers who use the component.
This is the reason for the protected implementation methods associated with each of the standard events. By overriding the implementation method, you can modify the internal event handling; and by calling the inherited method you can maintain the standard handling, including the event for the application developer's code.
The order in which you call the methods is significant. As a rule, call the inherited method first, allowing the application developer's event-handler to execute before your customizations (and in some cases, to keep the customizations from executing). There may be times when you want to execute your code before calling the inherited method, however. For example, if the inherited code is somehow dependent on the status of the component and your code changes that status, you should make the changes and then allow the user's code to respond to them.