Users Can Override Default Handling
Go Up to Defining the Handler Type
For some kinds of events, developers may want to replace the default handling or even suppress all responses. To allow this, you need to pass an argument by reference to the handler and check for a certain value when the handler returns.
This is in keeping with the rule that an empty handler should have the same effect as no handler at all. Because an empty handler will not change the values of arguments passed by reference, the default handling always takes place after calling the empty handler.