Introduction to Event_Dispatcher
Event_Dispatcher acts as a notification dispatch table.
It is used to notify other objects of interesting things.
This information is encapsulated in Event_Notification objects.
Client objects register themselves with the Event_Dispatcher as observers of
specific notifications posted by other objects. When an event occurs,
an object posts an appropriate notification to the Event_Dispatcher.
The Event_Dispatcher dispatches a message to each registered
observer, passing the notification as the sole argument.
event management. If an event is not handled by the dispatcher that triggered
the event, it may bubble up to the next dispatcher.