DELEGATION EVENT MODEL

• delegation event model is a specialized kind of Observer
• description: "An event is propagated from a 'Source' object to a 'Listener' object by invoking a method on the listener and passing in the instance of the event subclass which defines the event type generated." (from Sun's Java AWT: Delegation Event Model)
• called "delegation" event model because the event source "delegates" the processing of an event to a separate object (the event listener)
• participants (based on Java's implementation):
• event source
• registers/unregisters event listener objects
• broadcasts events to listeners (by invoking methods on them)
• pushes an "event object" to listeners (by passing it to the event methods)
• corresponding Observer pattern participant: Observable
• event object
• describes the event
• provides methods to access information about the event (e.g., MouseEvent.getX())
• passed to listener by event source
• corresponding Observer pattern participant: Observer.update() method's infoObj parameter
• event listener interface
• defines agreed upon event methods (the event source invokes these on the listener object)
• is implemented by event listener
• corresponding Observer pattern participant: Observer.update() method
• event listener object
• registers with event source to receive events
• reacts to events broadcast by event source
• implements methods in event listener interface
• corresponding Observer pattern participant: Observer