/17/ the 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