three participants in the event delegation model:

event source: class that broadcasts events
event listeners: classes that receive event notifications
event object: class that describes the event

general structure

event occurs (e.g., mouse click, ending of a game, new user joins a chat...etc)

event source broadcasts event by invoking an agreed-upon method on all event listeners

event object is passed to agreed-upon method as an argument

event listeners respond as they see fit (e.g., submit a form, show a game over screen, display a welcome message to the new chat user)


the event source

the event source includes two classes:

the class that broadcasts events (event source is user-defined)
EventListenerList: utility to manage event listeners

EventListenerList implemenatation

EventListenerList resides in "event" pacakge

class event.EventListenerList { }

EventListenerList also imports the contents of the event package

import event.*;

array of listener objects stored in instance property, listeners

private var listeners:Array;

constructor creates empty listeners array:

public function EventListenerList () { listeners = new Array(); }

addObj() method adds a listener to the listeners array

public function addObj (l:EventListener):Boolean { // Search for the specified listener. var len:Number = listeners.length; for (var i = len; --i >= 0; ) { if (listeners[i] == l) { return false; } } // The new listener is not already in the list, so add it. listeners.push(l); return true; }

removeObj() method removes a listener from the listeners array

public function removeObj (l:EventListener):Boolean { // Search for the specified listener. var len:Number = listeners.length; for (var i:Number = len; --i >= 0; ) { if (listeners[i] == l) { // We found the listener, so remove it. listeners.splice(i, 1); // Quit looking. return true; } } return false; }

getListeners() method returns a copy of the listener list, used to broadcast an event

public function getListeners ():Array { // Return a copy of the list, not the list itself. return listeners.slice(0); }

the event object

the event object includes two classes:

EventObject, a base class for all event objects
an EventObject subclass, whose properties and methods describe a specific event
the subclass is user-defined, per event implementation