ACTIONSCRIPT 3 Developer’s Guide en

ACTIONSCRIPT 3.0 DEVELOPER’S GUIDE
Handling events
Managing event listeners
Flash Player 9 and later, Adobe AIR 1.0 and later
You can manage your listener functions using the methods of the IEventDispatcher interface. The IEventDispatcher
interface is the ActionScript 3.0 version of the EventTarget interface of the DOM event model. Although the name
IEventDispatcher may seem to imply that its main purpose is to send (or dispatch) event objects, the methods of this
class are actually used much more frequently to register event listeners, check for event listeners, and remove event
listeners. The IEventDispatcher interface defines five methods, as shown in the following code:
package flash.events
{
public interface IEventDispatcher
{
function addEventListener(eventName:String,
listener:Object,
useCapture:Boolean=false,
priority:Integer=0,
useWeakReference:Boolean=false):Boolean;
}
}
function removeEventListener(eventName:String,
listener:Object,
useCapture:Boolean=false):Boolean;
function dispatchEvent(eventObject:Event):Boolean;
function hasEventListener(eventName:String):Boolean;
function willTrigger(eventName:String):Boolean;
The Flash Player API implements the IEventDispatcher interface with the EventDispatcher class, which serves as a
base class for all classes that can be event targets or part of an event flow. For example, the DisplayObject class inherits
from the EventDispatcher class. This means that any object on the display list has access to the methods of the
IEventDispatcher interface.
Adding event listeners
The addEventListener() method is the workhorse of the IEventDispatcher interface. You use it to register your
listener functions. The two required parameters are type and listener. You use the type parameter to specify the
type of event. You use the listener parameter to specify the listener function that will execute when the event occurs.
The listener parameter can be a reference to either a function or a class method.
Do not use parentheses when you specify the listener parameter. For example, the clickHandler() function is
specified without parentheses in the following call to the addEventListener() method:
addEventListener(MouseEvent.CLICK, clickHandler)
The useCapture parameter of the addEventListener() method allows you to control the event flow phase on which
your listener will be active. If useCapture is set to true, your listener will be active during the capture phase of the
event flow. If useCapture is set to false, your listener will be active during the target and bubbling phases of the event
flow. To listen for an event during all phases of the event flow, you must call addEventListener() twice, once with
useCapture set to true, and then again with useCapture set to false.
Last updated 6/6/2012
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