Listeners

Listeners are a different kind of event handler. Instead of overriding a method of the object, listeners "register" themselves with the object. Then when the event occurs, the object is responsible for notifying all the registered listeners by calling specific methods on the listener objects.

The main advantage of listeners over traditional event handlers is that more than one object can listen for a specific event. It supports the idea of "broadcasting" the eventone object trigging an event handler on multiple listening objects.

Many of the built-in classes support listeners such as the following: Key, Mouse, Stage, MovieClipLoader, FileReference, and so on.

In the following example, we will listen for an onResize event on the Stage class:

   function onResize() {      trace(Stage.width + " x " + Stage.height);    }    Stage.addListener(this);

In this simple example we are creating a function on our main Timeline called onResize. This is the name of the method the Stage class will attempt to call on all registered listeners whenever the stage size is changed.

Inside this listener function we are simply outputting the new dimensions of the stage using the trace command.

The final thing we do is register our Timeline as a listener to the Stage class using the addListener method. If we want to unregister our Timeline, we could use the removeListener method. In the following example, we will remove the listener after the first resize of the stage, which ensures that the stage is resized only once:

   function onResize() {      trace(Stage.width + " x " + Stage.height);      Stage.removeListener(this);    }    Stage.addListener(this);

There are many built-in classes that use listeners. The following is a list of all those classes in Flash 8:

Class	Listeners
flash.net.FileReference	onCancel
	onComplete
	onIOError
	onHTTPError
	onOpen
	onProgress
	onSecurityError
	onSelect
flash.net.FileReferenceList	onCancel
	onSelect
System.IME	onIMEComposition
Key	onKeyDown
	onKeyUp
Mouse	onMouseDown
	onMouseMove
	onMouseUp
	onMouseWheel
MovieClipLoader	onLoadComplete
	onLoadError
	onLoadInt
	onLoadProgress
	onLoadStart
Selection	onSetFocus
Stage	onResize
TextField	onChanged
	onScroller