ACTIONSCRIPT 3 Developer’s Guide en

ACTIONSCRIPT 3.0 DEVELOPER’S GUIDE
Handling events
Event handling in ActionScript 3.0
Flash Player 9 and later, Adobe AIR 1.0 and later
ActionScript 3.0 introduces a single event-handling model that replaces the many different event-handling
mechanisms that existed in previous versions of the language. The new event model is based on the Document Object
Model (DOM) Level 3 Events Specification. Although the SWF file format does not adhere specifically to the
Document Object Model standard, there are sufficient similarities between the display list and the structure of the
DOM to make implementation of the DOM event model possible. An object on the display list is analogous to a node
in the DOM hierarchical structure, and the terms display list object and node are used interchangeably throughout this
discussion.
The Flash Player and AIR implementation of the DOM event model includes a concept named default behaviors. A
default behavior is an action that Flash Player or AIR executes as the normal consequence of certain events.
Default behaviors
Developers are usually responsible for writing code that responds to events. In some cases, however, a behavior is so
commonly associated with an event that Flash Player or AIR automatically executes the behavior unless the developer
adds code to cancel it. Because Flash Player or AIR automatically exhibits the behavior, such behaviors are called
default behaviors.
For example, when a user enters text into a TextField object, the expectation that the text will be displayed in that
TextField object is so common that the behavior is built into Flash Player and AIR. If you do not want this default
behavior to occur, you can cancel it using the new event-handling system. When a user inputs text into a TextField
object, Flash Player or AIR creates an instance of the TextEvent class to represent that user input. To prevent Flash
Player or AIR from displaying the text in the TextField object, you must access that specific TextEvent instance and
call that instance’s preventDefault() method.
Not all default behaviors can be prevented. For example, Flash Player and AIR generate a MouseEvent object when a
user double-clicks a word in a TextField object. The default behavior, which cannot be prevented, is that the word
under the cursor is highlighted.
Many types of event objects do not have associated default behaviors. For example, Flash Player dispatches a connect
event object when a network connection is established, but there is no default behavior associated with it. The API
documentation for the Event class and its subclasses lists each type of event and describes any associated default
behavior, and whether that behavior can be prevented.
It is important to understand that default behaviors are associated only with event objects dispatched by Flash Player
or AIR, and do not exist for event objects dispatched programmatically through ActionScript. For example, you can
use the methods of the EventDispatcher class to dispatch an event object of type textInput, but that event object will
not have a default behavior associated with it. In other words, Flash Player and AIR will not display a character in a
TextField object as a result of a textInput event that you dispatched programmatically.
What’s new for event listeners in ActionScript 3.0
For developers with experience using the ActionScript 2.0 addListener() method, it may be helpful to point out the
differences between the ActionScript 2.0 event listener model and the ActionScript 3.0 event model. The following list
describes a few major differences between the two event models:
To add event listeners in ActionScript 2.0, you use addListener() in some cases and addEventListener() in
others, whereas in ActionScript 3.0, you use addEventListener() in all situations.
There is no event flow in ActionScript 2.0, which means that the addListener() method can be called only on the
object that broadcasts the event, whereas in ActionScript 3.0, the addEventListener() method can be called on
any object that is part of the event flow.
Last updated 6/6/2012
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