ACTIONSCRIPT 3 Developer’s Guide en

ACTIONSCRIPT 3.0 DEVELOPER’S GUIDE
Handling events
function eventResponse(eventObject:EventType):void
{
// Actions performed in response to the event go here.
}
eventTarget.addEventListener(EventType.EVENT_NAME, eventResponse);
This code does two things. First, it defines a function, which is the way to specify the actions that will be performed in
response to the event. Next, it calls the addEventListener() method of the source object, in essence “subscribing”
the function to the specified event so that when the event happens, the function’s actions are carried out. When the
event actually happens, the event target checks its list of all the functions and methods that are registered as event
listeners. It then calls each one in turn, passing the event object as a parameter.
You need to alter four things in this code to create your own event listener. First, you must change the name of the
function to the name you want to use (this must be changed in two places, where the code says eventResponse).
Second, you must specify the appropriate class name of the event object that is dispatched by the event you want to
listen for (EventType in the code), and you must specify the appropriate constant for the specific event
(EVENT_NAME in the listing). Third, you must call the addEventListener() method on the object that will
dispatch the event (eventTarget in this code). Optionally, you can change the name of the variable used as the
function’s parameter (eventObject in this code).
Important concepts and terms
The following reference list contains important terms that you will encounter when writing event-handling routines:
Bubbling Bubbling occurs for some events so that a parent display object can respond to events dispatched by its
children.
Bubbling phase The part of the event flow in which an event propagates up to parent display objects. The bubbling
phase occurs after the capture and target phases.
Capture phase The part of the event flow in which an event propagates down from the most general target to the most
specific target object. The capture phase occurs before the target and bubbling phases.
Default behavior Some events include a behavior that normally happens along with the event, known as the default
behavior. For example, when a user types text in a text field, a text input event is raised. The default behavior for that
event is to actually display the character that was typed into the text field—but you can override that default behavior
(if for some reason you don’t want the typed character to be displayed).
Dispatch To notify event listeners that an event has occurred.
Event Something that happens to an object that the object can tell other objects about.
Event flow When events happen to an object on the display list (an object displayed on the screen), all the objects that
contain the object are notified of the event and notify their event listeners in turn. This process starts with the Stage
and proceeds through the display list to the actual object where the event occurred, and then proceeds back to the Stage
again. This process is known as the event flow.
Event object An object that contains information about a particular event’s occurrence, which is sent to all listeners
when an event is dispatched.
Event target The object that actually dispatches an event. For example, if the user clicks a button that is inside a Sprite
that is in turn inside the Stage, all those objects dispatch events, but the event target is the one where the event actually
happened—in this case, the clicked button.
Listener An object or function that has registered itself with an object, to indicate that it should be notified when a
specific event takes place.
Last updated 6/6/2012
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