Chapter 13: Handling Events; Introduction To Event Handling - Adobe FLEX 2-PROGRAMMING ACTIONSCRIPT 3.0 Manual

Introduction to Event Handling

You can think of events as occurrences of any kind in your SWF file that are of interest to you
as a programmer. For example, most SWF files support user interaction of some sort—
whether it's something as simple as responding to a mouse click or something more complex,
such as accepting and processing data entered into a form. Any such user interaction with
your SWF file is considered an event. Events can also occur without any direct user
interaction, such as when data has finished loading from a server or when an attached camera
has become active.
In ActionScript 3.0, each event is represented by an event object, which is an instance of the
Event class or one of its subclasses. An event object not only stores information about a
specific event, but also contains methods that facilitate manipulation of the event object. For
example, when Flash Player detects a mouse click, it creates an event object to represent that
particular mouse click event. In this case, the event object is an instance of the MouseEvent
class.
After creating an event object, Flash Player dispatches it, which means that the event object is
passed to the object that is the target of the event. An object that serves as the destination for
a dispatched event object is called an event target. For example, when an attached camera
becomes active, Flash Player dispatches an event object directly to the event target, which in
this case is the object that represents the camera. If the event target is on the display list,
however, the event object is passed down through the display list hierarchy until it reaches the
event target. In some cases, the event object then "bubbles" back up the display list hierarchy
along the same route. This traversal of the display list hierarchy is called the event flow.
You can "listen" for event objects in your code using event listeners. Event listeners are the
functions or methods that you write to respond to specific events. To ensure that your
program responds to events, you must add event listeners either to the event target or to any
display list object that is part of an event object's event flow.
These three basic ideas form the structure of the new event model: the event flow, event
objects, and event listeners. A solid understanding of each concept is important if you want to
respond to events that occur in your SWF file. For advanced developers, the new event model
allows you to create and dispatch custom events, but this chapter focuses primarily on the
events that are defined by ActionScript 3.0 and dispatched directly by Flash Player.
346 Handling Events