component broadcasts events when a user interacts with it. When
a user clicks a Button, for example, it dispatches a MouseEvent.CLICK event
and when a user selects an item in a List, the List dispatches an
Event.CHANGE event. An event can also occur when
something significant happens to a component such as when content
finishes loading for a UILoader instance, generating an Event.COMPLETE event.
To handle an event, you write ActionScript code that executes when
the event occurs.
A component’s events include the events of any class from which
the component inherits. This means that all ActionScript 3.0 User
Interface components inherit events from the UIComponent class because
it is the base class for the ActionScript 3.0 User Interface components.
To see the list of events a component broadcasts, see the Events
section of the component’s class entry in the ActionScript 3.0 Reference for the Adobe
For a complete explanation of event handling in ActionScript
3.0, see the ActionScript 3.0 Developer's Guide.
About event listeners
The following key points apply to handling
events for ActionScript 3.0 components:
All events are broadcast by an instance of a component
class. The component instance is the broadcaster.
You register an event listener by calling the addEventListener() method for
the component instance. For example, the following line of code
adds a listener for the MouseEvent.CLICK event
to the Button instance aButton:
second parameter of the addEventListener() method
registers the name of the function, clickHandler,
to be called when the event occurs. This function is also referred
to as a callbackfunction.
You can register multiple listeners to one component instance.
You can register one listener to multiple component instances.
The event handler function is passed to an event object that
contains information about the event type and the instance that
broadcast the event. For more information, see About the event object.
The listener remains active until the application terminates
or you explicitly remove it using the removeEventListener() method.
For example, the following line of code removes the listener for
the MouseEvent.CLICK event on aButton:
About the event object
event object inherits from the Event object class and has properties
that contain information about the event that occurred, including
the target and type properties,
which provide essential information about the event:
A string indicating the type of the event.
A reference to the component instance broadcasting
When an event has additional properties, they are listed in the
event’s class description in the ActionScript 3.0 Reference for the Adobe
The event object is automatically generated and passed to the
event handler function when an event occurs.
You can use the event object inside the function to access the
name of the event that was broadcast or the instance name of the
component that broadcast the event. From the instance name, you
can access other component properties. For example, the following
code uses the target property of the evtObj event object
to access the label property of aButton and
display it in the Output panel:
var aButton:Button = new Button();
aButton.label = "Submit";
trace("The " + evtObj.target.label + " button was clicked");