ActionScript for Flash MX: The Definitive Guide, Second Edition

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Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

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Chapter 10: Events and Event Handling

Scope of onClipEvent( ) Handlers

Unlike event handler properties and event listener methods, on( ) and onClipEvent( )

handlers do not define a local scope! When we attach an onClipEvent( ) handler to a

clip, the scope of the handler is the clip, not the event handler. This means that all

variables are retrieved from the clip’s timeline. For example, if we attach an

onClipEvent(enterFrame) handler to a clip named

navigation and write trace(x);

inside the handler, the interpreter looks for the value of

x on navigation’s timeline:

onClipEvent (enterFrame) {

trace(x); // Displays the value of navigation.x

}

The interpreter does not consult a local scope first, because there is no local scope to

consult. If we write var y = 10; in the onClipEvent( ) handler,

y is defined on

navigation’s timeline, even though the var keyword ordinarily declares a local vari-

able when used in a function.

The easiest way to remember the scope rules of an onClipEvent( ) handler is to treat

the handler’s statements as though they were attached to a frame of the handler’s

clip. For example, suppose we have a clip named

ball on the main timeline that has a

variable called

xVelocity in it. To access xVelocity from inside ball’s onClipEvent( )

handler, we simply refer to it directly, ...