ActionScript for Flash MX: The Definitive Guide, Second Edition

This is the Title of the Book, eMatter Edition

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

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Chapter 12: Objects and Classes

for example, have properties specific to individual movie clips, such as their number

of frames (

_totalframes) and position on screen (_x and _y).

Traditionally, object properties are set via methods of the object to preserve the

encapsulated nature of an object. That is, in strict OOP, an object should set its own

properties. If an outside entity wants to set an object’s property, it should do so by

calling an appropriate method of the object. For example, a purist would frown on

setting the

_width property of a movie clip directly. The purist would argue that it is

best to let the movie clip object set the

_width property itself, and that code outside

of the object should do so only indirectly, by calling one of the movie clip object’s

methods (say, setWidth()). By moderating access to properties with method calls, the

maintainer of the MovieClip class could change the name of the

_width property to w

without adversely affecting other users of the class.

Instantiating Objects

Although it may sound backward, let’s assume that we have already created an

object class. This assumption isn’t too ludicrous, because ActionScript provides

many built-in classes, and the custom classes we’ll build will behave similarly.

Assuming we have an object class, we have ...