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 12: Objects and Classes

Drawing the current content of the screen is often the responsibility of a separate

class that deals specifically with the screen and graphical user interface. How the

screen is drawn, when it is drawn, and what draws the screen can vary greatly from

application to application. For example, in our Quiz at the end of this chapter, we’ll

update the screen from a QuizGUI class every time a question is displayed. But, in

other applications, a ball object would probably move around a lot, so it would need

to be displayed repeatedly via either MovieClip.onEnterFrame( ) or setInterval( ).

Here are the major issues to consider when designing an application’s display

system.

• First decide whether the display elements will render themselves or be rendered

by a central class. For example, will the Ball class provide its own drawBall( )

method, or will there be a drawBalls( ) or drawScreen( ) method on some other

class?

• Decide how often the elements should be displayed—either when some event

occurs (such as a mouse click) or repeatedly (as fast as frames are displayed in

the Flash Player).

• Decide on the rendering technique. Each visual element in a Flash movie must

be displayed in a movie clip. However, movie clips can be placed manually at

authoring time,