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.

Passing Information to Functions

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185

3. Place an instance of ballSymbol on Layer 1.

4. Name the ballSymbol instance

ball.

5. On frame 1 of Layer 1 of the main movie timeline, attach the following code:

// Store a portable reference to the main timeline.

_global.mainTimeline = this;

function moveBall () {

mainTimeline.ball._x += 10;

mainTimeline.ball._y += 10;

}

As a matter of good form, functions should not access external timeline variables

(such as ball) directly. Therefore, we access ball through our application’s global

reference to the movie’s main timeline. With the function moveBall( ) defined, we

can move the

ball clip diagonally by calling the moveBall( ) function. Add the follow-

ing code to the existing code on frame 1.

moveBall();

The ball moves diagonally down and to the right. (Note that the origin (0, 0) is in the

upper left of the main Stage. Increasing values of

_x move the ball to the right, but,

unlike the Cartesian coordinates, increasing values of

_y move the ball down, not up.)

Our moveBall() function is convenient, but it lacks flexibility. It works only on one

movie clip (

ball), it moves ball in only one direction, and it always moves ball the

same distance.

A well-designed function should work in many circumstances. We can generalize our

moveBall() function so that