ActionScript for Flash MX: The Definitive Guide, Second Edition

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

Classes and Object-Oriented Programming

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Ball.prototype.moveClipTo = function (x, y) {

// Notice the use of this to refer to the ball being moved.

this.xPosition = x;

this.yPosition = y;

};

We’ve now created the all properties and methods of our Ball class, so our work is

done! The final code is shown in Example 12-2. Attach the example code to frame 1

of a new movie; then choose Control

➝ Test Movie to see it in action.

Invoking a method and defining a method have similar syntax but very different

results. It’s crucial to understand the difference. When defining a method, omit the

parentheses:

SomeClass.prototype.someMethod = someFunction;

When invoking a method, include the parentheses:

someObj.someMethod( );

The difference in syntax is subtle. Make sure to get it right.

Example 12-2. The completed Ball class

// The Ball( ) class constructor.

function Ball (radius, color, xPosition, yPosition) {

// Create Ball properties.

this.radius = radius;

this.color = color;

this.xPosition = xPosition;

this.yPosition = yPosition;

}

// The Ball.getArea( ) method.

Ball.prototype.getArea = function () {

// The keyword this means the ball on which getArea( ) was invoked.

return Math.PI * this.radius * this.radius;

};

// The Ball.moveClipTo( ) method.

Ball.prototype.moveClipTo = function (x, y) {

// Notice ...