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

Object literals are used for the sake of terseness. For example, in the following code

we transfer the

speed and onEnterFrame properties to car_mc more succinctly (though

perhaps less legibly) than we did earlier:

this.attachMovie("CarSymbol", "car_mc", 1, {speed:3, onEnterFrame: function ( ) {

this._x += this.speed;}});

The Almighty Prototype Chain

In Chapter 2, we saw that the interpreter uses a scope chain to resolve (look up the

value of) unqualified variable references. To resolve property values, the interpreter

uses a prototype chain. We’ve already encountered the

prototype property in our

examination of methods and superclasses. Now let’s take a look “under the hood” of

ActionScript’s inheritance system to see how the

prototype property really works.

For the sake of the current discussion, the term property refers to object properties,

methods, and event handlers (the latter two of which are, technically, functions

stored in properties).

As we’ve seen, every class constructor function is automatically given a property

called

prototype. For example, when we create a Book class, the Book constructor

automatically defines

Book.prototype:

function Book ( ) {

}

trace(Book.prototype); // Displays: [object Object]

Initially, a constructor function’s prototype ...