classes

What are classes?

Classes are nothing more than a collection of functions (called methods in this context) that provide a blueprint for any number of instances that are created from it. By changing some variables (or in OOP terminology, properties) of a class instance, or by passing different values as arguments to its methods, the same underlying class can have widely different outcomes.

Another way of understanding a class is to compare the tasks performed in the Flash authoring environment. While ActionScript classes are created programmatically, they are basically analogous to the concept of importing assets and then working with symbols in the Library panel and the symbol copies (instances) on the Stage. The symbol instances on the Stage are similar to class instances. The different instances on the Stage—despite coming from the same symbol—can each individually have different properties settings that affect how they appear (such as color, alpha transparency, rotation, and so on).

The ability to generate multiple instances (that can appear and behave differently) from the same object is one of the reasons why classes are so powerful. An ActionScript class is even more powerful than a symbol in the Library because, as a developer, you have complete control over how an object looks and behaves. There are many properties and methods accessible using ActionScript 3.0 that you cannot access through the Flash authoring interface. Additionally, ActionScript classes make it possible to dynamically animate objects during runtime that are outside the scope of timeline-based animations. Finally, writing your own ActionScript classes promotes reusability because the functionality you create can be repurposed.

Let's see how this looks in code:

Brick.as

package com.adobe.ooas3 {
 public class Brick
  {
   public var color:String = "red";
   public function Brick() {
     trace("new "+ color +" brick created");
   }
 }
}

The Brick.as code above illustrates one of the most basic implementations of an ActionScript 3.0 class. The class holds just a single method and property. As you walk through each line of code, the following facts come to light:

• First, a package is defined. Packages allow you to structure your code in folders on your hard drive and prevent class name conflicts. In this example, the class is saved as Brick.as (the exact case-sensitive class name), inside a folder named "ooas3", which in turn is stored in an "adobe" folder that itself is contained within a folder called "com".

  By convention, package names are usually the domain name of the project in reverse order, followed by the project name. Doing this ensures that the package name is unique and there is no risk of any other project's classes conflicting with your own.

• Then the class is defined and set as public (more about access modifiers later). In our example the class is named "Brick".
• The next line of code declares a public property "color" of a type String that we will be using in the class. The value of color is set to red. There are better ways of working with class properties—as you'll learn about when I discuss encapsulation.

• Finally, the Brick method is defined. Note that this is not just any method. Every class has a constructor, which is a method that is automatically called when a new instance of the class is created.

  You can easily recognize constructor methods because they always use the same name as the class within which it is defined. Knowing this, you can easily see that, for the Brick class, this Brick method is its constructor.

  Inside the method we are tracing out a message to the Output panel that reads: "new red brick created". The trace statement incorporates the value of the variable color, which we defined with a default value of "red."

To quickly test this class, launch Flash CS3 Professional, create a new blank ActionScript 3.0 FLA, and write the following lines of code on the first keyframe on the main Timeline:

import com.adobe.ooas3.Brick;
var firstBrick:Brick = new Brick();

If you run Test Movie (select Control > Test Movie), you'll see that the Output panel now displays "new red brick created" (see Figure 1).

Figure 1. Output panel of Brick class getting instantiated

The traced message in the Output panel means that your ActionScript 3.0 class is functioning correctly. Now it is time to start doing some more advanced things with classes.