Learning ActionScript 3.0: Chapter 1, ActionScript Overview
Pages: 1, 2, 3

Let's start with a simulated chapter example that you might use in the timeline. It does nothing more than use the trace() method to place a word into the fOutput panel—an authoring-only panel that accepts text output from your file.

trace("Flash");

To create a document class, you're going to create a kind of wrapper that encloses the trace() method in the correct class syntax.

Create a new ActionScript file (rather than a new FLA document) and type the following document class shell:

1   package {
2
3      import flash.display.MovieClip;
4
5      public class Main extends MovieClip {
6
7          public function Main() {
8
9          }
10
11      }
12  }

The first line, along with the closing brace in line 12, defines the class's package. A package is a mandatory structure that ensures your class is known to the compiler. Next, you must import any classes that you need to use in your package.

A document class essentially serves as a shortcut for creating an instance of a movie clip or sprite (a new Flash object that is nothing more than a one-frame movie clip) and adding it to the display list so it can be displayed by Flash Player. (This is true even when there is nothing to display, as in this case. We will cover manipulating the display list in Chapter 4.)

All document classes must be derived from either the MovieClip or Sprite class. (Other custom classes that are not document classes do not need to be extended from MovieClip or Sprite if that is not appropriate.) This example uses MovieClip so you must import the MovieClip class, as seen in line 3.

Line 5, along with its closing brace on line 11, is the class definition. Its name is arbitrary but, when naming it, you should follow a few basic rules and conventions. The name should be one word that does not already exist in ActionScript, it should start with an alpha character (rather than a number or other character), and it is typically capitalized. The class must be public, meaning that other classes can access the constructor, and it must extend MovieClip or Sprite, as described previously.

Line 7, along with its closing brace on line 9, is the class constructor. This is the main function that automatically runs when creating an instance of this class. It, too, must be public and must have the same name as the class. Other functions (if any) can, and must, have unique names. All that remains is to add the lone method required in this case. The constructor must trace "Flash" to the Output panel, so add the following to line 8:

7        public function Main() {
8            trace("Flash");
9        }

Once finished, you must save the file in the same directory as your FLA file for now. (Later on, you'll learn how to place your class files in other locations.) You must give the file the same name as the class, but add an .as extension. Therefore, this file should be named Main.as. Now create a new FLA file, choosing ActionScript 3.0 as its programming language version, and save it in the same directory as your previously created class file. The name of the FLA is unimportant.

Finally, open the Properties Inspector and add the name of your document class, not the name of the document itself, in the Document Class field. In this case, type Main instead of Main.as, as seen in this figure.

Figure : Adding a document class to your FLA

Now preview your file. Doing so will create an instance of the Main class (which extends MovieClip and, therefore, behaves like a movie clip) and add it to the display list. The class will trace "Flash" to the output panel, and your test application will be complete.

This excerpt is from Learning ActionScript 3.0. Learning ActionScript 3.0 gives you a solid foundation in the Flash language and demonstrates how you can use it for practical, everyday projects. The book does more than give you a handful of sample scripts, defining how ActionScript and Flash work. It gives you a clear look into essential topics such as logic, event handling, displaying content, migrating legacy projects to ActionScript 3.0, classes, and much more. Written for those new to the language, this book doesn't rely exclusively on prior knowledge of object-oriented programming (OOP). Instead, it helps you expand your skillset by first focusing on clear, concise examples in the timeline, evolving into OOP examples over time-allowing you to choose the programming approach with which you are most comfortable.

Hereafter, you can try any of our timeline code in a document class of your own. Initially, you probably won't know which classes to import or how to make any possible changes to variables or similar structures to conform to the class syntax. However, all the sample code will come with an accompanying class file for testing. You can use those files whenever you wish until you get used to the document class format.

Section 1.5: Legacy Code Compatibility

I'd like to end this chapter with a small caveat. You cannot mix ActionScript 1.0 or 2.0 code with ActionScript 3.0 code in the same SWF. You are unlikely to do this if you're learning from scratch, but you may run into this situation if you attempt to update legacy projects by adding ActionScript 3.0 code.

If you ever have the need to run a discrete mixture of ActionScript 3.0 and a prior version of the language, such as showing a legacy file within a new demo interface shell, you can do so by loading a SWF. An ActionScript 3.0 file can load a SWF created in ActionScript 1.0 or 2.0, but it cannot access the older SWF's variables or functions. For all intents and purposes, the same is not true in reverse. An older SWF cannot load an ActionScript 3.0 SWF.

In Chapter 13, we will discuss how to communicate between these two discrete SWFs using a special process. For now, however, just remind yourself again that you cannot combine ActionScript 3.0 with older versions of the language in the same file.

Section 1.6: What's Next?

Now that you know a little more about ActionScript 3.0 and the Flash Platform, it's time for a look at some of the fundamentals of the language. By reviewing version-independent concepts at the outset, we can focus on new syntax in subsequent chapters. If you have a lot of experience with ActionScript 1.0 or 2.0, you may wish to skim this material.

In the next chapter, we'll discuss:

• Basic concepts to bring you up to speed quickly, including using the trace() method as a diagnostic tool to see immediate feedback from your scripts
• Using variables to store data, including arrays and custom objects that allow you to easily manage more than one value, and data typing those values to improve error reporting
• Logical structures such as conditionals for decision making and loops for simplifying repetitive tasks
• Functions that can isolate code into convenient blocks that will be executed only when instructed
• Ways to address Flash objects with ActionScript, including using absolute and relative paths, and the shortcut identifier this

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