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Learning ActionScript 3.0 by Zevan Rosser, Rich Shupe

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The Document Class

If you decide you would like to start thinking in OOP terms right away, we will show you how to easily take a step in that direction. Flash CS3 introduced a new feature that simplifies associating a main class, or application entry point with your FLA. It is called the document class and it does all the work of instantiating the class for you. This means you don't need any code in the timeline at all, and can edit all examples in Flash or the external text editor or development environment of your choice.

Note

If you don't plan to start using OOP until we roll it out in later chapters, feel free to skip this section as it will be repeated in Chapter 6. We will provide minimal explanation here just to get you going using the document class, and will explain this material in greater detail in later chapters throughout the book.

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 Output 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 Figure 1-1.

Adding a document class to your FLA

Figure 1-1. 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.

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.

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