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Learning ActionScript 2.0 in Adobe Flash > Classes > Example: Writing custom classes

Example: Writing custom classes

Now that you've explored the basics of a class file, and what kinds of things it contains, it's time to learn some of the general guidelines for creating a class file. The first example in this chapter shows you how to write classes and package them. The second example shows you how to use those class files with a FLA file.

As discussed in Writing custom class files, a class consists of two main parts: the declaration and the body. The class declaration consists minimally of the class statement, followed by an identifier for the class name, and then left and right curly braces ({}). Everything inside the braces is the class body, as shown in the following example:

class className {
  // class body
}

Remember: you can define classes only in external ActionScript files. For example, you can't define a class in a frame script in a FLA file. Therefore, you create a new file for this example.

In its most basic form, a class declaration consists of the class keyword, followed by the class name (Person, in this case), and then left and right curly braces ({}). Everything between the braces is called the class body and is where the class's properties and methods are defined.

By the end of this example, the basic ordering of your class files is as follows:

• Documentation comments
• Class declaration
• Constructor function
• Class body

You do not write subclasses in this chapter. For more information on inheritance and subclassing, see Inheritance.

This example includes the following topics:

• About general guidelines for creating a class
• Creating and packaging your class files
• Writing the constructor function
• Adding methods and properties
• Controlling member access in your classes
• Documenting the classes

For samples that demonstrates how to create a dynamic menu with XML data and a custom class file, see the Flash Samples page at www.adobe.com/go/learn_fl_samples. The sample calls the ActionScript XmlMenu() constructor and passes it two parameters: the path to the XML menu file and a reference to the current timeline. Download and decompress the Samples zip file and navigate to the ActionScript2.0/XML_Menu folder to access these samples:

• XmlMenu.as
• xmlmenu.fla

About general guidelines for creating a class

The following points are guidelines to follow when you write custom class files. They help you write correct and well-formed classes. You practice these guidelines in upcoming examples.

• In general, place only one declaration per line, and do not place either the same or different types of declarations on a single line. Format your declarations as the following example shows:

  private var SKU:Number; // product SKU (identifying) number
  private var quantity:Number; // quantity of product
  

• Initialize local variables when you declare them, unless that initial value is determined by a calculation. For information on initializing variables, see Adding methods and properties.
• Declare variables before you first use them (including loops). For example, the following code predeclares the loop iterator variable (i) before using it in the for loop:

  var my_array:Array = new Array("one", "two", "three");
  var i:Number;
  for (i = 0 ; i < my_array.length; i++) {
      trace(i + " = " + my_array[i]);
  }
  

• Avoid using local declarations that hide higher-level declarations. For example, do not declare a variable twice, as the following example shows:

  // bad code
  var counter:Number = 0;
  function myMethod() {
      var counter:Number;
      for (counter = 0; counter <= 4; counter++) {
          // statements;
      }
  }
  

  This code declares the same variable inside an inner block.

• Do not assign many variables to a single value in a statement, because it is difficult to read, as you can see in the following ActionScript code samples:

  // bad form
  xPos = yPos = 15;
  

  or

  // bad form
  class User {
      private var m_username:String, m_password:String;
  }
  

• Have a good reason for making public instance variables, or public static, class, or member variables. Make sure that these variables are explicitly public before you create them this way.
• Set most member variables to private unless there is a good reason to make them public. It is much better from a design standpoint to make member variables private and allow access only to those variables through a small group of getter and setter functions.

About naming class files

Class names must be identifiers--that is, the first character must be a letter, underscore (_), or dollar sign ($), and each subsequent character must be a letter, number, underscore, or dollar sign. As a preferred practice, try to always limit class names to letters.

The class name must exactly match the name of the ActionScript file that contains it, including capitalization. In the following example, if you create a class called Rock, the ActionScript file that contains the class definition must be named Rock.as:

// In file Rock.as
class Rock {
  // Rock class body
}

You name and create a class definition in the following section. See the section Creating and packaging your class files to create, name, and package the class files. For more information on naming class files, see Naming classes and objects.

Flash CS3