Flash CS3 Documentation

Learning ActionScript 2.0 in Adobe Flash > Classes > About working with custom classes in an application > About class members > Using class members

Using class members

One use of class (static) members is to maintain state information about a class and its instances. For example, suppose you want to keep track of the number of instances that have been created from a particular class. An easy way to do this is to use a class property that increments each time a new instance is created.

In the following example, you'll create a class called Widget that defines a single, static instance counter named widgetCount. Each time a new instance of the class is created, the value of widgetCount increments by 1 and the current value of widgetCount is displayed in the Output panel.

To create an instance counter using a class variable:

1. Select File > New and then select ActionScript File, and then click OK.
2. Type the following code into the Script window:

   class Widget {
       //Initialize the class variable
       public static var widgetCount:Number = 0; 
       public function Widget() {
           Widget.widgetCount++;
           trace("Creating widget #" + Widget.widgetCount);
       }
   }
   

   The widgetCount variable is declared as static, so it initializes to 0 only once. Each time the Widget class's constructor statement is called, it adds 1 to widgetCount and then shows the number of the current instance that's being created.

3. Save your file as Widget.as.
4. Select File > New and then select Flash Document to create a new FLA, and save it as widget_test.fla in the same directory as Widget.as.
5. In widget_test.fla, type the following code into Frame 1 of the Timeline:

   // Before you create any instances of the class,
   // Widget.widgetCount is zero (0).
   trace("Widget count at start: " + Widget.widgetCount); // 0
   var widget1:Widget = new Widget(); // 1
   var widget2:Widget = new Widget(); // 2
   var widget3:Widget = new Widget(); // 3
   trace("Widget count at end: " + Widget.widgetCount); // 3 
   

6. Save the changes to widget_test.fla.
7. Select Control > Test Movie to test the file.

   Flash displays the following information in the Output panel:

   Widget count at start: 0
   Creating widget # 1
   Creating widget # 2
   Creating widget # 3
   Widget count at end: 3
   

Flash CS3