MACROMEDIA FLASH 8-LEARNING ACTIONSCRIPT 2.0 IN FLASH, Manual

Page: 305 / 830

About writing subclasses in Flash 305
5. In SubWidget.as, type the following code into the Script window:
class SubWidget extends Widget {
public function SubWidget() {
trace("Creating subwidget #" + Widget.widgetCount);
}
}
6. Save your changes to SubWidget.as.
7. Create a new FLA file, and save it as subWidgetTest.fla in the same directory as the
previous ActionScript class files.
8.
In the subWidgetTest.fla file, type the following code into Frame 1 of the main Timeline:
var sw1:SubWidget = new SubWidget();
var sw2:SubWidget = new SubWidget();
trace("Widget.widgetCount = " + Widget.widgetCount);
trace("SubWidget.widgetCount = " + SubWidget.widgetCount);
The previous code creates two instances of the SubWidget class: sw1 and sw2. Each call to
the
SubWidget
constructor traces the current value of the static
Widget.widgetCount
property. Because the SubWidget class is a subclass of the Widget class, you can access the
widgetCount
property through the SubWidget class, and the compiler rewrites the
reference (in the bytecode, not in your ActionScript file) as
Widget.widgetCount
. If you
try to access the static
widgetCount
property off of instances of the Widget or SubWidget
class, like sw1 or sw2, the compiler throws an error.
9. Save your changes to the document.
10. Select Control > Test Movie to test the Flash document.
The Output panel displays the following output:
Creating subwidget #1
Creating subwidget #2
Widget.widgetCount = 2
SubWidget.widgetCount = 2
You see this output because even though the Widget class’s constructor is never explicitly
called, the SubWidget class’s constructor calls it for you. This causes the Widget class’s
constructor to increment the Widget class’s static
widgetCount
variable.
The ActionScript 2.0 compiler can resolve static member references within
class definitions.