ActionScript 3.0 Design Patterns

Using Flash’s Built-in Composite Structure: the Display List | 237

Unlike theShapeclass,Spritedoes inherit fromInteractiveObject. Therefore, com-

posite components in the airplane can respond to user interface events. We can now

develop the component and composite classes that make up the airplane.

Creating the Fuselage, Wings, and Engines

Now that theComponentandCompositeclasses have been developed, we can create

the nodes that make up the airplane. Example 6-21 through Example 6-24 show the

Fuselage,MainWing,TailWing, andEngineclasses. TheFuselageandMainWingclasses

represent composite nodes that hold other components and extend theComposite

class (see Example 6-20). TheTailWingandEngineclasses are leaf nodes and extend

theComponentclass (see Example 6-19). The implementation of all these subclasses is

very similar to each other. The component is drawn with simple lines, using the

graphics property of theDisplayObjectclass. The first parameter of the constructor

method is the weight, and the second is the initial damage (defaults to zero). Note

the call to the superclass constructor using thesuperkeyword in the last line of the

constructor. This ensures proper initialization of properties defined in the superclass.

Example 6-21. Fuselage.as

package
{
import flash.events.*;

public class Fuselage extends Composite
{
public function Fuselage(weight:Number, damage:Number = 0)
{
graphics.lineStyle(40, 0xC0C0C0);
graphics.moveTo(0, 0);
graphics.lineTo(0, 150);
super(weight, damage);
}
}
}

Example 6-22. MainWing.as

package
{
public class MainWing extends Composite
{
public function MainWing(weight:Number, damage:Number = 0)
{
graphics.lineStyle(25, 0x999999);
graphics.moveTo(0, 0);
graphics.lineTo(100, 0);
super(weight, damage);
}
}
}