13.3 Avatar: The Composition Version

Now that we've seen how to implement Avatar as a MovieClip subclass, let's consider an alternative using object composition. This time, the Avatar class does not inherit from MovieClip . Instead, it creates an instance of AvatarSymbol and stores it in an instance property, av_mc . (Well, technically, the ActionScript object representation of that instance is stored in av_mc , but here is an example in which making a distinction between the symbol instance and the ActionScript object would make the discussion awkward , albeit more technically accurate. My editor tells me to "Avoid precision where it merely sacrifices clarity." So I will here and elsewhere in this book.)

Example 13-2 shows a revised version of the Avatar class. This version defines a constructor because, now that Avatar is no longer a subclass of MovieClip , instances are instantiated with new . The AvatarSymbol clip instance is created in the Avatar constructor, which replaces the createAvatar( ) method. Because initialization is usually performed in a class's constructor function, we put some of the code from init( ) in the constructor and move the rest to a new method, setPosition( ) .

Example 13-2. The composition-based Avatar class

 class Avatar {   public static var HAPPY:Number = 0;   public static var SAD:Number = 1;   public static var IDLE:Number = 2;   private var av_mc:MovieClip;   public function Avatar (name:String, target:MovieClip,                            depth:Number, x:Number, y:Number) {     av_mc = target.attachMovie("AvatarSymbol", name, depth);     setState(Avatar.HAPPY);     setPosition(x, y);   }   public function setState(newState:Number):Void {     switch (newState) {       case Avatar.HAPPY:         av_mc.gotoAndStop("HAPPY");         break;       case Avatar.SAD:         av_mc.gotoAndStop("SAD");         break;       case Avatar.IDLE:         av_mc.gotoAndStop("IDLE");         break;     }   }   public function setPosition (x:Number, y:Number):Void {     av_mc._x = x;     av_mc._y = y;   } } 

In many ways, the composition version of the Avatar class is cleaner and more flexible than its MovieClip subclass counterpart ”cleaner because it uses fewer methods and a traditional constructor, more flexible because it can easily change the av_mc to some other movie clip symbol if required. However, the MovieClip subclass version would still be preferable in the following situations:

• When Avatar instances need to belong to the MovieClip datatype (e.g., for the sake of polymorphism)

• When the Avatar class and the AvatarSymbol movie clip symbol need to be packaged together as a component for distribution

• When Avatar instances need most of the methods of the MovieClip class (in which case the "Is-A" relationship is legitimate )