Æliens

    private function init():void
    {
     stage.scaleMode = StageScaleMode.NO_SCALE;
     stage.align=StageAlign.TOP_LEFT;
  
     ball = new animation_ch07_Ball();
     ball.x = stage.stageWidth / 2;
     ball.y = stage.stageHeight / 2;
     vx = Math.random() * 10 - 5;
     vy = -10;
     addChild(ball);
     ball.addEventListener(MouseEvent.MOUSE_DOWN, onMouseDown);
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }
   

    private function onEnterFrame(event:Event):void
    {
     vy += gravity;
     ball.x += vx;
     ball.y += vy;
     var left:Number = 0;
     var right:Number = stage.stageWidth;
     var top:Number = 0;
     var bottom:Number = stage.stageHeight;
     
     if(ball.x + ball.radius > right)
     {
      ball.x = right - ball.radius;
      vx *= bounce;
     }
     else if(ball.x - ball.radius < left)
     {
      ball.x = left + ball.radius;
      vx *= bounce;
     }
     if(ball.y + ball.radius > bottom)
     {
      ball.y = bottom - ball.radius;
      vy *= bounce;
     }
     else if(ball.y - ball.radius < top)
     {
      ball.y = top + ball.radius;
      vy *= bounce;
     }
    }
   

    private function onMouseDown(event:MouseEvent):void
    {
     oldX = ball.x;
     oldY = ball.y;
     stage.addEventListener(MouseEvent.MOUSE_UP, onMouseUp);
     ball.startDrag();
     removeEventListener(Event.ENTER_FRAME, onEnterFrame);
     addEventListener(Event.ENTER_FRAME, trackVelocity);
    }
    
    private function onMouseUp(event:MouseEvent):void
    {
     stage.removeEventListener(MouseEvent.MOUSE_UP, onMouseUp);
     ball.stopDrag();
     removeEventListener(Event.ENTER_FRAME, trackVelocity);
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }
   

    private function trackVelocity(event:Event):void
    {
     vx = ball.x - oldX;
     vy = ball.y - oldY;
     oldX = ball.x;
     oldY = ball.y;
    }
   }
  }