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    private function init():void
    {
     balls = new Array();
     for(var i:uint = 0; i < numBalls; i++)
     {
      var ball:animation_ch08_Ball = new animation_ch08_Ball(20);
      addChild(ball);
      balls.push(ball);
     }
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }
  
   

    private function onEnterFrame(event:Event):void
    {
     graphics.clear();
     graphics.lineStyle(1);
     graphics.moveTo(mouseX, mouseY);
     moveBall(balls[0], mouseX, mouseY);
     graphics.lineTo(balls[0].x, balls[0].y);
     
     for(var i:uint = 1; i < numBalls; i++)
     {
      var ballA:animation_ch08_Ball = balls[i-1];
      var ballB:animation_ch08_Ball = balls[i];
      moveBall(ballB, ballA.x, ballA.y);
      graphics.lineTo(ballB.x, ballB.y);
     }
    }
   

    private function moveBall(ball:animation_ch08_Ball, targetX:Number, targetY:Number):void
    {
     ball.vx += (targetX - ball.x) * spring;
     ball.vy += (targetY - ball.y) * spring;
     ball.vy += gravity;
     ball.vx *= friction;
     ball.vy *= friction;
     ball.x += ball.vx;
     ball.y += ball.vy;
    }
   }
  }