Æliens

    private function init():void
    {
     stage.scaleMode = StageScaleMode.NO_SCALE;
     stage.align = StageAlign.TOP_LEFT;
     particles = new Array();
     for(var i:uint = 0; i < numParticles; i++)
     {
      var size:Number = Math.random() * 10 + 2;
      var particle:animation_ch12_Ball = new animation_ch12_Ball(size, 0xffffff);
      particle.x = Math.random() * stage.stageWidth;
      particle.y = Math.random() * stage.stageHeight;
      particle.vx = Math.random() * 6 - 3;
      particle.vy = Math.random() * 6 - 3;
      particle.mass = size;
      addChild(particle);
      particles.push(particle);
     }
     
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }
  

    private function onEnterFrame(event:Event):void
    {
     graphics.clear();
     for(var i:uint = 0; i < numParticles; i++)
     {
      var particle:animation_ch12_Ball = particles[i];
      particle.x += particle.vx;
      particle.y += particle.vy;
      if(particle.x > stage.stageWidth)
      {
       particle.x = 0;
      }
      else if(particle.x < 0)
      {
       particle.x = stage.stageWidth;
      }
      if(particle.y > stage.stageHeight)
      {
       particle.y = 0;
      }
      else if(particle.y < 0)
      {
       particle.y = stage.stageHeight;
      }
     }
       
     for(i=0; i < numParticles - 1; i++)
     {
      var partA:animation_ch12_Ball = particles[i];
      for(var j:uint = i + 1; j < numParticles; j++)
      {
       var partB:animation_ch12_Ball = particles[j];
       spring(partA, partB);
      }
     }
    }
   

    private function spring(partA:animation_ch12_Ball, partB:animation_ch12_Ball):void
    {
     var dx:Number = partB.x - partA.x;
     var dy:Number = partB.y - partA.y;
     var dist:Number = Math.sqrt(dx * dx + dy * dy);
     if(dist < minDist)
     {
      graphics.lineStyle(1, 0xffffff, 1 - dist / minDist);
      graphics.moveTo(partA.x, partA.y);
      graphics.lineTo(partB.x, partB.y);
      var ax:Number = dx * springAmount;
      var ay:Number = dy * springAmount;
      partA.vx += ax / partA.mass;
      partA.vy += ay / partA.mass;
      partB.vx -= ax / partB.mass;
      partB.vy -= ay / partB.mass;
     }
    }
   }
  }