Problem Creating a Background

programming flash
1

Hi everyone. I’m working in FlashDevelop and I have a question about an issue I’ve run into. (I posted this question in the FlashDevelop community and they recommended I bring the discussion over here).

I’m trying to design a space-like background in which particles fly down the screen. I have created a Background class and I’m using sprites to generate the particles. When they fly off-screen (say, when y > 420) I want them to disappear, but I can’t seem to figure out how. Here is the code for the Background class:

package
{
    import flash.display.Sprite;
    import flash.events.Event;
    import flash.display.MovieClip;
    
    public class Background extends Sprite
    {
        private var background:Sprite = new Sprite;
        private var speed:int = 2;
        private var _root:Object;
        
        public function Background():void
        {
            _root = MovieClip(root);
            addEventListener(Event.ADDED, create);
            addEventListener(Event.ENTER_FRAME, move);
        }
        
        private function create(e:Event):void
        {
            var size:int = Math.random() * 10;
            var position:int = Math.random() * 600;
            background.graphics.drawRect(0, 0, size,size);
            background.graphics.beginFill(0xFFED09, 1);
            background.x = position;
            background.y = -20;
            addChild(background);
        }
        
        private function move(e:Event):void
        {
            this.y += speed;
            if (this.y > 120)
            {
                removeEventListener(Event.ENTER_FRAME, move);
                _root.removeChild(this);
            }
        }
    }
}

The program will crash when y>120. (I’ve set 120 for ease of testing.)