Development Flash ActionScript

package
{
    import flash.display.Sprite;
    import flash.events.Event;
    import flash.events.MouseEvent;
    import flash.geom.Rectangle;
    public class Main extends Sprite
    {
        public static const SEEKING:String = "seeking";
        public static const FINISHED_SEEKING:String = "finishedSeeking";
        public var playhead:Sprite;
        public function Main()
        {
            this.graphics.beginFill(0xCCCCCC, 1);
            this.graphics.drawRect(0, 0, 100, 20);
            playhead = new Sprite();
            playhead.graphics.beginFill(0x0000ff, 1);
            playhead.graphics.drawRect(0, 0, 20, 20);
            addChild(playhead);
            playhead.x = -10;
            playhead.addEventListener(MouseEvent.MOUSE_DOWN, beginDrag);
        }
        private function beginDrag(mouseEvent:MouseEvent):void
        {
            playhead.startDrag(false, new Rectangle(0, 0, width, height));
            playhead.addEventListener(MouseEvent.MOUSE_UP, stopDragPlayhead);
            var event:Event = new Event("seeking");
            dispatchEvent(event);
        }
        private function stopDragPlayhead(mouseEvent:MouseEvent):void
        {
            playhead.stopDrag();
            var event:Event = new Event("finishedSeeking");
            dispatchEvent(event);
        }
        public function updatePlayhead(number:Number):void
        {
            playhead.x = number;
        }
    }
}