请问NetStream在什么时候,bytesTotal,bytesLoaded的值大于0

做个播放的,下载的进度条,但是播放的好做,
做到下载的时候碰到了ns.bytesTotal=bytesLoaded=0,
请问
设置NetStream预先加载的属性是哪个?
bytesTotal,bytesLoaded在什么时候调用才有值?

package {
    private var connStr:String = "rtmp://192.168.1.145/vod";
     private var ns:NetStream;
    private var nc:NetConnection;
    private var video:Video;

     private function videoStatus():void{
          trace(ns.bytesTotal);trace(ns.bytesLoaded);
     }

    private function conn(connstr:String):void {
        nc = new NetConnection();
        nc.connect(connstr);
    }

   private function connectStream():void {
       ns = new NetStream(nc);
       ns.bufferTime = 8;
       video = new Video(stage.stageWidth,stage.stageHeight-22);
       video.attachNetStream(ns);
       video.smoothing = true;
       ns.play(videoURL);
       addChild(video);
     
      videoInterval = setInterval(videoStatus,1000);
  }
}

请帮我看看,谢谢.