jwplayer - 如何在播放前获取视频时长？

我试图在 jwplayer 开始播放之前获取视频的持续时间。我尝试在onReady事件回调中调用getDuration()，但它返回-1。当我在 onPlay 事件回调中调用 getDuration () 时，我得到了正确的值。有任何想法吗？

这是我的代码：

<video src="movie.mp4" id="video" width="800" height="600"></video>
<script type="text/javascript">
jwplayer ('video').setup ({
    flashplayer: '/js/mediaplayer-5.10/player.swf',
    width: 600,
    height: 400,
    events: {
        onReady: function () {
            var duration = this.getDuration();
            alert ('ready, duration: ' + duration);
        },
        onPlay: function (state) {
            alert ('play, duration: ' + this.getDuration());
        }
    }
});
</script>

我大概是这样处理这个问题的：

var duration = 0;
jwplayer().onReady(function() {
  if (duration == 0) {
    // we don't have a duration yet, so start playing
    jwplayer().play();
  }
});

jwPlayer().onTime(function() {
  if (duration == 0) {
    // we don't have a duration, so it's playing so we can discover it...
    duration = jwplayer().getDuration();
    jwplayer().stop();
    // do something with duration here
  } else {
    ...
  }
}