Ios m3u8视频的AVURLAsset持续时间为0

我正在尝试获取m3u8格式的远程视频（通过http流传输）的持续时间。对于某些本地持续时间较长的视频，无法获取资源的持续时间。 我目前正在使用KVO捕获AVPlayerItem的.Ready状态，然后调用委托方法

let asset = AVURLAsset(URL: url)
let playerItem = AVPlayerItem(asset: asset!)
let player = AVPlayer(playerItem: playerItem!)

....

override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
    if object as? AVPlayerItem == player?.currentItem {
        if keyPath == "status" {
            guard let status = player?.currentItem?.status
                else { return }

            delegate?.videoViewController(self, statusDidChange: status)
        }
    }
}

print(asset?.duration)
print(playerItem?.duration)
print(playerItem?.asset.duration)
print(playerItem?.tracks[0].assetTrack.asset?.duration)