我有一个函数，它可以播放一首歌，我在它的参数中给出了URL。我正在使用AVPlayer来做这件事。

func playSong(url:URL) {
        self.playerItem = AVPlayerItem(url: url)
        self.avPlayer = AVPlayer(playerItem: playerItem)
        self.avPlayer.automaticallyWaitsToMinimizeStalling = false
        self.avPlayer!.volume = 1.0
        self.avPlayer.play()
        let duration = (self.avPlayer.currentItem?.asset.duration)!
        let totalDuration : CMTime = duration
        let seconds : Float64 = CMTimeGetSeconds(totalDuration)
        self.totalAudioPlaybackTime.text = self.stringFromTimeInterval(interval: seconds)
        self.sliderProgress.maximumValue = Float(seconds)
        self.timeObserver = self.avPlayer.addPeriodicTimeObserver(forInterval: CMTime.init(value: 1, timescale: 1), queue: .main, using: { time in
            if let duration = self.avPlayer.currentItem?.duration{
                let time : Float64 = CMTimeGetSeconds(self.avPlayer!.currentTime())
                self.sliderProgress.value = Float(time)
                self.currentTime.text = self.stringFromTimeInterval(interval: time)
                }
            })
    }

字符串
其他一切都很好，但唯一的问题是在线路

let duration = (self.avPlayer.currentItem?.asset.duration)!

型
当这一行被执行时，它需要5到6秒（有时甚至10秒）来计算持续时间，而当这发生时，UI只是冻结，直到计算出持续时间。我不知道为什么会这样。请让我知道我做错了什么