如何在swift 3中将AVPlayer从4秒循环到8秒？

import AVFoundation

// 创建一个AVPlayerItem对象
let playerItem = AVPlayerItem(url: yourVideoURL)

// 创建一个AVPlayer对象
let player = AVPlayer(playerItem: playerItem)

// 监听播放进度
player.addPeriodicTimeObserver(forInterval: CMTime(seconds: 1, preferredTimescale: CMTimeScale(NSEC_PER_SEC)), queue: DispatchQueue.main) { time in
    // 当播放时间达到8秒时，将播放时间设置为4秒
    if time.seconds >= 8 {
        player.seek(to: CMTime(seconds: 4, preferredTimescale: CMTimeScale(NSEC_PER_SEC)))
    }
}

// 播放视频
player.play()