从后台转换到活动状态后，如何恢复mp4视频？Xcode 10.1，Swift 4.2

import AVFoundation

class VideoPlayerViewController: UIViewController {
    var player: AVPlayer?
    var playerItem: AVPlayerItem?
    var playerView: AVPlayerLayer?
    var playerURL: URL?
    var isPlaying = false
    var currentTime: CMTime?

    override func viewDidLoad() {
        super.viewDidLoad()
        // 视频播放器视图
        playerView = AVPlayerLayer()
        playerView?.frame = view.bounds
        view.layer.addSublayer(playerView!)
        
        // 视频播放器
        player = AVPlayer()
        playerView?.player = player
        
        // 监听应用进入前台
        NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)
    }
    
    @objc func willEnterForeground() {
        // 恢复视频播放
        if let currentTime = currentTime {
            player?.seek(to: currentTime)
            player?.play()
        }
    }
    
    func playVideo(url: URL) {
        playerURL = url
        playerItem = AVPlayerItem(url: playerURL!)
        player?.replaceCurrentItem(with: playerItem)
        player?.play()
        isPlaying = true
    }
    
    func pauseVideo() {
        player?.pause()
        isPlaying = false
        currentTime = player?.currentTime()
    }
    
    func stopVideo() {
        player?.pause()
        player?.seek(to: CMTime.zero)
        isPlaying = false
        currentTime = nil
    }
}