在Swift 4中，AVPlayerLoop不会提供无缝循环的功能。然而，您可以使用以下代码示例创建一个自定义循环播放器：

import AVFoundation

class SeamlessLoopPlayer {
    private var player: AVPlayer?
    
    init(url: URL) {
        let playerItem = AVPlayerItem(url: url)
        player = AVPlayer(playerItem: playerItem)
        
        // 监听播放结束事件
        NotificationCenter.default.addObserver(self, selector: #selector(playerDidFinishPlaying), name: NSNotification.Name.AVPlayerItemDidPlayToEndTime, object: player?.currentItem)
    }
    
    func play() {
        player?.play()
    }
    
    func pause() {
        player?.pause()
    }
    
    @objc private func playerDidFinishPlaying(note: NSNotification) {
        // 重置播放时间到起始点
        player?.seek(to: CMTime.zero)
        player?.play()
    }
}

使用示例：

let url = Bundle.main.url(forResource: "yourVideo", withExtension: "mp4")!
let loopPlayer = SeamlessLoopPlayer(url: url)
loopPlayer.play()

这个自定义循环播放器会在播放结束后将播放时间重置到起始点，并重新开始播放，从而实现无缝循环的效果。