AVPlayerViewController下一步/上一步按钮不工作吗？

import AVKit

class CustomPlayerViewController: AVPlayerViewController {
    var playerItems: [AVPlayerItem] = [] // 播放列表
    var currentItemIndex: Int = 0 // 当前播放项索引
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 添加下一步按钮
        let nextButton = UIButton(type: .system)
        nextButton.setTitle("下一步", for: .normal)
        nextButton.addTarget(self, action: #selector(nextButtonTapped), for: .touchUpInside)
        nextButton.frame = CGRect(x: 0, y: 0, width: 80, height: 40)
        nextButton.center = CGPoint(x: view.bounds.width - 60, y: view.bounds.height - 60)
        view.addSubview(nextButton)
        
        // 添加上一步按钮
        let previousButton = UIButton(type: .system)
        previousButton.setTitle("上一步", for: .normal)
        previousButton.addTarget(self, action: #selector(previousButtonTapped), for: .touchUpInside)
        previousButton.frame = CGRect(x: 0, y: 0, width: 80, height: 40)
        previousButton.center = CGPoint(x: 60, y: view.bounds.height - 60)
        view.addSubview(previousButton)
    }
    
    @objc func nextButtonTapped() {
        if currentItemIndex < playerItems.count - 1 {
            currentItemIndex += 1
            player?.replaceCurrentItem(with: playerItems[currentItemIndex])
            player?.play()
        }
    }
    
    @objc func previousButtonTapped() {
        if currentItemIndex > 0 {
            currentItemIndex -= 1
            player?.replaceCurrentItem(with: playerItems[currentItemIndex])
            player?.play()
        }
    }
}