在初始化 AvPlayer 时，可以使用 KVO 观察 "currentItem" 属性，并在属性更改时添加和删除 Observer。这样，当您更改 AvPlayer 的项目时，您的 Observer 也会相应地更改。

示例代码：

class ViewController: UIViewController { var player: AVPlayer!

override func viewDidLoad() {
    super.viewDidLoad()
    // Initialize player with first item
    let url = URL(string: "https://example.com/video1.mp4")!
    player = AVPlayer(url: url)

    // Add observer for currentItem
    player.addObserver(self, forKeyPath: "currentItem", options: [.new, .initial], context: nil)
}

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if keyPath == "currentItem" {
        // Your code here
    }
}

deinit {
    // Remove observer
    player.removeObserver(self, forKeyPath: "currentItem")
}

func playNextItem() {
    // Change to next item
    let url = URL(string: "https://example.com/video2.mp4")!
    let item = AVPlayerItem(url: url)
    player.replaceCurrentItem(with: item)
}

}