AVPlayerItem的duration属性是一个计算属性，表示当前项目的持续时间。为了保证duration属性正确，需要等待项目的基础媒体数据准备完毕，否则duration将返回0。解决方案是在使用duration属性前，先确保项目的基础媒体数据已经准备完毕。

以下是一个使用AVPlayer和AVPlayerItem的示例代码：

let url = URL(string: "http://example.com/video.mp4")
let playerItem = AVPlayerItem(url: url!)
let player = AVPlayer(playerItem: playerItem)

playerItem.addObserver(self, forKeyPath: #keyPath(AVPlayerItem.status), options: [.new], context: nil)

...

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
   if keyPath == #keyPath(AVPlayerItem.status) {
      let status: AVPlayerItem.Status
      
      if let statusNumber = change?[.newKey] as? NSNumber {
         status = AVPlayerItem.Status(rawValue: statusNumber.intValue)!
      } else {
         status = .unknown
      }
      
      switch status {
      case .readyToPlay:
          let duration = playerItem.duration
          //使用duration属性
      case .failed:
          //处理错误
      case .unknown:
          break
      }
   }
}