ios - Swift:URLSessionDownload 文件说它存在但实际上不存在？

Question

好的，我已经成功下载了各种 m4a 文件并通过 URLSession 删除了它们。我的问题是 URLSessionDownloadDelegate 要求的最终“完成”功能，有时我会在控制台打印以下内容，即使我检查我的下载功能(在下载之前)文件是否存在于目录中。很困惑。这是消息:

File download succesfully
    “CFNetworkDownload_1wGgxs.tmp” couldn’t be moved to “Documents” because an item with the same name already exists.
    The task finished successfully

这是下载功能，我在其中明确检查文件是否存在:

func goDownload()
    {
        if let audioUrl = downloadUrl { //set at beginning

            let documentsDirectoryURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
            let destinationUrl = documentsDirectoryURL.appendingPathComponent(audioUrl.lastPathComponent)
            print(destinationUrl)

            // to check if it exists before downloading it
            if FileManager.default.fileExists(atPath: destinationUrl.path) {
                print("********** The file already exists at path")

                // if the file doesn't exist
            } else {

                print("---------------> Starting Download")


                currentTask = session.downloadTask(with: audioUrl)
                currentTask.resume()
            }
        }
    }

这是完成函数:

func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {

        print("File download succesfully")
        let documentsDirectoryURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!

        // lets create your destination file url
        let destinationUrl = documentsDirectoryURL.appendingPathComponent((downloadUrl?.lastPathComponent)!)

        do {
            try FileManager.default.moveItem(at: location, to: destinationUrl)

            //success
            print("************** SUCCESS File moved to documents folder", downloadUrl)
            playModeStreaming = false

            self.pausePlay()
            AudioPlayerManager.shared.play(url: downloadUrl)

        } catch let error as NSError {
            print(error.localizedDescription)
        }


    }

我什至实现了一个检查函数，它返回文件是否存在，在收到上述消息后，它返回 false:

func checkIfExists(url: URL)->Bool
    {
        return FileManager.default.fileExists(atPath: url.path)
    }

这是什么原因造成的？我怎样才能确保它下载以便它可以播放 m4a？

Answer 1

您的代码非常接近 Apple's sample code在标题为“处理下载完成和错误”的部分中。但不完全是……您的 print("File download successfully") 做出了很大的假设!

我在 urlSession:... didFinishDownloadingTo: 中看到的差异是您跳过了 downloadTask.response.statusCode 的检查以确保下载成功。 Apple 检查它在 200...299 范围内。

您可以尝试添加此 HTTP 状态代码检查，以防您收到的错误消息是虚假的并且您实际上遇到了网络故障。

苹果网站的校验码:

...
didFinishDownloadingTo location: URL) {

guard let httpResponse = downloadTask.response as? HTTPURLResponse,
    (200...299).contains(httpResponse.statusCode) else {
        print ("server error")
        return
}

... now do the FileManager code