未调用 URLSession 委托成功方法，但没有错误

Question

我正在使用 URLSession dataTask 下载文件，并将 URLSessionDownloadDelegate 作为结果处理程序。但是，urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) 永远不会被调用。相反，我得到 urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) 错误为零。当使用 completionHandler 方法代替执行任务时，一切正常。

这是我的代码：

import UIKit

class ViewController: UIViewController, URLSessionDownloadDelegate {

var downloadTask: URLSessionDataTask?

override func viewDidLoad() {
    super.viewDidLoad()

    let configuration = URLSessionConfiguration.default
    let session = URLSession(configuration: configuration, delegate: self, delegateQueue: OperationQueue.main)

    let url = URL(string: "https://unsplash.it/200/300/?random")!
    //downloadTask = session.dataTask(with: request)
    downloadTask = session.dataTask(with: url)
    downloadTask!.resume()
}


@IBAction func cancelButtonTapped(_ sender: Any) {
    downloadTask?.cancel()
}

func urlSession(_ session: URLSession, didBecomeInvalidWithError error: Error?) {
    print("session: didBecomeInvalidWithError: \(error?.localizedDescription)")
}
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
    print("Your data is here!")
}

func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
    let progress = Float(totalBytesWritten / totalBytesExpectedToWrite)
    print("Making progress: \(progress)")
}

func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
    print("session: task: didCompleteWithError: \(error?.localizedDescription)")
    session.finishTasksAndInvalidate()
}

}

模拟器输出为

session: task: didCompleteWithError: nil
session: didBecomeInvalidWithError: nil

提前致谢。

Answer 1

您应该使用 URLSessionDownloadTask 而不是 URLSessionDataTask 并使用 URLSessionConfiguration 的背景，如下所示：

var downloadTask: URLSessionDownloadTask?
var session: URLSession?

override func viewDidLoad() {
        super.viewDidLoad()


        let configuration = URLSessionConfiguration.background(withIdentifier: "backgroundSession")
        session = URLSession(configuration: configuration, delegate: self, delegateQueue: OperationQueue.main)

        let url = URL(string: "https://unsplash.it/200/300/?random")!
        downloadTask = session?.downloadTask(with: url)
        downloadTask!.resume()

}

未调用 URLSession 委托成功方法，但没有错误

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