URLSession委託成功方法未調用，但沒有錯誤

Question

我正在使用URLSession dataTask來下載帶有URLSessionDownloadDelegate作爲結果處理程序的文件。但是，從來沒有調用urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL)。相反，我得到urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?)與錯誤爲零。當使用completionHandler方法來執行任務時，一切正常。

這裏是我的代碼：

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() 
} 

} 

模擬器輸出

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

預先感謝您。

Answer 1

您應該使用URLSessionDownloadTask代替URLSessionDataTask和使用背景URLSessionConfiguration如下：

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() 

}