URLSession downloadTask 在后台运行时的行为？

Question

我有一个应用程序需要下载一个可能相当大的文件（可能大到 20 MB）。我一直在阅读 URLSession downloadTasks 以及当应用程序进入后台或被 iOS 终止时它们是如何工作的。我希望继续下载，从我读过的内容来看，这是可能的。我找到了一篇博文here，详细讨论了这个话题。

根据我的阅读，我首先创建了一个下载管理器类，如下所示：

class DownloadManager : NSObject, URLSessionDownloadDelegate, URLSessionTaskDelegate {

    static var shared = DownloadManager()

    var backgroundSessionCompletionHandler: (() -> Void)?

    var session : URLSession {
        get {
            let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
            return URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue())
        }
    }

    private override init() {
    }

    func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
        DispatchQueue.main.async {
            if let completionHandler = self.backgroundSessionCompletionHandler {
                self.backgroundSessionCompletionHandler = nil
                completionHandler()
            }
        }
    }

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        if let sessionId = session.configuration.identifier {
            log.info("Download task finished for session ID: \(sessionId), task ID: \(downloadTask.taskIdentifier); file was downloaded to \(location)")

            do {
                // just for testing purposes
                try FileManager.default.removeItem(at: location)
                print("Deleted downloaded file from \(location)")
            } catch {
                print(error)
            }         
        }
    }

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
        if totalBytesExpectedToWrite > 0 {
            let progress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
            let progressPercentage = progress * 100
            print("Download with task identifier: \(downloadTask.taskIdentifier) is \(progressPercentage)% complete...")
        }
    }

    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
        if let error = error {
            print("Task failed with error: \(error)")
        } else {
            print("Task completed successfully.")
        }
    }
}

我也在我的 AppDelegate 中添加了这个方法：

    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {

        DownloadManager.shared.backgroundSessionCompletionHandler = completionHandler

        // if the app gets terminated, I need to reconstruct the URLSessionConfiguration and the URLSession in order to "re-connect" to the previous URLSession instance and process the completed download tasks
        // for now, I'm just putting the app in the background (not terminating it) so I've commented out the lines below
        //let config = URLSessionConfiguration.background(withIdentifier: identifier)
        //let session = URLSession(configuration: config, delegate: DownloadManager.shared, delegateQueue: OperationQueue.main)

        // since my app hasn't been terminated, my existing URLSession should still be around and doesn't need to be re-created
        let session = DownloadManager.shared.session

        session.getTasksWithCompletionHandler { (dataTasks, uploadTasks, downloadTasks) -> Void in

            // downloadTasks = [URLSessionDownloadTask]
            print("There are \(downloadTasks.count) download tasks associated with this session.")
            for downloadTask in downloadTasks {
                print("downloadTask.taskIdentifier = \(downloadTask.taskIdentifier)")
            }
        }
    }

最后，我像这样开始我的测试下载：

    let session = DownloadManager.shared.session

    // this is a 100MB PDF file that I'm using for testing
    let testUrl = URL(string: "https://scholar.princeton.edu/sites/default/files/oversize_pdf_test_0.pdf")!            
    let task = session.downloadTask(with: testUrl)

    // I think I'll ultimately need to persist the session ID, task ID and a file path for use in the delegate methods once the download has completed

    task.resume()

当我运行此代码并开始下载时，我看到委托方法被调用，但我还看到一条消息：

A background URLSession with identifier com.example.testapp.background already exists!

我认为这是因为 application:handleEventsForBackgroundURLSession:completionHandler:

中的以下调用而发生的

let session = DownloadManager.shared.session

我的 DownloadManager 类中的 session 属性的 getter（我直接从前面引用的博客文章中获取）总是尝试使用后台配置创建一个新的 URLSession。据我了解，如果我的应用程序已被终止，那么这将是“重新连接”到原始 URLSession 的适当行为。但是由于可能应用程序没有被终止，而只是进入后台，所以当调用 application:handleEventsForBackgroundURLSession:completionHandler: 时，我应该引用现有的 URLSession 实例。至少我认为这就是问题所在。谁能为我澄清这种行为？谢谢！

Answer 1

您的问题是每次引用会话变量时都在创建一个新会话：

var session : URLSession {
        get {
            let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
            return URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue())
        }
    }

相反，将会话保留为实例变量，然后获取它：

class DownloadManager:NSObject {

    static var shared = DownloadManager()
    var delegate = DownloadManagerSessionDelegate()
    var session:URLSession

    let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")

    override init() {
        session = URLSession(configuration: config, delegate: delegate, delegateQueue: OperationQueue())
        super.init()
    }
}

class DownloadManagerSessionDelegate: NSObject, URLSessionDelegate {
    // implement here
}

当我在操场上这样做时，它表明重复调用会产生相同的会话，并且没有错误：

会话不在进程中，它是操作系统的一部分。每次访问写入的会话变量时都会增加引用计数，这会导致错误。