swift - 当应用程序在iOS12中进入后台时，URLSessionDelegate的didWriteData不会被调用

我想实现下载功能，可以用百分比显示下载任务的完成状态。我可以做到，但问题是当应用程序移到后台并返回前台时，didWriteData中没有调用delegate方法iOS12。有人能帮我吗？这是我的密码

protocol DownloadDelagate {
    func downloadingProgress(value:Float)
    func downloadCompleted(identifier: Int,url: URL)
}

class DownloadManager : NSObject, URLSessionDelegate, URLSessionDownloadDelegate {

    static var shared = DownloadManager()
    var delegate: DownloadDelagate?
    var backgroundSessionCompletionHandler: (() -> Void)?

    var session : URLSession {
        get {

            let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
            config.isDiscretionary = true
            config.sessionSendsLaunchEvents = true
            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) {
        delegate?.downloadCompleted(identifier: downloadTask.taskIdentifier, url: location)
    }

    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
            delegate?.downloadingProgress(value: progressPercentage)
            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.")
        }
    }
}

最佳答案

基于this thread这是NSURLSesstion中的一个bug。目前有已知的解决方法（经苹果工程师批准）：

var session: URLSession?
...
func applicationDidBecomeActive(_ application: UIApplication) {
    session?.getAllTasks { tasks in
        tasks.first?.resume() // It is enough to call resume() on only one task
        // If it didn't work, you can try to resume all
        // tasks.forEach { $0.resume() }
    }
}