应用程序处于后台时，Alamofire后台上载不会打印进度

Question

我正在使用PDF将Alamofire文件上传到服务器。我创建了backgroundSessionManager以确保app在应用程序处于后台时上传文件。当我从backgroundCompletionHandler方法AppDelegate调用handleEventsForBackgroundURLSession时，代码完美无缺。

进展是我正在显示进度，当应用程序进入后台时，进度被卡住，.uploadProgress完成也没有调用。因此，当用户进入后台并且在返回一段时间之后，进度不会增加。

共享实例：

class Networking {
    static let sharedInstance = Networking()
    public var backgroundSessionManager: Alamofire.SessionManager // your web services you intend to keep running when the system backgrounds your app will use this
    private init() {
        self.backgroundSessionManager = Alamofire.SessionManager(configuration: URLSessionConfiguration.background(withIdentifier: "com.dw.myapp"))

        var backgroundCompletionHandler: (() -> Void)? {
            get {
                return backgroundSessionManager.backgroundCompletionHandler
            }
            set {
                backgroundSessionManager.backgroundCompletionHandler = newValue
            }
        }
    }
}

上传文件代码

Networking.sharedInstance.backgroundSessionManager.upload(multipartFormData: { (multipartData) in
 }, usingThreshold: UInt64.init(), to: "\(url)", method: .post, headers: headers) { (result) in
            switch result {
            case .success(let upload, _, _):
                upload.uploadProgress(closure: { (progress) in
                //Does not call while app is in background
                        Helper.dispatchMain {
                            print(progress)
                            print("Upload Time ", Date())
                        }
                    })
                    upload.responseJSON { response in
                        print("Response Time ", Date())
                        print("Response Time ", Date())
                    }
                case .failure(let error):
                    completion!(["message" : error],false)
                }
            }

我搜索了SO和github页面但是没有找到解决方案。如果我犯了任何错误，任何人都可以指导吗？