ios - 标识符为 backgroundSession 的后台 URLSession 已经存在

Question

我正在使用 NSURLSession 和 NSURLSessionDelegate 从服务器获取 xml 数据。取决于我与服务器连接的某些条件。如果我正在连接服务器，一切正常，没有任何错误，但如果我没有连接(取决于条件)到服务器并移动到另一个 View Controller (通过使用 Storyboard？.instantiateViewControllerWithIdentifier(id))，我得到以下信息IOS 错误:

'具有标识符 backgroundSession 的后台 URLSession 已经存在!'

这是我的代码:

class MainClass: UITableViewController, NSURLSessionDelegate {     

   var task_service = NSURLSessionDataTask?()

   override func viewDidLoad() {
      super.viewDidLoad()

      if(condition) {
        getXMLFromServer()
      }

   }

   func getXMLFromServer(){

     task_service = getURLSession().dataTaskWithRequest() {

        (data, response, error) -> Void in

        dispatch_async(dispatch_get_main_queue(), {

         // Fetching data from server 

         // In the end
         self.session.invalidateAndCancel()
       }
    }

  }

 func getURLSession() -> NSURLSession {

    let configuration =      NSURLSessionConfiguration.defaultSessionConfiguration()

    configuration.timeoutIntervalForRequest = 30.0

    session = NSURLSession(configuration: configuration, delegate: self, delegateQueue: NSOperationQueue.mainQueue())

    return session
  }

 func URLSession(session: NSURLSession, task: NSURLSessionTask, didReceiveChallenge challenge: NSURLAuthenticationChallenge, completionHandler: (NSURLSessionAuthChallengeDisposition, NSURLCredential?) -> Void) {

    completionHandler(NSURLSessionAuthChallengeDisposition.UseCredential, NSURLCredential(forTrust: challenge.protectionSpace.serverTrust!)) // Bypassing SSL error
 }   
}

编辑:找到错误原因。

由于在调用的 View Controller 中创建 NSURLSession 而发生错误。调用的 VC 包含从服务器下载 PDF 的代码。但我不知道如何解决这个问题。下面是调用VC的代码

class MainFormsController: UIViewController, UIPickerViewDelegate, UITextFieldDelegate, NSURLSessionDownloadDelegate, UIDocumentInteractionControllerDelegate, MFMailComposeViewControllerDelegate{

 var download_task = NSURLSessionDownloadTask?()
 var backgroundSession = NSURLSession()

 override func viewDidLoad() {
    super.viewDidLoad()

     createNSURLSession()
 }               

 /** Error occurred while creating this NSURLSession **/

    func createNSURLSession() { 

        let backgroundSessionConfiguration =  NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier("backgroundSession")

       backgroundSession = NSURLSession(configuration:   backgroundSessionConfiguration, delegate: self, delegateQueue:   NSOperationQueue.mainQueue())
    }

  func downloadPDF() {

     //Download PDF
     download_task = backgroundSession.downloadTaskWithURL(url)
     download_task?.resume()
  }

}

Answer 1

在您的 MainFormsController 中添加此代码:

deinit {
        self.backgroundSession.finishTasksAndInvalidate();
    }