Swift3 同步发送多个URLSession dataTask请求并等待结束

我需要向服务器发送多个POST请求，但在执行此操作时遇到了同步问题

我需要做以下工作：

• 显示进度条（正在下载…）
• 发送第一个POST请求
• 每发送n张图片
  • 发送图片
• 结束

发送每篇文章时，禁用进度条 显示状态弹出消息

以下是我使用的代码：

• 调用第一个POST请求的func

  self.envoi{ (response) in
  if let result = response as? Bool {
  if(result == true){
      //now we are sending the Photos !
      var i = 0;
      while(i <      selectedPictures.sharedInstance.selectedCells.count){
          self.envoiPhoto(obj: PhotoGallery.sharedInstance.photoGallery[item], pic: self.Image, num: item){ (result) -> () in
                  print("Envoi Photo \(i): \(result)")
          }
          i=i+1;
      }//end loop while for each pic
  
      print("we have sent all the pictures")
  
      alertController.dismiss(animated: true, completion: ({
      }))
      self.envoiSuccess()
  }
  else{
      print("erreur d'envoi")
      self.envoiError()
  }
  }
  }
  

  第二张func环境照片：

      func envoiPhoto(obj: Photo, pic: UIImage, num: Int, completion: @escaping (_ result: Bool)->()){
  
      let url = URL(string: "\(KAIROS_SERVER)/MobileCreatePhoto")
      var request = URLRequest(url: url!)
      request.httpMethod = "POST"
      let boundary = self.generateBoundaryString()
      request.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
  
      let image_data = UIImagePNGRepresentation(self.fixOrientation(img: self.constatImage))
  
      let body = NSMutableData()
  
      //preparing values for pictures
      let fname = "Constat-\(reference!)-\(num).png"
      let mimetype = "image/png"
      let CRLF = "\r\n";
  
      //send parameter id
      body.append("--\(boundary)\(CRLF)".data(using: String.Encoding.utf8)!)
      body.append("Content-Disposition: form-data; name=\"id\"\(CRLF)".data(using: String.Encoding.utf8)!)
      body.append("Content-Type: text/plain; charset=UTF-8\(CRLF)\(CRLF)".data(using: String.Encoding.utf8)!)
      body.append("\(id!)\(CRLF)".data(using: String.Encoding.utf8)!)
  
      body.append("--\(boundary)\(CRLF)".data(using: String.Encoding.utf8)!)
      body.append("Content-Disposition: form-data; name=\"file\"; filename=\"\(fname)\"\(CRLF)".data(using: String.Encoding.utf8)!)
      body.append("Content-Type: \(mimetype)\(CRLF)\(CRLF)".data(using: String.Encoding.utf8)!)
      body.append(image_data!)
  
      body.append("\(CRLF)".data(using: String.Encoding.utf8)!)
  
      body.append("--\(boundary)--\(CRLF)".data(using: String.Encoding.utf8)!)
  
      request.httpBody = body as Data
  
      let configuration = URLSessionConfiguration.default
      let session = Foundation.URLSession(configuration: configuration,
                                          delegate: self,
                                          delegateQueue:OperationQueue.main)
  
      let task = session.dataTask(with: request, completionHandler: {
          (
          data, response, error) in
  
          guard let _:Data = data, let _:URLResponse = response, error == nil else {
              print("error")
              completion(false)
              return
          }
  
          var id = "0";
          var result = "";
          // Print out response string
          //print("response -- \(response)")
          if let response = response as? HTTPURLResponse {
              if response.statusCode == 200 {
                  print("Create Photo Success")
              }
              id = response.allHeaderFields["id"]! as! String
              print("ID Photo created: \(id)");
              result = response.allHeaderFields["result"]! as! String;
              print("Result: \(result)");
          }
          completion(true)
  
      })
      task.resume() 
  }
  

  今天我主要关心的是确保在向最终用户显示进度条时同步发送每个HTTP POST。 当第一个HTTP POST和每个发送图片的HTTP POST完成时，我需要禁用进度条并显示另一个详细说明操作的alertView
  到今天为止，这些操作尚未同步，即使操作尚未完成，代码也会尝试禁用进度条。 照片上传的启动是在第一次HTTP POST之后启动的（因为它在回调中启动），但由于我需要为每个要发送的图片实现一个循环，我不知道如何在不阻塞URLSession数据任务的情况下同步每个上传（如果我使用DispatchGroup，则是这种情况）
  任何帮助都将不胜感激
  ------------------------------------------------------------------------- 编辑1
  我试图在代码中实现DispatchGroup，但没有按预期同步任何内容
  代码如下：

              var myGroup = DispatchGroup();
  
          self.docSent = false;
          self.allPhotosSent = false
          self.myGroup.enter()
          print("First sending the doc")
  
          self.envoi{ (response) in
              if let result = response as? Bool {
                  if(result == true){
                      self.docSent = true
                      print("Doc sent, now sending photos")
  
                      //now we are sending the Photos !
                      for it in selectedPictures.sharedInstance.selectedCells{
                          print("Sending selected pic item:\(it.item)");
                          print("retrieving picture: \(fileName!)")
                          self.constatImage = self.getSavedImage(named: fileName!)!                            
                          self.envoiPhoto(obj: PhotoGallery.sharedInstance.photoGallery[it.item], pic: self.Image, num: it.item){ (result) -> () in
                              print("Envoi Photo \(it.item): \(result)")
                              if(it.item == selectedPictures.sharedInstance.selectedCells.count-1){
                                  self.allPhotosSent = true
                              }
                          }
                      }
                  }
                  //Case when doc sending is failed
                  else{
                      self.docSent = false
                  }
  
              }
          }//end envoi doc
          self.myGroup.leave()
  
          self.myGroup.notify(queue: .main, execute: {
              print("entering in notify")
              print("we have sent all the pictures")
              if(self.docSent && self.allPhotosSent){
                  //now we need to leave the progress bar...
                  alertController.dismiss(animated: true, completion: ({
                      print("now the alert is dismissed !")
                      self.envoiSuccess()
                  }))  
              }
              else{
                  print("erreur d'envoi de doc")
                  alertController.dismiss(animated: true, completion: ({
                      print("now the alert is dismissed !")
                      self.envoiError()
                  }))
              }
  
          })
  

  以及日志：

  First sending the doc
  entering in notify
  we have sent all the pictures
  erreur d'envoi de doc
  Result: success
  doc sent, now sending photos
  Sending selected pic item:0
  retrieving picture: IMG_0241
  Sending selected pic item:1
  retrieving picture: IMG_1265
  now the alert is dismissed !
  Result: success
  Envoi Photo 0: true
  Result: success
  Envoi Photo 1: true
  

  ---

  当然，我错过了一些东西，因为GCD似乎没有像我期望的那样工作。
  不要等待。使用

  DispatchGroup

  在最后一个请求完成时获得通知。在Swift-never

  while

  线性索引循环中，我需要使用什么来代替while？一个

  for-in

  循环

  First sending the doc
  entering in notify
  we have sent all the pictures
  erreur d'envoi de doc
  Result: success
  doc sent, now sending photos
  Sending selected pic item:0
  retrieving picture: IMG_0241
  Sending selected pic item:1
  retrieving picture: IMG_1265
  now the alert is dismissed !
  Result: success
  Envoi Photo 0: true
  Result: success
  Envoi Photo 1: true