视图控制器弹出后需触发S3上传回调及XMPP消息发送

基于XMPP实现的聊天功能运行正常，当前开发图片上传功能，流程为：将图片上传至S3存储桶，上传完成后通过XMPP将S3图片URL作为消息发送给对方。该功能在停留在聊天页面时可正常运行，但如果在图片上传过程中退出聊天视图控制器（VC），控制器会被释放，导致S3上传完成后的回调无法触发，进而无法调用sendMessage方法发送消息。需要实现即使切换到其他页面，仍能触发上传完成回调并完成消息发送。

@IBAction func didPressMedia(_ sender: Any) {
        ImagePickerManager().pickImage(self) { [weak self] pickedImage in
        guard let self = self else { return }

        let compressedImage = pickedImage.resizedImage(newSize: CGSize(width: 120, height: 120))
        
        let messageId = XMPPHelper.shared.xmppStream.generateUUID

        let chat = ChatListModel()
        chat.roomId = ""
        chat.message = ""
        chat.messageId = messageId
        chat.time = String(Date().currentTimeMillis())
        chat.to = self.viewModel.otherUser
        chat.from = self.viewModel.myJid ?? ""
        chat.type = "chat/image"
        chat.imageLocalPath = messageId
        chat.msgDeliveryStatus = MsgDeliveryStatus.sending.rawValue
        chat.isSeenSync = true
        chat.isUploading = "pending"
        
        self.viewModel.setChatDataInRealm(item: chat, isAfterPickingImage: true)

        //This is completion handler which get called after image upload to s3.
        self.viewModel.didImageUploaded = { [weak self] isImgUploaded , imgUrl, imgMessageId in
            guard let self = self else { return }
            if isImgUploaded {
                print("success uploading image")
                self.viewModel.sendMessage(message: imgUrl, isImage: true, messageId: imgMessageId) // send image url as message to other user

            }else{
                print("error uploading image")
                self.viewModel.updateRealmAfterImageUpload(uploadStatus: "failed", uploadedUrl: "", messsageId: imgMessageId)
            }
        }

        self.viewModel.uploadImage(localPath: "\(Helper.shared.getDocumentDirectoryPath(filename: messageId)).png" ,image: compressedImage, messageId: messageId)
    }
}

核心思路是让上传逻辑和后续的消息发送逻辑脱离ViewController的生命周期，确保即使VC被释放，上传任务和回调处理依然能正常执行。

• 将上传后的回调逻辑移至ViewModel内部
  移除ViewModel中依赖VC的didImageUploaded闭包，直接在ViewModel的uploadImage方法内部处理上传完成后的逻辑（发送消息、更新Realm）。这样不需要VC参与后续流程，即使VC被释放，ViewModel只要存活就能完成后续操作。

  重构后的ViewModel示例：

  class ChatViewModel {
      // 移除原有的didImageUploaded闭包
  
      func uploadImage(localPath: String, image: UIImage, messageId: String) {
          // 执行S3上传操作（以AWS SDK为例）
          let uploadRequest = AWSS3TransferManagerUploadRequest()
          uploadRequest?.bucket = "your-bucket-name"
          uploadRequest?.key = "images/\(messageId).png"
          uploadRequest?.body = URL(fileURLWithPath: localPath)
          uploadRequest?.contentType = "image/png"
  
          AWSS3TransferManager.default().upload(uploadRequest!).continueWith { [weak self] task in
              guard let self = self else { return nil }
              if let error = task.error {
                  // 上传失败，更新Realm状态
                  self.updateRealmAfterImageUpload(uploadStatus: "failed", uploadedUrl: "", messsageId: messageId)
                  print("Image upload failed: \(error.localizedDescription)")
              } else {
                  // 上传成功，获取S3文件URL并发送消息
                  let imgUrl = "https://your-bucket-name.s3.your-region.amazonaws.com/images/\(messageId).png"
                  self.sendMessage(message: imgUrl, isImage: true, messageId: messageId)
                  // 更新Realm状态为成功
                  self.updateRealmAfterImageUpload(uploadStatus: "success", uploadedUrl: imgUrl, messsageId: messageId)
                  print("Image uploaded successfully")
              }
              return nil
          }
      }
  }
  

• 简化ViewController中的代码
  VC只需要负责选择图片、创建聊天记录模型并保存到Realm，然后调用ViewModel的上传方法即可，无需设置回调：

  @IBAction func didPressMedia(_ sender: Any) {
      ImagePickerManager().pickImage(self) { pickedImage in
          let compressedImage = pickedImage.resizedImage(newSize: CGSize(width: 120, height: 120))
          
          let messageId = XMPPHelper.shared.xmppStream.generateUUID
  
          let chat = ChatListModel()
          chat.roomId = ""
          chat.message = ""
          chat.messageId = messageId
          chat.time = String(Date().currentTimeMillis())
          chat.to = self.viewModel.otherUser
          chat.from = self.viewModel.myJid ?? ""
          chat.type = "chat/image"
          chat.imageLocalPath = messageId
          chat.msgDeliveryStatus = MsgDeliveryStatus.sending.rawValue
          chat.isSeenSync = true
          chat.isUploading = "pending"
          
          self.viewModel.setChatDataInRealm(item: chat, isAfterPickingImage: true)
  
          // 直接调用上传，无需绑定回调到VC
          self.viewModel.uploadImage(localPath: "\(Helper.shared.getDocumentDirectoryPath(filename: messageId)).png", image: compressedImage, messageId: messageId)
      }
  }
  

• 确保ViewModel生命周期稳定
  要保证ViewModel不会随VC一起被释放，可以将ViewModel交由全局的聊天管理类持有，或者使用单例模式（如果是全局聊天逻辑），避免ViewModel被提前销毁。

• 配置后台任务延长上传时间
  如果上传可能在APP进入后台时完成，需要使用UIApplication的后台任务API延长执行时间，避免系统终止上传任务：

  func uploadImage(localPath: String, image: UIImage, messageId: String) {
      var backgroundTaskId: UIBackgroundTaskIdentifier = .invalid
      backgroundTaskId = UIApplication.shared.beginBackgroundTask {
          UIApplication.shared.endBackgroundTask(backgroundTaskId)
          backgroundTaskId = .invalid
      }
  
      // 执行S3上传操作...
  
      // 上传完成后结束后台任务
      defer {
          if backgroundTaskId != .invalid {
              UIApplication.shared.endBackgroundTask(backgroundTaskId)
          }
      }
  }
  

内容的提问来源于stack exchange，提问作者rauldhasmana