如何知道异步请求何时使用闭包完成？

Question

我正在使用Alamofire下载数据并使用JSON进行解析。我知道为了宣布数据可用，我们应该使用闭包并且不使用通知中心。我不明白关闭。一旦请求完成，我将如何使用闭包来重新加载表视图？ 这是代码。

func downloadEvents() { 
    let coreDataObject = CoreDataMethods() 
    Alamofire.request(URL(string:URL)!) 
     .responseJSON { response in 
      switch response.result { 
      case .success: 
       // we create the model object 
       let downloadedEvent = EventModel() 
       /* we have new data remove old data from core data*/ 
       coreDataObject.deleteData(entityArgument: "Event") 
       events.removeAll() // remove the data from array this is no longer needed FIX 
       for JSONData in response.result.value as! [Dictionary<String, Any>] { 
        /* get the data from JSON and store in the event model*/ 
        downloadedEvent.eTitle = JSONData[downloadedEvent.titleString] as? String 
        downloadedEvent.eDate = JSONData[downloadedEvent.dateString] as? String 
        downloadedEvent.eDescription = JSONData[downloadedEvent.descriptionString] as? String 
        downloadedEvent.eLocation = JSONData[downloadedEvent.locationline1String] as? String 
        downloadedEvent.eLocation2 = JSONData[downloadedEvent.locationline2String] as? String 
        /* if the event has an image save the url*/ 
        if let image = JSONData[downloadedEvent.imageString] as? String { 
         downloadedEvent.eImageURL = image 
        } else { 
         /* else we save the default image url */ 
         downloadedEvent.eImageURL = downloadedEvent.defaultImageURL 
        } 
        coreDataObject.save(eventParam: downloadedEvent) 
       } 
       /* post notification to reload table view FIX */ 
       NotificationCenter.default.post(name: RELOAD_NOTIFICATION, object: nil) 
      case .failure(let error): 
       print("ALAMO REQUEST FIALED: \(error)") 
      } 
    } 
} 

Answer 1

这里是downloadEvents函数来通知呼叫者它是成功的能力：

func downloadEvents(completion: @escaping (Bool, String?)-> Void) { 
    let coreDataObject = CoreDataMethods() 
    Alamofire.request(URL(string:URL)!) 
     .responseJSON { response in 

      switch response.result { 
      case .success: 
       // we create the model object 
       let downloadedEvent = EventModel() 
       /* we have new data remove old data from core data*/ 
       coreDataObject.deleteData(entityArgument: "Event") 
       events.removeAll() // remove the data from array this is no longer needed FIX 
       for JSONData in response.result.value as! [Dictionary<String, Any>] { 
        /* get the data from JSON and store in the event model*/ 
        downloadedEvent.eTitle = JSONData[downloadedEvent.titleString] as? String 
        downloadedEvent.eDate = JSONData[downloadedEvent.dateString] as? String 
        downloadedEvent.eDescription = JSONData[downloadedEvent.descriptionString] as? String 
        downloadedEvent.eLocation = JSONData[downloadedEvent.locationline1String] as? String 
        downloadedEvent.eLocation2 = JSONData[downloadedEvent.locationline2String] as? String 
        /* if the event has an image save the url*/ 
        if let image = JSONData[downloadedEvent.imageString] as? String { 
         downloadedEvent.eImageURL = image 
        } else { 
         /* else we save the default image url */ 
         downloadedEvent.eImageURL = downloadedEvent.defaultImageURL 
        } 
        coreDataObject.save(eventParam: downloadedEvent) 
       } 

       completion(true, nil) 

       /* post notification to reload table view FIX */ 
       //NotificationCenter.default.post(name: RELOAD_NOTIFICATION, object: nil) 
      case .failure(let error): 
       print("ALAMO REQUEST FIALED: \(error)") 
       completion(false, "ALAMO REQUEST FIALED: \(error)") 
      } 
    } 
} 

你可以这样调用这样的功能：

func reloadTable(){ 

    downloadEvents { (success, errMsg) in 
     if success{ 
      DispatchQueue.main.async { 
       self.tableView.reloadData() 
      } 
     } 
     else{ 
      let alertMessage: String 
      if let err = errMsg{ 
       alertMessage = err 
      } 
      else{ 
       alertMessage = "An unknown error occurred." 
      } 
      let alert = UIAlertController.init(title: "Request Failed", message: alertMessage, preferredStyle: .alert) 
      alert.addAction(UIAlertAction(title: "Ok", style: UIAlertActionStyle.cancel, handler: nil)) 
      DispatchQueue.main.async { 
       self.present(alert, animated: true, completion: nil) 
      } 
     } 
    } 
}