从API（JSON格式）中提取数据不会将函数调用以外的数据保存到

Question

我想从JSON格式的API中获取给定时间段内的温度数组。我能够通过完成处理程序检索数组，但我无法将其保存到函数调用之外的另一个变量（使用完成处理程序的变量）。这是我的代码。请参阅评论区域。

class WeatherGetter { 

    func getWeather(_ zip: String, startdate: String, enddate: String, completion: @escaping (([[Double]]) -> Void)) { 
     // This is a pretty simple networking task, so the shared session will do. 
     let session = URLSession.shared 

     let string = "api address" 

     let url = URL(string: string) 
     var weatherRequestURL = URLRequest(url:url! as URL) 
     weatherRequestURL.httpMethod = "GET" 


     // The data task retrieves the data. 
     let dataTask = session.dataTask(with: weatherRequestURL) { 
      (data, response, error) -> Void in 
      if let error = error { 
       // Case 1: Error 
       // We got some kind of error while trying to get data from the server. 
       print("Error:\n\(error)") 
      } 
      else { 
       // Case 2: Success 
       // We got a response from the server! 
       do { 
        var temps = [Double]() 
        var winds = [Double]() 
        let weather = try JSON(data: data!) 
        let conditions1 = weather["data"] 
        let conditions2 = conditions1["weather"] 
        let count = conditions2.count 
        for i in 0...count-1 { 
         let conditions3 = conditions2[i] 
         let conditions4 = conditions3["hourly"] 
         let count2 = conditions4.count 
         for j in 0...count2-1 { 
          let conditions5 = conditions4[j] 
          let tempF = conditions5["tempF"].doubleValue 
          let windspeed = conditions5["windspeedKmph"].doubleValue 
          temps.append(tempF) 
          winds.append(windspeed) 
         } 
        } 
        completion([temps, winds]) 
       } 
       catch let jsonError as NSError { 
        // An error occurred while trying to convert the data into a Swift dictionary. 
        print("JSON error description: \(jsonError.description)") 
       } 
      } 
     } 
     // The data task is set up...launch it! 
     dataTask.resume() 
    } 
} 


I am calling this method from my view controller class. Here is the code. 

    override func viewDidLoad() { 
     super.viewDidLoad() 

     let weather = WeatherGetter() 
     weather.getWeather("13323", startdate: "2016-10-01", enddate: "2017-04-30") { (weatherhandler: [[Double]]) in 
      //It prints out the correct array here 
      print(weatherhandler[0]) 
      weatherData = weatherhandler[0] 
     } 
     //Here it prints out an empty array 
     print(weatherData) 

    } 

Answer 1

的问题是，API需要一些时间来恢复数据，当数据返回“完成监听器”之称，并它进入“getWeather”方法实现中，在那里它打印数组的数据。但是，当您的外部打印方法被调用时，API尚未返回数据。所以它显示空阵列。如果您尝试在某个时间后打印数据表单“weatherData”对象，它将起作用。

我可以建议你，最好的办法是更新这样的“的GetWeather”方法实现内部与数据的UI：

override func viewDidLoad() { 
    super.viewDidLoad() 

    let weather = WeatherGetter() 
    weather.getWeather("13323", startdate: "2016-10-01", enddate: "2017-04-30") { (weatherhandler: [[Double]]) in 
     //It prints out the correct array here 
     print(weatherhandler[0]) 
     weatherData = weatherhandler[0] 

     // Update your UI here. 

    } 
    //Here it prints out an empty array 
    print(weatherData) 

} 

Answer 2

这是不是一个错误，当你的controller得到加载阵列仍然是空的，因为你的getWeather还在做它的事（指访问API，解码JSON）当它完成了callback将有data返回到您的控制器。

例如，如果你使用的是的tableView，你将有reloadData()刷新UI，分配数据weatherData

后，或者你可以放置一个property Observer，你宣布你weatherData财产。

var weatherData:[Double]? = nil { 
    didSet { 
     guard let data = weatherData else { return } 
     // now you could do soemthing with the data, to populate your UI 
    } 
} 

现在数据被分配到wheaterData后，didSet将被调用。

希望帮助，并且还您jsonParsing逻辑放到一个`结构:)