Swift 为什么可以'；是否更改此会话中的实例变量。dataTask（）？

我试图直接将从URLsession接收到的数据更新到实例变量中。在playgroud中尝试了下面的代码，直到

self.cityName=weatherdecoded.name

代码似乎工作正常，但是作为实例变量的

self.cityName

没有更新。结果是

nil

s。希望了解原因，我犯了什么错误。谢谢

import UIKit
class WeatherManager {
    var cityName: String?
    var conditionID: Int?
    var temp: Double?


    func fetchData(cityName: String) {
        let session = URLSession(configuration: .default)
        let urlStr = "https://api.openweathermap.org/data/2.5/weather?units=metric&appid=8da179fa1c83749056ec6a5385cabb04&q=" + cityName.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)!
        print(urlStr)
        let url = URL(string: urlStr)!

        let weatherDataSession = session.dataTask(with: url, completionHandler: getdata(data:response:error:))
        weatherDataSession.resume()
    }

    func getdata(data: Data?, response: URLResponse?, error: Error?) {
        if error != nil {
                print(error!)
                return
            }

            if let safedata = data {
                let decoder = JSONDecoder()
                do {
                   let weatherdecoded = try decoder.decode(Weatherdata.self, from: safedata)

                    self.cityName = weatherdecoded.name
                    self.conditionID = weatherdecoded.weather[0].id
                    self.temp = weatherdecoded.main.temp

                } catch {
                    print(error)
                }
            }
        }
}

struct Weatherdata: Decodable {
    let weather : [Weather]
    let main: Main
    let name: String
}

struct Weather: Decodable {
    let id: Int
    let description: String
}

struct Main: Decodable {
    let temp: Double
}

let weathermanager = WeatherManager()
weathermanager.fetchData(cityName: "beijing")

print(weathermanager.cityName)
print(weathermanager.conditionID)
print(weathermanager.temp)

---

原因很简单，因为您正在请求实际完成之前打印这些值。您必须使用完成处理程序：

class WeatherManager {
    func fetchObject<T: Decodable>(urlPath: String, onCompletion: @escaping (Result<T, Error>) -> Void) {
        let session = URLSession(configuration: .default)
        let url = URL(string: urlPath)!

        let task = session.dataTask(with: url) { data, _, error in
            if let error = error {
                onCompletion(.failure(error))
                return
            }

            let decoder = JSONDecoder()
            do {
               let decoded = try decoder.decode(T.self, from: data ?? Data())
                onCompletion(.success(decoded))
            } catch {
                onCompletion(.failure(error))
            }
        }

        task.resume()
    }


    func fetchWeather(cityName: String, onCompletion: @escaping (Result<Weatherdata, Error>) -> Void) {
        let urlPath = "https://api.openweathermap.org/data/2.5/weather?units=metric&appid=8da179fa1c83749056ec6a5385cabb04&q=" + cityName.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)!

        fetchObject(urlPath: urlPath, onCompletion: onCompletion)
    }
}

struct Weatherdata: Decodable {
    let weather : [Weather]
    let main: Main
    let name: String
}

struct Weather: Decodable {
    let id: Int
    let description: String
}

struct Main: Decodable {
    let temp: Double
}

let weathermanager = WeatherManager()
weathermanager.fetchWeather(cityName: "beijing") { result in
    switch result {
    case .success(let weatherData):
        print(weatherData.name)
        print(weatherData.weather[0].id)
        print(weatherData.main.temp)
    case .failure(let error):
        print(error)
    }
}

类天气管理器{
func fetchObject（urlPath:String，onCompletion:@escaping（Result）->Void）{
let session=URLSession（配置：。默认值）
让url=url（字符串：urlPath）！
让task=session.dataTask（with:url）{data，\中出现错误
如果let error=error{
完成（.failure（error））
返回
}
let decoder=JSONDecoder（）
做{
let decoded=尝试decoder.decode（T.self，from:data？？data（））
完成（.success（已解码））
}抓住{
完成（.failure（error））
}
}
task.resume（）
}
func fetchWeather（cityName:String，onCompletion:@escaping（Result）->Void）{
让urlPath=”https://api.openweathermap.org/data/2.5/weather?units=metric&appid=8da179fa1c83749056ec6a5385cabb04&q=“+cityName.addingPercentEncoding（使用允许的字符：.urlQueryLowed）！
fetchObject（urlPath:urlPath，onCompletion:onCompletion）
}
}
结构Weatherdata：可解码{
让天气：[天气]
让main：main
let name:String
}
结构天气：可解码{
让id:Int
let description:字符串
}
主结构：可解码{
温度：双倍
}
让weathermanager=weathermanager（）
weathermanager.fetchWeather（城市名称：“北京”）{结果
切换结果{
成功案例（让weatherData）：
打印（weatherData.name）
打印（weatherData.weather[0].id）
打印（天气数据主温度）
案例。失败（let错误）：
打印（错误）
}
}

URLSession.dataTask

是异步的。从阅读Swift中的异步执行和完成处理程序开始