Why can't I change the instance variable inside this session.dataTask()? - swift

Question

I tried to directly get the data recieved from a URLsession updated into the instance variables. Tried the code below in playgroud, I can see until the self.cityName = weatherdecoded.name the code seems working fine, but the self.cityName which intended to be the instance's variable didn't got updated. The results are nils. Hope to understand the reason, what is the mistake i made. Thanks!
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)

Answer 1

The simple reason is the fact that you are printing the values before the request actually completes. You have to use a completion handler:
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)
}
}