transfer data with protocol - ios

Question

I try to decode weather api
this is my struct class weatherModal :
import Foundation
struct WeatherModel:Decodable{
var main:Main?
}
struct Main:Decodable {
var temp : Double?
var feels_like : Double?
var temp_min:Double?
var temp_max:Double?
var pressure , humidity: Int?
}
I am trying to learn protocols. So this is where a make api call manager class :
protocol WeatherManagerProtocol:AnyObject {
func weatherData(weatherData:WeatherModel)
}
class WeatherManager{
var weather : WeatherModel?
weak var delegate :WeatherManagerProtocol?
public func callWeather(city:String) {
let url = "http://api.openweathermap.org/data/2.5/weather?q=\(city)&appid=1234"
URLSession.shared.dataTask(with:URL(string: url)!) { (data, response, err) in
if err != nil {
print(err!.localizedDescription)
} else {
do {
self.weather = try JSONDecoder().decode(WeatherModel.self, from: data!)
self.delegate?.weatherData(weatherData: self.weather!)
} catch {
print(error.localizedDescription)
}
}
}.resume()
}
}
In my ViewController what I want to do is user write city name on textfield and If user clicked the process button print the information about weather.
class ViewController: UIViewController {
#IBOutlet weak var textField: UITextField!
var weatherManager = WeatherManager()
var data : WeatherModel?
override func viewDidLoad() {
super.viewDidLoad()
weatherManager.delegate = self
// Do any additional setup after loading the view.
}
#IBAction func processButtonClicked(_ sender: Any) {
if textField.text != "" {
weatherManager.callWeather(city: textField.text ?? "nil")
print(data?.main?.humidity) // it print nil
} else{
print("empty")
}
}
extension ViewController: WeatherManagerProtocol{
func weatherData(weatherData: WeatherModel) {
self.data = weatherData
print(self.data.main)
// in here I can show my data
}
}
When I clicked process button it always print nil. Why ? What am I doing wrong?

Answer 1

You seem not to understand how your own code is supposed to work. The whole idea of the protocol-and-delegate pattern you've set up is that the "signal" round-trips thru the weather manager on a path like this:
You (the ViewController) say weatherManager.callWeather
The weather manager does some networking.
The weather manager calls its own delegate's weatherData.
You (the ViewController) are that delegate, so your weatherData is called and that is where you can print.
So that is the signal path:
#IBAction func processButtonClicked(_ sender: Any) {
weatherManager.callWeather(city: textField.text ?? "nil") // < TO wm
}
func weatherData(weatherData: WeatherModel) { // < FROM wm
// can print `weatherData` here
}
You cannot short circuit this path by trying to print the weather data anywhere else. Stay on the path. You cannot turn this into a "linear" simple path; it is asynchronous.
If you do want it to look more like a "linear" simple path, use a completion handler instead of a delegate callback. That's what I do in my version of this same experiment, so my view controller code looks like this:
self.jsonTalker.fetchJSON(zip:self.currentZip) { result in
DispatchQueue.main.async {
// and now `result` contains the weather data, or an error
Even better, use the Combine framework (or wait until Swift 6 implements async/await).

Answer 2

You call to weatherManager.callWeather that start URLSession.shared.dataTask. the task is executed asynchronously, but callWeather return immediately. So, when you print data?.main?.humidity, the task did not finish yet, and data is still nil. After the task finish, you call weatherData and assign the response to data.