How to return JSON data from Swift URLSession

Question

I'm trying to return json data from an api call. I'm able to access the json data successfully but am struggling to find a way / the best way to return it for access in my app. Thanks for any ideas!

import UIKit

class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // make the api call and obtain data
        let data = self.loadData()
        print("inside viewDidLoad", data) // prints 'inside viewDidLoad emptyString'
    }
    
    func loadData() -> String {
        var circData = "emptyString"
        let session = URLSession.shared
        let url = URL(string: "https://us-east-1.aws.webhooks.mongodb-realm.com/api/client/v2.0/app/cirdata-khyvx/service/cirData/incoming_webhook/cirlAPI")!
        let task = session.dataTask(with: url, completionHandler: { data, response, error in
            if let json = try? JSONSerialization.jsonObject(with: data!, options: []) {
//                print("json: ", json) // prints the whole json file, verifying the connection works. Some 300kb of data.
//                print("json file type: ", type(of: json)) // prints '__NSArrayI'
                
                let jsonString = "\(json)"
                circData = jsonString
//                print("circData", circData) // prints the whole json file, verifying that the json string has been assigned to 'circData'
            }
        })
        task.resume()
//        print("after: ", circData) // prints 'after: emptyString'. It's as if the reassignment didn't take place.
        return circData
    }
}

Run a search. Similar questions are asked almost daily. – El Tomato Jun 27 '21 at 03:51 — El Tomato, Jun 27 '21 at 03:51

score 3 · Answer 1 · answered Jun 26 '21 at 21:27

You can't return a value synchronously becuase the api call that is fetching json data is asynchronous. You need to use a completion handler instead.

You can put breakpoints in different places inside the code to understand how the flow executes.

import UIKit

class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.loadData(completion: { [weak self] (result, error) in
            if let error = error {
                print(error.localizedDescription)
            }
            if let result = result {
                print(result)
            }
        })
    }
    
    func loadData(completion: @escaping (_ data: Any?, _ error: Error?) -> Void) {
        let url = URL(string: "https://us-east-1.aws.webhooks.mongodb-realm.com/api/client/v2.0/app/cirdata-khyvx/service/cirData/incoming_webhook/cirlAPI")!
        let task = URLSession.shared.dataTask(with: url, completionHandler: { data, response, error in
            if let error = error {
                completion(nil, error)
                return
            }
            do {
                if let data = data {
                    let json = try JSONSerialization.jsonObject(with: data, options: [.allowFragments])
                    completion(json, nil)
                } else {
                    completion(nil, nil)
                }
            } catch {
                completion(nil, error)
            }
        })
        task.resume()
    }
}

This has been asked and answered *many* times already. – Martin R Jun 26 '21 at 21:36 — Martin R, Jun 26 '21 at 21:36

How to return JSON data from Swift URLSession

1 Answers1