Codable in Swift - Convert JSON Response to Swift Object - Part 1

Parsing JSON object to native Swift model object and vice-versa is a very common task while working with REST API in iOS Applications. Codable protocol made this job very easy. You can simply conform Codable for your model structs and classes then use JSONEncoder and JSONDecoder to convert them.

In the following example, I used APIs from JSONPlaceholder to demonstrate converting JSON to Swift object.

For user API: https://jsonplaceholder.typicode.com/users/1, JSON response is like below for user id: 1:

{
  "id": 1,
  "name": "Leanne Graham",
  "username": "Bret",
  "email": "Sincere@april.biz",
  "address": {
    "street": "Kulas Light",
    "suite": "Apt. 556",
    "city": "Gwenborough",
    "zipcode": "92998-3874",
    "geo": {
      "lat": "-37.3159",
      "lng": "81.1496"
    }
  },
  "phone": "1-770-736-8031 x56442",
  "website": "hildegard.org",
  "company": {
    "name": "Romaguera-Crona",
    "catchPhrase": "Multi-layered client-server neural-net",
    "bs": "harness real-time e-markets"
  }
}

Let’s first create model structures based on the above JSON:

struct Geo: Codable {
    let lat: String
    let long: String

    enum CodingKeys: String, CodingKey {
        case lat
        case long = "lng"
    }
}

struct Address: Codable {
    let street: String
    let suite: String
    let city: String
    let zipcode: String
    let geo: Geo
}

struct Company: Codable {
    let name: String
    let catchPhrase: String
    let business: String

    enum CodingKeys: String, CodingKey {
        case name, catchPhrase
        case business = "bs"
    }
}

struct User: Codable {
    let id: Int
    let name: String
    let nickname: String
    let email: String
    let address: Address
    let phone: String
    let website: String
    let company: Company

    enum CodingKeys: String, CodingKey {
        case id, name, email, address, phone, website, company
        case nickname = "username"
    }
}

You can see, our structures are pretty simple: just conformed Codable protocol. Generally, property names will become keys in JSON but in few cases we defined CodingKeys enum where keys differs from the property names.

Now, let’s create a simple client for user API: https://jsonplaceholder.typicode.com/users/1. I created the client class so that it can be extended later for other APIs (users, posts, etc.).

class TypicodeClient {
    let host = "jsonplaceholder.typicode.com"

    enum Api {
        case user(id: Int)
        // ...
    }

    func getUrl(api: Api) -> URL {
        var urlComponents = URLComponents()
        urlComponents.host = host
        urlComponents.scheme = "https"

        let path: String
        switch api {
            case .user(let id):
                path = "/users/\(id)"
        }

        urlComponents.path = path
        return urlComponents.url!
    }

    func fetchUser(id: Int, completion: @escaping (User?) -> Void) {
        var urlRequest = URLRequest(url: getUrl(api: .user(id: id)))
        urlRequest.httpMethod = "GET"

        let task = URLSession.shared.dataTask(with: urlRequest) { data, response, error in
            if let jsonData = data {
                do {
                    let jsonDecoder = JSONDecoder()
                    let user = try jsonDecoder.decode(User.self, from: jsonData)
                    completion(user)
                } catch {
                    completion(nil)
                }
            } else {
                completion(nil)
            }
        }
        task.resume()
    }

    // ...

}

Here, we used JSONDecoder.decode(_:from:) method to convert the JSON response we got from the URLRequest to our User model.

let jsonDecoder = JSONDecoder()
let user = try jsonDecoder.decode(User.self, from: jsonData)

Finally, for demo purpose, we populated a UITextView, using the user information.

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var tvUserInfo: UITextView!

    var user: User?

    override func viewDidLoad() {
        super.viewDidLoad()

        loadData()
    }

    private func loadData() {
        let client = TypicodeClient()
        client.fetchUser(id: 1) { (user) in
            if let user = user {
                DispatchQueue.main.async {
                    self.user = user
                    self.updateUI()
                }
            }
        }
    }

    private func updateUI() {
        guard let user = self.user else { return }

        let userInfo = """
            Id: \(user.id)
            Name: " \(user.name)
            Nickname: \(user.nickname)
            Email: \(user.email)
            Address:
                Street: \(user.address.street)
                Suite: \(user.address.suite)
                City: \(user.address.city)
                Zipcode: \(user.address.zipcode)
                Geo-location:
                    lat: \(user.address.geo.lat)
                    long: \(user.address.geo.long)
            Phone: \(user.phone)
            Website: \(user.website)
            Company:
                Name: \(user.company.name)
                Catch Phrase: \(user.company.catchPhrase)
                Business: \(user.company.business)
            """

        tvUserInfo.text = userInfo
    }

}

You can check source code of this demo here https://github.com/mkhrussell/SwiftRestClientDemo.