swift - serializing json

import Foundation
import CoreFoundation

// create a Codable struct called "POST" with the correct properties
struct Post: Codable {
    let userId: Int
    let title: String
    let body: String
}   

// create an instance of the Post struct with your own values
let post = Post(userId: 123,title: "Title",  body: "body")

// create a URLRequest by passing in the URL
var request = URLRequest(url: URL(string: "https://jsonplaceholder.typicode.com/posts")!)

// set the HTTP method to POST
request.httpMethod = "POST"

// set the HTTP body to the encoded "Post" struct
request.httpBody = try! JSONEncoder().encode(post)

// set the appropriate HTTP header fields
request.addValue("application/json", forHTTPHeaderField: "Content-Type")

// HACK: this line allows the workspace or an Xcode playground to execute the request, but is not needed in a real app
let runLoop = CFRunLoopGetCurrent()
// task for making the request
let task = URLSession.shared.dataTask(with: request) {data, response, error in
    print(String(data: data!, encoding: .utf8))
    // also not necessary in a real app
    CFRunLoopStop(runLoop)
}
task.resume()
// not necessary
CFRunLoopRun()


//prints: Optional("{\n \"userId\": 123,\n \"body\": \"body\",\n \"title\": \"Title\",\n \"id\": 101\n}")

class func taskForPOSTRequest<RequestType: Encodable, ResponseType: Decodable>(url: URL, responseType: ResponseType.Type, body: RequestType, completion: @escaping (ResponseType?, Error?) -> Void) {
    var request = URLRequest(url: url)
    request.httpMethod = "POST"
    request.httpBody = try! JSONEncoder().encode(body)
    request.addValue("application/json", forHTTPHeaderField: "Content-Type")
    let task = URLSession.shared.dataTask(with: request) { data, response, error in
        guard let data = data else {
            DispatchQueue.main.async {
                completion(nil, error)
            }
            return
        }
        let decoder = JSONDecoder()
        do {
            let responseObject = try decoder.decode(ResponseType.self, from: data)
            DispatchQueue.main.async {
                completion(responseObject, nil)
            }
        } catch {
            DispatchQueue.main.async {
                completion(nil, error)
            }
        }
    }
    task.resume()
}

Here's an example of how one would call the above method:

class func login(username: String, password: String, completion: @escaping (Bool, Error?) -> Void) {
    let body = LoginRequest(username: username, password: password, requestToken: Auth.requestToken)
    
    taskForPOSTRequest(url: Endpoints.login.url, responseType: RequestTokenResponse.self, body: body) { (response, error) in
        if let response = response {
            Auth.requestToken = response.requestToken
            completion(true, nil)
        }
        else {
            completion(false, error)
        }
    }
}