mvc_example_playground.swift

import UIKit
import PlaygroundSupport

// API CLient
typealias Resource = (url: String, method: String, data: Data)
class APIClient {
    
    func request(_ res: Resource, _ result: @escaping (Result<Data, Error>) -> Void) {
        /*
         All network code
         - request
         - task
         - data retrivement
         */
        result(Result.success("505".data(using: .utf8)!))
    }
    
}

// Storage
struct User: Codable {
    let id: String
    let un: String
    let pw: String
}

enum RepoError: Error {
    case decoding
}

class UserRepo {
    
    private let apiClient = APIClient()
    
    func signup(_ user: User, _ result: @escaping (Result<Int, Error>) -> Void) {
        let resource = Resource("https://...", "POST", try! JSONEncoder().encode(user))
        apiClient.request(resource) { (res) in
            switch res {
            case .success(let data):
                guard let id = try? JSONDecoder().decode(Int.self, from: data) else {
                    result(.failure(RepoError.decoding))
                    return
                }
                result(.success(id))
            case .failure(let err):
                result(.failure(err))
            }
        }
    }
    
    // e.g.
    func getUser(_ id: Int, _ result: @escaping (Result<User, Error>) -> Void) {
        // TODO: - implement
        /*
         - get from DB
         - perform request to the server using APIClient
         */
    }
    
}

// Logic
typealias SignUpResult = (Error?) -> Void
class RegModel {
    
    let repository = UserRepo()
    
    func validate(un: String?, pw: String?) throws {
        // Validate input data
    }
    
    func signup(un: String?, pw: String?, _ result: @escaping SignUpResult) {
        let user = User(id: "0", un: un!, pw: pw!)
        repository.signup(user) { (res) in
            switch res {
            case .success(let id):
                // - create a new one User struct with received id and store into local storage
                result(nil)
            case .failure(let err):
                result(err)
            }
        }
    }
    
}

// View & Controller
class RegVC: UIViewController {
    
    private let model = RegModel()
    
    @IBAction func signup() {
        // 1 - validation
        let un: String = "", pw: String = ""
        do {
            try model.validate(un: un, pw: pw)
        } catch {
            // Display error e.g. UIAlertController
            return
        }
        // 2 - Registration
        // ->>>> SHOW HUD (ActivityIndicator)
        model.signup(un: un, pw: pw) { (err) in
            // - >>> HIDE HUD
            if err != nil {
                // Display error e.g. UIAlertController
            } else {
                // Navigate to the next screen
            }
        }
    }
    
}