A sample code to demonstrate how to use protocols to facilitate unit testing in iOS app

TestableNetworkServiceExample.swift

//
//  TestableNetworkServiceExample.swift
//  KayakCodingChallenge
//
//  Created by Jayesh Kawli on 11/28/23.
//

import Foundation
import XCTest

struct Name: Decodable {
    let firstName: String
    let lastName: String
}

protocol NetworkServiceableProtocol: AnyObject {
    func loadSampleData(completion: @escaping (Result<Name, Error>) -> Void)
}

final class NetworkServiceClass: NetworkServiceableProtocol {

    func loadSampleData(completion: @escaping (Result<Name, Error>) -> Void) {

        let url = URL(string: "www.google.com")!

        let task = URLSession.shared.dataTask(with: url) { (data, response, error) in

            let decoder = JSONDecoder()
            let responsePayload = try! decoder.decode(Name.self, from: data!)
            completion(.success(responsePayload))

        }
        task.resume()
    }
}

//View Model

final class MyViewModel {
    
    private let networkService: NetworkServiceable

    init(networkService: NetworkServiceable) {
        self.networkService = networkService
    }

    func loadData() {

        networkService.loadData(with: "www.google.com") { result in

        }

    }
}

//Test framework

class MockNetworkService: NetworkServiceableProtocol {
    func loadSampleData(completion: @escaping (Result<Name, Error>) -> Void) {
        completion(.success(Name(firstName: "Tom", lastName: "Bob")))
    }
}

class NetworkServiceTest: XCTestCase {

    let mockNetworkService = MockNetworkService()

    override func setup() {
        super.setUp()
        let viewModel = MyViewModel(networkService: mockNetworkService)
        viewModel.loadData()
    }
}