UnitTesting Asynchronous code

DispatchInMainQueueTests.swift

//
//  DispatchInMainQueueTests.swift
//  DispatchInMainQueueTests
//
//  Created by Kumar, Sanjay (623) on 29/09/22.
//

import XCTest
@testable import DispatchInMainQueue

class DispatchInMainQueueTests: XCTestCase {
    
    func test_viewDidLoad_rendersLabelTextFromService() {
        let service = ServiceSpy()
        let sut = ViewController.make(service: service)
        sut.loadViewIfNeeded()
        XCTAssertEqual(sut.label.text, "Loading...")
        
        service.completion?("Loaded")
        XCTAssertEqual(sut.label.text, "Loaded")
    }
}

private class ServiceSpy: Service {
    var completion: ((String) -> Void)?
    func load(completion: @escaping (String) -> Void) {
        self.completion = completion
    }
}

SceneDelegate.swift

//
//  SceneDelegate.swift
//  DispatchInMainQueue
//
//  Created by Kumar, Sanjay (623) on 29/09/22.
//

import UIKit

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        
        guard let scene = (scene as? UIWindowScene) else { return }
        let service = MainQueueDispatchDecorator(AsyncService())
        let vc = ViewController.make(service: service)
        window = UIWindow(windowScene: scene)
        window?.rootViewController = vc
        window?.makeKeyAndVisible()
    }
}


final class AsyncService: Service {
    func load(completion: @escaping (String) -> Void) {
        DispatchQueue.global().asyncAfter(deadline: .now() + 2) {
            completion("Hello swift dev")
        }
    }
}

private final class MainQueueDispatchDecorator: Service {
    private let decoratee: Service
    init(_ decoratee: Service) {
        self.decoratee = decoratee
    }
    
    func load(completion: @escaping (String) -> Void) {
        decoratee.load { result in
            executeInMainThread {
                completion(result)
            }
        }
    }
}

func executeInMainThread(_ work: @escaping () -> Void) {
    if Thread.isMainThread {
        work()
    } else {
        DispatchQueue.main.async(execute: work)
    }
}

ViewController.swift

//  ViewController.swift
import UIKit

class ViewController: UIViewController {
    
    @IBOutlet weak var label: UILabel!
    private var service: Service!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        self.label.text = "Loading..."
        
        service.load { [weak self] text in
            self?.label.text = text
        }
    }
}

extension ViewController {
    static func make(service: Service) -> ViewController {
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        return storyboard.instantiateViewController(identifier: "ViewController") {
            let vc = ViewController(coder: $0)
            vc?.service = service
            return vc
        }
        
    }
}