VIPER: Key Concepts, the View-Interactor-Presenter-Entity-Router Pattern

// Protocol — View
protocol UserViewProtocol: AnyObject {
    func display(name: String)
    func display(email: String)
    func showLoading()
    func hideLoading()
}

// Protocol — Interactor
protocol UserInteractorProtocol {
    func fetchUser(id: Int, completion: @escaping (Result<User, Error>) -> Void)
}

// Protocol — Router
protocol UserRouterProtocol {
    func navigateToProfile(userId: Int)
}

// Interactor — business logic
final class UserInteractor: UserInteractorProtocol {
    private let service: UserService

    init(service: UserService) { self.service = service }

    func fetchUser(id: Int, completion: @escaping (Result<User, Error>) -> Void) {
        service.fetchUser(id: id, completion: completion)
    }
}

// Presenter — data preparation
final class UserPresenter {
    private weak var view: UserViewProtocol?
    private let interactor: UserInteractorProtocol
    private let router: UserRouterProtocol

    init(interactor: UserInteractorProtocol, router: UserRouterProtocol) {
        self.interactor = interactor
        self.router = router
    }

    func setView(_ view: UserViewProtocol) {
        self.view = view
    }

    func viewDidLoad() {
        view?.showLoading()
        interactor.fetchUser(id: 42) { [weak self] result in
            guard let self else { return }
            self.view?.hideLoading()
            switch result {
            case .success(let user):
                self.view?.display(name: user.name)
                self.view?.display(email: user.email)
            case .failure(let error):
                // error handling
            }
        }
    }
}

// Router — navigation
final class UserRouter: UserRouterProtocol {
    private weak var viewController: UIViewController?

    func setViewController(_ vc: UIViewController) {
        viewController = vc
    }

    func navigateToProfile(userId: Int) {
        let profileModule = ProfileModuleBuilder.build(userId: userId)
        viewController?.navigationController?.pushViewController(profileModule, animated: true)
    }
}

// Builder — module assembly
enum UserModuleBuilder {
    static func build(userId: Int) -> UIViewController {
        let service = UserService()
        let interactor = UserInteractor(service: service)
        let router = UserRouter()
        let presenter = UserPresenter(interactor: interactor, router: router)
        let viewController = UserViewController(presenter: presenter)
        presenter.setView(viewController)
        router.setViewController(viewController)
        return viewController
    }
}

import XCTest

final class UserPresenterTests: XCTestCase {
    func testViewDidLoad_callsFetchUserAndUpdatesView() {
        // Given
        let view = MockUserView()
        let interactor = MockUserInteractor()
        let router = MockUserRouter()
        let presenter = UserPresenter(interactor: interactor, router: router)
        presenter.setView(view)
        let expectedUser = User(id: 42, name: "John", email: "john@test.com")
        interactor.result = .success(expectedUser)

        // When
        presenter.viewDidLoad()

        // Then
        XCTAssertEqual(interactor.capturedUserId, 42)
        XCTAssertEqual(view.displayedName, "John")
        XCTAssertTrue(view.didShowLoading)
    }
}