[TDD] Writing Unit Tests for a ViewModel

Let’s say we have a UserViewModel that interacts with AuthManager and SessionManager. Here's a simple example of the ViewModel that you want to test:

final class UserViewModel: ObservableObject {
  @Published var isUserSignIn: Bool = false
  @Published var user: User?
  
  private let authManager: AuthManagerProtocol
  private let sessionManager: SessionManagerProtocol
  
  init(authManager: AuthManagerProtocol, sessionManager: SessionManagerProtocol) {
    self.authManager = authManager
    self.sessionManager = sessionManager
    
    self.loadUser()
  }
  
  func loadUser() {
    if let user = sessionManager.loadUser() {
      self.user = user
      self.isUserSignIn = true
    }
  }
  
  func signIn() {
    authManager.signIn { [weak self] user in
      guard let user = user else {
        return
      }
      self?.sessionManager.saveUser(user)
      self?.user = user
      self?.isUserSignIn = true
    }
  }
  
  func signOut() {
    authManager.signOut()
    sessionManager.clearUser()
    user = nil
    isUserSignIn = false
  }
}

To test the ViewModel, you need to mock the AuthManager and SessionManager behaviors. This allows you to isolate and test the ViewModel's logic without calling real APIs or writing to disk.

// Mock for AuthManager
class MockAuthManager: AuthManager {
    var signInResult: Bool = true

    override func signIn(callback: @escaping (Bool) -> Void) {
        callback(signInResult)
    }

    override func signOut() {
        // Simulate sign-out, no need for further logic
    }
}

// Mock for SessionManager
class MockSessionManager: SessionManager {
    var mockUser: User?

    override func saveUser(_ user: User) {
        mockUser = user
    }

    override func loadUser() -> User? {
        return mockUser
    }

    override func clearUser() {
        mockUser = nil
    }
}

Now, let’s write unit tests for UserViewModel using XCTest. The goal is to test the behavior of UserViewModel during sign-in and sign-out, ensuring it correctly updates the isUserSignedIn and currentUser properties.

import XCTest
@testable import YourApp

class UserViewModelTests: XCTestCase {
  var viewModel: UserViewModel!
  var mockAuthManager: MockAuthManager!
  var mockSessionManager: MockSessionManager!
  
  override func setUp() {
    super.setUp()
    mockAuthManager = MockAuthManager()
    mockSessionManager = MockSessionManager()
    viewModel = UserViewModel(
      authManager: mockAuthManager,
      sessionManager: mockSessionManager
    )
  }
  
  func testUserSignedIn() {
    let user = User(id: 123, name: "John Doe", email: "john@example.com")
    mockSessionManager.saveUser(user)
    
    viewModel.loadUser()
    
    XCTAssertTrue(viewModel.isUserSignIn, "User should be sign in")
    XCTAssertEqual(viewModel.user?.id, user.id, "User should match")
  }
  
  func testSignInSuccess() {
    let user = User(id: 123, name: "John Doe", email: "john@example.com")
    mockAuthManager.signInUser = user
    
    let expectation = self.expectation(description: "Sign in should complete")
    viewModel.signIn()
    
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
      XCTAssertTrue(self.viewModel.isUserSignIn, "User should be sign in")
      XCTAssertNotNil(self.viewModel.user, "User should be present")
      expectation.fulfill()
    }
    
    wait(for: [expectation], timeout: 1.0)
  }
  
  func testSignInFailed() {
    let expectation = self.expectation(description: "Sign in should fail")
    viewModel.signIn()
    
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
      XCTAssertFalse(self.viewModel.isUserSignIn, "User should not be sign in")
      XCTAssertNil(self.viewModel.user, "User should be nil")
      expectation.fulfill()
    }
    
    wait(for: [expectation], timeout: 1.0)
  }
  
  func testSignOut() {
    let user = User(id: 123, name: "John Doe", email: "john@example.com")
    mockSessionManager.saveUser(user)
    
    viewModel.loadUser()
    
    viewModel.signOut()
    
    XCTAssertFalse(viewModel.isUserSignIn, "User should be sign out")
    XCTAssertNil(viewModel.user, "User should be nil")
  }
}

class MockAuthManager: AuthManagerProtocol {
  var signInUser: User?
  
  func signIn(callback: @escaping (User?) -> Void) {
    callback(signInUser)
  }
  
  func signOut() {
    signInUser = nil
  }
}

class MockSessionManager: SessionManagerProtocol {
  var signInUser: User?
  
  func saveUser(_ user: User) {
    signInUser = user
  }
  
  func loadUser() -> User? {
    return signInUser
  }
  
  func clearUser() {
    signInUser = nil
  }
}

• Mocking dependencies (e.g., AuthManager, SessionManager) ensures that the ViewModel is isolated and only tested for its logic.
• Asynchronous behavior is handled using XCTestExpectation, allowing us to wait for the signIn method to complete before validating the results.
• Edge cases like sign-in failures are tested to ensure the ViewModel reacts correctly.