iOS Testing

Comprehensive testing strategies for robust iOS applications.

Testing Pyramid

          /\
         /  \     UI Tests (10%)
        /----\    Integration Tests (20%)
       /      \   
      /--------\  Unit Tests (70%)
     /          \

Unit Testing

Setup with XCTest

import XCTest
@testable import MyApp

final class UserServiceTests: XCTestCase {
    
    var sut: UserService!  // System Under Test
    var mockNetworkService: MockNetworkService!
    
    override func setUpWithError() throws {
        try super.setUpWithError()
        mockNetworkService = MockNetworkService()
        sut = UserService(networkService: mockNetworkService)
    }
    
    override func tearDownWithError() throws {
        sut = nil
        mockNetworkService = nil
        try super.tearDownWithError()
    }
    
    // MARK: - Tests
    
    func test_fetchUser_success() async throws {
        // Given
        let expectedUser = User(id: "1", name: "John")
        mockNetworkService.result = .success(expectedUser)
        
        // When
        let user = try await sut.fetchUser(id: "1")
        
        // Then
        XCTAssertEqual(user.id, "1")
        XCTAssertEqual(user.name, "John")
    }
    
    func test_fetchUser_failure() async {
        // Given
        mockNetworkService.result = .failure(NetworkError.notFound)
        
        // When/Then
        do {
            _ = try await sut.fetchUser(id: "1")
            XCTFail("Expected error to be thrown")
        } catch {
            XCTAssertEqual(error as? NetworkError, .notFound)
        }
    }
}

Test Naming Conventions

// Pattern: test_[method]_[scenario]_[expectedResult]

func test_login_withValidCredentials_returnsUser() { }
func test_login_withInvalidPassword_throwsAuthError() { }
func test_calculateTotal_withEmptyCart_returnsZero() { }
func test_formatDate_withNilDate_returnsPlaceholder() { }

Async Testing

// Modern async/await
func test_asyncOperation() async throws {
    let result = try await sut.fetchData()
    XCTAssertFalse(result.isEmpty)
}

// With timeout
func test_asyncWithTimeout() async throws {
    try await withTimeout(seconds: 5) {
        let result = try await sut.slowOperation()
        XCTAssertNotNil(result)
    }
}

// Combine testing
func test_publisher() {
    let expectation = expectation(description: "Publisher completes")
    var receivedValues: [String] = []
    
    sut.dataPublisher
        .sink(
            receiveCompletion: { _ in expectation.fulfill() },
            receiveValue: { receivedValues.append($0) }
        )
        .store(in: &cancellables)
    
    wait(for: [expectation], timeout: 2)
    XCTAssertEqual(receivedValues, ["expected"])
}

Mocking & Dependency Injection

Protocol-Based Mocking

// Production code
protocol UserRepositoryProtocol {
    func getUser(id: String) async throws -> User
    func saveUser(_ user: User) async throws
}

// Mock for testing
class MockUserRepository: UserRepositoryProtocol {
    var getUserResult: Result<User, Error>!
    var saveUserCalled = false
    var savedUser: User?
    
    func getUser(id: String) async throws -> User {
        switch getUserResult! {
        case .success(let user): return user
        case .failure(let error): throw error
        }
    }
    
    func saveUser(_ user: User) async throws {
        saveUserCalled = true
        savedUser = user
    }
}

Dependency Injection

// Constructor injection (preferred)
class UserViewModel {
    private let repository: UserRepositoryProtocol
    
    init(repository: UserRepositoryProtocol = UserRepository()) {
        self.repository = repository
    }
}

// In tests
func test_example() {
    let mockRepo = MockUserRepository()
    let viewModel = UserViewModel(repository: mockRepo)
    // Test with mock
}

UI Testing

Basic UI Test

import XCTest

final class LoginUITests: XCTestCase {
    
    var app: XCUIApplication!
    
    override func setUpWithError() throws {
        try super.setUpWithError()
        continueAfterFailure = false
        app = XCUIApplication()
        app.launchArguments = ["--uitesting"]
        app.launch()
    }
    
    func test_login_withValidCredentials_showsHomeScreen() {
        // Given
        let emailField = app.textFields["email-field"]
        let passwordField = app.secureTextFields["password-field"]
        let loginButton = app.buttons["login-button"]
        
        // When
        emailField.tap()
        emailField.typeText("test@example.com")
        
        passwordField.tap()
        passwordField.typeText("password123")
        
        loginButton.tap()
        
        // Then
        XCTAssertTrue(app.staticTexts["Welcome"].waitForExistence(timeout: 5))
    }
}

Accessibility Identifiers

// In production code
TextField("Email", text: $email)
    .accessibilityIdentifier("email-field")

Button("Login") { }
    .accessibilityIdentifier("login-button")

Page Object Pattern

// LoginPage.swift
struct LoginPage {
    let app: XCUIApplication
    
    var emailField: XCUIElement { app.textFields["email-field"] }
    var passwordField: XCUIElement { app.secureTextFields["password-field"] }
    var loginButton: XCUIElement { app.buttons["login-button"] }
    var errorMessage: XCUIElement { app.staticTexts["error-message"] }
    
    func login(email: String, password: String) {
        emailField.tap()
        emailField.typeText(email)
        passwordField.tap()
        passwordField.typeText(password)
        loginButton.tap()
    }
}

// In test
func test_login() {
    let loginPage = LoginPage(app: app)
    loginPage.login(email: "test@example.com", password: "password")
    XCTAssertTrue(app.staticTexts["Welcome"].exists)
}

Snapshot Testing

Using swift-snapshot-testing

import SnapshotTesting
import SwiftUI
import XCTest
@testable import MyApp

final class HomeViewSnapshotTests: XCTestCase {
    
    func test_homeView_lightMode() {
        let view = HomeView()
            .preferredColorScheme(.light)
        
        assertSnapshot(
            matching: view,
            as: .image(layout: .device(config: .iPhone13Pro))
        )
    }
    
    func test_homeView_darkMode() {
        let view = HomeView()
            .preferredColorScheme(.dark)
        
        assertSnapshot(
            matching: view,
            as: .image(layout: .device(config: .iPhone13Pro))
        )
    }
    
    func test_homeView_dynamicTypeXXL() {
        let view = HomeView()
            .environment(\.sizeCategory, .accessibilityExtraExtraLarge)
        
        assertSnapshot(
            matching: view,
            as: .image(layout: .device(config: .iPhone13Pro))
        )
    }
}

Device Configurations

// Test across devices
func test_responsiveLayout() {
    let view = MyView()
    
    assertSnapshot(matching: view, as: .image(layout: .device(config: .iPhoneSe)))
    assertSnapshot(matching: view, as: .image(layout: .device(config: .iPhone13Pro)))
    assertSnapshot(matching: view, as: .image(layout: .device(config: .iPhone13ProMax)))
    assertSnapshot(matching: view, as: .image(layout: .device(config: .iPadPro11)))
}

Test Coverage

Enable Code Coverage

1. Edit Scheme → Test → Options
2. Check "Gather coverage for:"
3. Select targets

# View coverage
Product → Test → Show Test Report → Coverage tab

Coverage Targets

Recommended minimums:
- Models: 90%+
- ViewModels: 80%+
- Services: 80%+
- Utilities: 90%+
- Views: 60%+ (UI tests)

Overall target: 70-80%

Testing Best Practices

Test Isolation

// Each test should be independent
// Use setUp/tearDown for fresh state
// Never rely on test execution order

override func setUp() {
    // Create fresh instance
    sut = ViewModel()
}

override func tearDown() {
    // Clean up
    sut = nil
}

Test Data Builders

// UserBuilder.swift
struct UserBuilder {
    private var id = "default-id"
    private var name = "Default User"
    private var email = "default@example.com"
    
    func withId(_ id: String) -> Self {
        var copy = self
        copy.id = id
        return copy
    }
    
    func withName(_ name: String) -> Self {
        var copy = self
        copy.name = name
        return copy
    }
    
    func build() -> User {
        User(id: id, name: name, email: email)
    }
}

// Usage
let user = UserBuilder()
    .withName("John")
    .build()

Flakey Test Prevention

// Avoid:
- sleep() or fixed delays
- Real network calls
- File system dependencies
- Date/time dependencies

// Use instead:
- Proper async/await
- Mocked services
- In-memory storage
- Injected date providers

Running Tests

Command Line

# Run all tests
xcodebuild test \
    -workspace MyApp.xcworkspace \
    -scheme MyApp \
    -destination 'platform=iOS Simulator,name=iPhone 15'

# Run specific test
xcodebuild test \
    -only-testing:MyAppTests/UserServiceTests/test_fetchUser_success \
    ...

# With coverage
xcodebuild test \
    -enableCodeCoverage YES \
    ...

Xcode Shortcuts

Cmd + U           Run all tests
Ctrl + Opt + Cmd + U   Run test under cursor
Ctrl + Opt + Cmd + G   Re-run last test

Test Organization

Tests/
├── UnitTests/
│   ├── Models/
│   │   └── UserTests.swift
│   ├── ViewModels/
│   │   └── HomeViewModelTests.swift
│   ├── Services/
│   │   └── UserServiceTests.swift
│   └── Mocks/
│       ├── MockUserRepository.swift
│       └── MockNetworkService.swift
├── IntegrationTests/
│   └── APIIntegrationTests.swift
├── SnapshotTests/
│   └── HomeViewSnapshotTests.swift
└── UITests/
    ├── Pages/
    │   └── LoginPage.swift
    └── Flows/
        └── LoginFlowTests.swift

Resources

See references/testing-patterns.md for advanced patterns. See references/tdd-guide.md for TDD workflow.