Swift Testing

Swift Testing Framework

Apple's modern testing framework replacing XCTest for unit tests. Ships with Xcode 16+ and Swift 6.

Quick Reference

import Testing

@Test func basicTest() {
    #expect(2 + 2 == 4)
}

@Test("User can log in with valid credentials")
func loginWithValidCredentials() async throws {
    let result = try await AuthService.login(user: "test", pass: "valid")
    #expect(result.isSuccess)
}

Core Macros

@Test - Define a test

// Basic test
@Test func simpleTest() { }

// With display name
@Test("Validates email format correctly")
func emailValidation() { }

// Async test
@Test func asyncOperation() async throws { }

// With traits (see Traits section)
@Test(.enabled(if: ProcessInfo.processInfo.environment["CI"] != nil))
func ciOnlyTest() { }

#expect - Assert conditions

// Boolean expectation
#expect(value == expected)
#expect(array.isEmpty)
#expect(result != nil)

// With custom message
#expect(count > 0, "Count should be positive")

// Throwing expectation
#expect(throws: ValidationError.self) {
    try validator.validate(invalidInput)
}

// Specific error expectation
#expect(throws: NetworkError.timeout) {
    try await client.fetch(slowEndpoint)
}

// No throw expectation
#expect(throws: Never.self) {
    try safeOperation()
}

#require - Fatal assertions

// Unwrap optionals (test fails immediately if nil)
let user = try #require(await fetchUser(id: 123))
#expect(user.name == "Alice")

// Boolean requirement (test fails immediately if false)
try #require(database.isConnected)
try #require(file.exists, "Test fixture missing")

Parameterized Tests

Run the same test with multiple inputs:

@Test(arguments: ["hello", "world", "swift"])
func stringIsNotEmpty(input: String) {
    #expect(!input.isEmpty)
}

@Test(arguments: [
    (2, 3, 5),
    (0, 0, 0),
    (-1, 1, 0),
    (100, 200, 300)
])
func addition(a: Int, b: Int, expected: Int) {
    #expect(a + b == expected)
}

// Zip two collections (paired arguments)
@Test(arguments: zip(["a", "b"], [1, 2]))
func pairedTest(letter: String, number: Int) {
    #expect(!letter.isEmpty)
    #expect(number > 0)
}

// Cartesian product (all combinations)
@Test(arguments: ["light", "dark"], ["iPhone", "iPad"])
func themeDeviceMatrix(theme: String, device: String) {
    // Runs 4 times: light+iPhone, light+iPad, dark+iPhone, dark+iPad
}

Traits

Modify test behavior with traits:

// Disabled test
@Test(.disabled("Known bug: #123"))
func brokenFeature() { }

// Conditional execution
@Test(.enabled(if: ProcessInfo.processInfo.environment["LIVE_TESTS"] != nil))
func liveAPITest() { }

// Bug reference
@Test(.bug("https://github.com/org/repo/issues/456"))
func workaroundTest() { }

// Tags for filtering
@Test(.tags(.ui, .slow))
func renderingTest() { }

// Time limit
@Test(.timeLimit(.minutes(2)))
func longRunningTest() async { }

// Serial execution (no parallelism)
@Test(.serialized)
func databaseMigrationTest() { }

// Combine traits
@Test(
    "Network timeout handling",
    .tags(.network),
    .timeLimit(.seconds(30)),
    .bug("https://example.com/issue/789", "Flaky on CI")
)
func timeoutTest() async throws { }

Custom Tags

extension Tag {
    @Tag static var ui: Self
    @Tag static var slow: Self
    @Tag static var network: Self
    @Tag static var integration: Self
}

@Test(.tags(.ui, .slow))
func animationTest() { }

Test Suites (@Suite)

Group related tests:

@Suite("User Authentication")
struct AuthTests {
    @Test func loginSucceeds() { }
    @Test func loginFailsWithBadPassword() { }
    @Test func logoutClearsSession() { }
}

// Nested suites
@Suite struct NetworkTests {
    @Suite struct RequestTests {
        @Test func getRequest() { }
        @Test func postRequest() { }
    }

    @Suite struct ResponseTests {
        @Test func parseJSON() { }
        @Test func handleError() { }
    }
}

// Suite-level traits (apply to all tests)
@Suite(.serialized)
struct DatabaseTests {
    @Test func migration() { }
    @Test func rollback() { }
}

Setup and Teardown

@Suite struct ViewModelTests {
    var viewModel: ViewModel!

    // Per-test setup (init runs before each test)
    init() {
        viewModel = ViewModel()
    }

    // Per-test teardown
    deinit {
        viewModel = nil
    }

    @Test func initialState() {
        #expect(viewModel.items.isEmpty)
    }
}

// Async setup
@Suite struct AsyncSetupTests {
    var database: Database

    init() async throws {
        database = try await Database.connect()
    }
}

Confirmation (Async Event Verification)

Wait for async events/callbacks:

@Test func notificationReceived() async {
    await confirmation { confirm in
        NotificationCenter.default.addObserver(
            forName: .userLoggedIn,
            object: nil,
            queue: .main
        ) { _ in
            confirm()
        }

        await loginUser()
    }
}

// Expected count
@Test func multipleCallbacks() async {
    await confirmation(expectedCount: 3) { confirm in
        for _ in 0..<3 {
            asyncOperation { confirm() }
        }
    }
}

Migrating from XCTest

Migration Example

Before (XCTest):

class UserServiceTests: XCTestCase {
    var service: UserService!

    override func setUp() {
        super.setUp()
        service = UserService()
    }

    func testFetchUserReturnsUser() async throws {
        let user = try await service.fetch(id: 1)
        XCTAssertEqual(user.name, "Alice")
        XCTAssertNotNil(user.email)
    }

    func testFetchInvalidUserThrows() async {
        do {
            _ = try await service.fetch(id: -1)
            XCTFail("Expected error")
        } catch {
            XCTAssertTrue(error is UserError)
        }
    }
}

After (Swift Testing):

@Suite struct UserServiceTests {
    let service = UserService()

    @Test func fetchUserReturnsUser() async throws {
        let user = try await service.fetch(id: 1)
        #expect(user.name == "Alice")
        #expect(user.email != nil)
    }

    @Test func fetchInvalidUserThrows() async {
        #expect(throws: UserError.self) {
            try await service.fetch(id: -1)
        }
    }
}

Running Tests

Xcode

• Cmd+U: Run all tests
• Click diamond in gutter: Run single test
• Test Navigator (Cmd+6): Browse and run

Command Line

# Run all tests
swift test

# Run specific test
swift test --filter "UserServiceTests/fetchUser"

# Run tagged tests
swift test --filter ".tags(.network)"

# Parallel execution (default)
swift test --parallel

# Serial execution
swift test --no-parallel

Best Practices

1. Use descriptive names - @Test("Login fails with expired token") over @Test func testLoginFail()

2. Prefer #expect over #require - Use #require only when subsequent assertions depend on the unwrapped value

3. Parameterize repetitive tests - Don't copy-paste tests with different inputs

4. Tag for CI filtering - Use .tags(.slow) for tests to skip in fast feedback loops

5. Keep suites focused - One suite per logical unit (ViewModel, Service, etc.)

6. Avoid shared mutable state - Struct suites with init() > class with setUp()

Common Gotchas

• Mixing frameworks: Swift Testing and XCTest can coexist but don't share state
• No setUp/tearDown: Use init/deinit instead (or async init)
• Parallel by default: Tests run concurrently unless .serialized applied
• #expect vs #require: #expect logs and continues; #require halts the test
• Confirmation timeout: Default is 60 seconds; use timeout: parameter for slower operations