Swift Testing

Swift Testing is the modern testing framework for Swift (Xcode 16+, Swift 6+). Prefer it for new unit tests. Keep XCTest where migration is still in progress, and use XCTest for UI automation, performance APIs, Objective-C exception tests, and common snapshot-test tooling.

Contents

• Basic Tests
• @Test Traits
• #expect and #require
• @Suite and Test Organization
• Execution Model
• XCTest Migration Boundaries
• Known Issues
• Additional Patterns
• Common Mistakes
• Test Attachments
• Exit Testing
• Version-Gated APIs
• Review Checklist
• References

Basic Tests

import Testing

@Test("User can update their display name")
func updateDisplayName() {
    var user = User(name: "Alice")
    user.name = "Bob"
    #expect(user.name == "Bob")
}

@Test Traits

@Test("Validates email format")                                    // display name
@Test(.tags(.validation, .email))                                  // tags
@Test(.disabled("Server migration in progress"))                   // disabled
@Test(.enabled(if: ProcessInfo.processInfo.environment["CI"] != nil)) // conditional
@Test(.bug("https://github.com/org/repo/issues/42"))               // bug reference
@Test(.timeLimit(.minutes(1)))                                     // time limit
@Test("Timeout handling", .tags(.networking), .timeLimit(.seconds(30))) // combined

#expect and #require

// #expect records failure but continues execution
#expect(result == 42)
#expect(name.isEmpty == false)
#expect(items.count > 0, "Items should not be empty")

// #expect with error type checking
#expect(throws: ValidationError.self) {
    try validate(email: "not-an-email")
}

// #expect with specific error value
#expect {
    try validate(email: "")
} throws: { error in
    guard let err = error as? ValidationError else { return false }
    return err == .empty
}

// #require records failure AND stops test (like XCTUnwrap)
let user = try #require(await fetchUser(id: 1))
#expect(user.name == "Alice")

// #require for optionals -- unwraps or fails
let first = try #require(items.first)
#expect(first.isValid)

Rule: Use `#require` when subsequent assertions depend on the value. Use `#expect` for independent checks.

@Suite and Test Organization

See references/testing-patterns.md for suite organization, confirmation patterns, known-issue handling, and execution-model details.

Execution Model

Swift Testing runs tests in parallel by default. Do not assume test order, shared suite instances, or exclusive access to mutable state unless you explicitly design for it.

@Suite(.serialized)
struct KeychainTests {
    @Test func storesToken() throws { /* ... */ }
    @Test func deletesToken() throws { /* ... */ }
}

Use .serialized when a test or suite must run one-at-a-time because it touches shared external state. It does not make unrelated tests outside that scope run serially.

Rules:

• Each test must set up its own state.
• Shared mutable globals are a bug unless protected or intentionally serialized.
• @Suite(.serialized) is for exclusive execution, not for expressing logical ordering between tests.
• If tests depend on sequence, combine them into one test or move the sequence into shared helper code.

XCTest Migration Boundaries

Swift Testing unit tests do not inherit from XCTestCase. Declare @Test on free functions or methods on suite types such as struct, class, or actor; use static or class methods when instance fixtures are unnecessary.

XCTest and Swift Testing can coexist during migration. Migrate one file or suite at a time, compare discovery/pass/fail/skip counts, and keep UI automation, performance benchmarks, and common snapshot flows on XCTest/XCUITest or snapshot tooling. Separate files or targets when that makes runner expectations clearer.

For Xcode 27-era mixed helpers, check the configured interoperability mode rather than claiming cross-framework APIs are forbidden. Older test plans inherit limited; new projects use complete; strict and none are also available. Prefer complete or strict during migration and use SWIFT_TESTING_XCTEST_INTEROP_MODE for SwiftPM when needed. See references/testing-advanced.md for the mode matrix and toolchain gates.

Do not mechanically replace every XCTest assertion with #expect; preserve required unwraps and unconditional failures with these migration defaults:

• XCTAssert* -> #expect(...)
• XCTUnwrap or any value required by later checks -> try #require(...)
• XCTFail("...") or manual unconditional issues -> Issue.record("...")
• UI tests, performance benchmarks, and common snapshot-test flows stay on XCTest/XCUITest or snapshot tooling.
• Put @available on individual @Test functions, not on suite types or their containing types.

See references/testing-patterns.md for migration examples and references/testing-advanced.md for Swift/Xcode version gates.

Known Issues

Mark expected failures so they do not cause test failure:

withKnownIssue("Propane tank is empty") {
    #expect(truck.grill.isHeating)
}

// Intermittent / flaky failures
withKnownIssue(isIntermittent: true) {
    #expect(service.isReachable)
}

// Conditional known issue
withKnownIssue {
    #expect(foodTruck.grill.isHeating)
} when: {
    !hasPropane
}

If no known issues are recorded, Swift Testing records a distinct issue notifying you the problem may be resolved.

Additional Patterns

See references/testing-patterns.md for parameterized tests, tags and suites, async testing, traits, and execution-model details.

Test Attachments

Attach diagnostic data to test results for debugging failures. See references/testing-patterns.md for full examples.

@Test func generateReport() async throws {
    let report = try generateReport()
    Attachment.record(report.data, named: "report.json")
    #expect(report.isValid)
}

For image attachments and their toolchain gate, use the canonical table in Version-Gated APIs.

Exit Testing

Test code that calls exit(), fatalError(), or preconditionFailure() on a supported runtime. State the exact gate from Version-Gated APIs when correcting exit-test code.

@Test func invalidInputCausesExit() async {
    await #expect(processExitsWith: .failure) {
        processInvalidInput()  // calls fatalError()
    }
}

Version-Gated APIs

For advanced APIs, state the exact toolchain and runtime gate beside the correction. This is the canonical summary; references/testing-advanced.md contains the detailed matrix and examples.

@Test func exitsWithCapturedCode() async {
    let expectedCode: Int32 = 42
    await #expect(processExitsWith: .failure) { [expectedCode] in
        exit(expectedCode)
    }
}

Common Mistakes

1. Testing implementation, not behavior. Test what the code does, not how.
2. No error path tests. If a function can throw, test the throw path.
3. Flaky async tests or sleeps. Use confirmation, clock injection, or concurrency primitives instead of sleeping.
4. Shared mutable state between tests. Each test sets up its own state via init() in @Suite.
5. Missing accessibility identifiers in UI tests. XCUITest queries rely on them.
6. Not testing cancellation. If code supports Task cancellation, verify it cancels cleanly.
7. Unclear migration boundaries. Follow XCTest Migration Boundaries instead of treating either framework as an all-or-nothing choice.
8. Non-Sendable helpers shared across tests. Make shared helpers Sendable; annotate MainActor-dependent test code with @MainActor.
9. Treating serialization as ordering. Tests run in parallel by default; .serialized protects exclusive state but does not make one test feed another.

Review Checklist

• [ ] All new tests use Swift Testing (@Test, #expect), not XCTest assertions
• [ ] Test names describe behavior (fetchUserReturnsNilOnNetworkError not testFetchUser)
• [ ] Error paths have dedicated tests
• [ ] Async tests use confirmation(), not Task.sleep
• [ ] Parameterized tests used for repetitive variations
• [ ] Tags applied for filtering (.critical, .slow)
• [ ] Mocks conform to protocols, not subclass concrete types
• [ ] No shared mutable state between tests
• [ ] Tests do not rely on declaration order or shared suite instances
• [ ] .serialized used only for truly exclusive state, not to model workflow sequencing
• [ ] Cancellation tested for cancellable async operations

References

• Testing patterns: references/testing-patterns.md
• Advanced testing (warnings, cancellation, image attachments): references/testing-advanced.md