Testing Async Code — Swift Testing Patterns

Purpose: Modern patterns for testing async/await code with Swift Testing framework Swift Version: Swift 6+ Xcode Version: Xcode 16+

When to Use This Skill

✅ Use this skill when:

• Writing tests for async functions
• Testing callback-based APIs with Swift Testing
• Migrating async XCTests to Swift Testing
• Testing MainActor-isolated code
• Need to verify events fire expected number of times

❌ Do NOT use this skill for:

• XCTest-only project (use XCTestExpectation)
• UI automation tests (use XCUITest)
• Performance testing with metrics (use XCTest)

Key Differences from XCTest

XCTest	Swift Testing
XCTestExpectation	confirmation { }
wait(for:timeout:)	await confirmation
@MainActor implicit	@MainActor explicit
Serial by default	Parallel by default

Key Patterns

Simple Async Function

@Test func fetchUser() async throws {
    let user = try await api.fetchUser(id: 1)
    #expect(user.name == "Alice")
}

Callback with confirmation

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

Multiple Callbacks

@Test func delegateCalledThreeTimes() async {
    await confirmation(expectedCount: 3) { confirm in
        delegate.onProgress = { _ in confirm() }
        startDownload()
    }
}

MainActor Tests

@Test @MainActor func viewModelUpdates() async {
    let vm = ViewModel()
    await vm.load()
    #expect(vm.items.count > 0)
}

Common Mistakes

Using sleep Instead of confirmation

// ❌ Flaky
try await Task.sleep(for: .seconds(1))
#expect(eventReceived)

// ✅ Deterministic
await confirmation { confirm in
    onEvent = { confirm() }
    triggerEvent()
}

Related Skills

• Swift Testing — Complete Swift Testing guide
• Swift Concurrency — Async/await patterns

Resources

• WWDC: Meet Swift Testing (10179)
• WWDC: Go further with Swift Testing (10195)
• Docs: Testing Framework