Testing Async Code — Swift Testing Patterns

Modern patterns for testing async/await code with Swift Testing framework.

When to Use

✅ Use 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

❌ Don't use when:

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

Key Differences from XCTest

Patterns

Pattern 1: Simple Async Function

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

Pattern 2: Completion Handler → Continuation

For APIs without async overloads:

@Test func legacyAPI() async throws {
    let result = try await withCheckedThrowingContinuation { continuation in
        legacyFetch { result, error in
            if let result {
                continuation.resume(returning: result)
            } else {
                continuation.resume(throwing: error!)
            }
        }
    }
    #expect(result.isValid)
}

Pattern 3: Single Callback with confirmation

When a callback should fire exactly once:

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

Pattern 4: Multiple Callbacks with expectedCount

@Test func delegateCalledMultipleTimes() async {
    await confirmation(expectedCount: 3) { confirm in
        delegate.onProgress = { progress in
            confirm()  // Called 3 times
        }
        startDownload()  // Triggers 3 progress updates
    }
}

Pattern 5: Verify Callback Never Fires

@Test func noErrorCallback() async {
    await confirmation(expectedCount: 0) { confirm in
        delegate.onError = { _ in
            confirm()  // Should never be called
        }
        performSuccessfulOperation()
    }
}

Pattern 6: MainActor Tests

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

Pattern 7: Timeout Control

@Test(.timeLimit(.seconds(5)))
func slowOperation() async throws {
    try await longRunningTask()
}

Pattern 8: Testing Throws

@Test func invalidInputThrows() async throws {
    await #expect(throws: ValidationError.self) {
        try await validate(input: "")
    }
}

// Specific error
@Test func specificError() async throws {
    await #expect(throws: NetworkError.notFound) {
        try await api.fetch(id: -1)
    }
}

Pattern 9: Optional Unwrapping with #require

@Test func firstVideo() async throws {
    let videos = try await videoLibrary.videos()
    let first = try #require(videos.first)  // Fails if nil
    #expect(first.duration > 0)
}

Pattern 10: Parameterized Async Tests

@Test("Video loading", arguments: [
    "Beach.mov",
    "Mountain.mov",
    "City.mov"
])
func loadVideo(fileName: String) async throws {
    let video = try await Video.load(fileName)
    #expect(video.isPlayable)
}

Arguments run in parallel automatically.

Parallel Test Execution

Swift Testing runs tests in parallel by default (unlike XCTest).

Handling Shared State

// ❌ Shared mutable state — race condition
var sharedCounter = 0

@Test func test1() async {
    sharedCounter += 1  // Data race!
}

@Test func test2() async {
    sharedCounter += 1  // Data race!
}

// ✅ Each test gets fresh instance
struct CounterTests {
    var counter = Counter()  // Fresh per test

    @Test func increment() {
        counter.increment()
        #expect(counter.value == 1)
    }
}

Forcing Serial Execution

When tests must run sequentially:

@Suite("Database tests", .serialized)
struct DatabaseTests {
    @Test func createRecord() async { /* ... */ }
    @Test func readRecord() async { /* ... */ }  // After create
    @Test func deleteRecord() async { /* ... */ }  // After read
}

Note: Other unrelated tests still run in parallel.

Common Mistakes

Mistake 1: Using sleep Instead of confirmation

// ❌ Flaky — arbitrary wait time
@Test func eventFires() async {
    setupEventHandler()
    try await Task.sleep(for: .seconds(1))  // Hope it happened?
    #expect(eventReceived)
}

// ✅ Deterministic — waits for actual event
@Test func eventFires() async {
    await confirmation { confirm in
        onEvent = { confirm() }
        triggerEvent()
    }
}

Mistake 2: Forgetting @MainActor on UI Tests

// ❌ Data race — ViewModel may be MainActor
@Test func viewModel() async {
    let vm = ViewModel()
    await vm.load()  // May cause data race warnings
}

// ✅ Explicit isolation
@Test @MainActor func viewModel() async {
    let vm = ViewModel()
    await vm.load()
}

Mistake 3: Missing confirmation for Callbacks

// ❌ Test passes immediately — doesn't wait for callback
@Test func callback() async {
    api.fetch { result in
        #expect(result.isSuccess)  // Never executed before test ends
    }
}

// ✅ Waits for callback
@Test func callback() async {
    await confirmation { confirm in
        api.fetch { result in
            #expect(result.isSuccess)
            confirm()
        }
    }
}

Mistake 4: Not Handling Parallel Execution

// ❌ Tests interfere with each other
@Test func writeFile() async {
    try! "data".write(to: sharedFileURL, atomically: true, encoding: .utf8)
}

@Test func readFile() async {
    let data = try! String(contentsOf: sharedFileURL)  // May fail!
}

// ✅ Use unique files or .serialized
@Test func writeAndRead() async {
    let url = FileManager.default.temporaryDirectory
        .appendingPathComponent(UUID().uuidString)
    try! "data".write(to: url, atomically: true, encoding: .utf8)
    let data = try! String(contentsOf: url)
    #expect(data == "data")
}

Migration from XCTest

XCTestExpectation → confirmation

// XCTest
func testFetch() {
    let expectation = expectation(description: "fetch")
    api.fetch { result in
        XCTAssertNotNil(result)
        expectation.fulfill()
    }
    wait(for: [expectation], timeout: 5)
}

// Swift Testing
@Test func fetch() async {
    await confirmation { confirm in
        api.fetch { result in
            #expect(result != nil)
            confirm()
        }
    }
}

Async setUp → Suite init

// XCTest
class MyTests: XCTestCase {
    var service: Service!

    override func setUp() async throws {
        service = try await Service.create()
    }
}

// Swift Testing
struct MyTests {
    let service: Service

    init() async throws {
        service = try await Service.create()
    }

    @Test func example() async {
        // Use self.service
    }
}

Resources

WWDC: 2024-10179, 2024-10195

Docs: /testing, /testing/confirmation

Skills: axiom-swift-testing, axiom-ios-testing