Swift Testing Basics

Core Components

@Test

@Test func userCanLogin() { }
@Test func emptyInput_returnsError() { }
@Test("User can update their profile") func updateProfile() { }

Requirements:

Must be a function (not a property or subscript)
Can be async and/or throws
Can be instance method or free function
No parameters unless using parameterized testing

@Suite

@Suite struct AuthenticationTests {
    @Test func login() { }
    @Test func logout() { }
}

Suite features:

Can be struct, class, actor, or enum
struct recommended (value semantics, fresh instance per test)
Can nest suites for hierarchy
Supports traits for suite-wide configuration

#expect

#expect(value == 42)
#expect(array.isEmpty)
#expect(string.contains("hello"))
#expect(!isDisabled)

Soft assertion—test continues on failure.

Comparisons:

#expect(a == b)
#expect(a != b)
#expect(a > b)
#expect(a === b)

Errors:

#expect(throws: (any Error).self) {
    try riskyOperation()
}

#expect(throws: NetworkError.self) {
    try fetch()
}

#expect {
    try validate("")
} throws: { error in
    error as? ValidationError == .empty
}

#expect(throws: Never.self) {
    try safeOperation()
}

#require

Hard assertion—test stops on failure. Use for unwrapping.

let user = try #require(await fetchUser())
#expect(user.name == "John")

try #require(array.count > 0)
let first = array[0]

let viewModel = try #require(controller.viewModel as? ProfileViewModel)

Lifecycle

@Suite struct ServiceTests {
    let service: MyService
    let mockRepository: MockRepository

    init() {
        mockRepository = MockRepository()
        service = MyService(repository: mockRepository)
    }

    @Test func fetchData() async { }
}

init() runs before each test. Each test gets a fresh instance.

Teardown (class only)

@Suite class ResourceTests {
    var tempFile: URL?

    init() throws {
        tempFile = try createTempFile()
    }

    deinit {
        if let tempFile {
            try? FileManager.default.removeItem(at: tempFile)
        }
    }
}

Async

@Test func asyncFetch() async throws {
    let data = try await api.fetchData()
    #expect(data.count > 0)
}

@Test(.timeLimit(.seconds(5)))
func mustCompleteFast() async { }

@Suite(.serialized)
struct OrderDependentTests {
    @Test func first() { }
    @Test func second() { }
}

Running Tests

Xcode: Cmd+U (all), click diamond (individual)

Command line:

swift test
swift test --filter UserTests
swift test --filter .tags:slow
swift test --skip .tags:slow

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