gh-Dimillian-Skills/swift-testing/SKILL.md

name	description
swift-testing	Swift Testing framework (@Test, @Suite,

Swift Testing

Overview

Write unit tests using Apple's Swift Testing framework with @Test, @Suite, #expect, and #require. Replaces XCTest for non-UI tests.

Quick reference

XCTest	Swift Testing
import XCTest	import Testing
class FooTests: XCTestCase	@Suite struct FooTests
func testBar()	@Test func bar()
XCTAssertEqual(a, b)	#expect(a == b)
XCTAssertNil(x)	#expect(x == nil)
XCTAssertThrowsError	#expect(throws:)
XCTUnwrap(x)	try #require(x)
setUpWithError()	init() throws
tearDown()	deinit

Workflow

1. Set up the test suite

Create a struct with @Suite and use init() for setup:

import Testing

@Suite struct UserServiceTests {
    let sut: UserService
    let mockRepo: MockRepository

    init() {
        mockRepo = MockRepository()
        sut = UserService(repository: mockRepo)
    }
}

2. Write test functions

Add @Test to test functions. Remove the test prefix:

@Test func fetchUser_returnsValidUser() async throws {
    let user = try await sut.fetchUser(id: "123")
    #expect(user.name == "John")
}

3. Use assertions

• #expect(condition) — soft assertion, test continues on failure
• try #require(optional) — hard assertion, test stops if nil

#expect(result == expected)
#expect(isValid)
let value = try #require(optional)

#expect(throws: ValidationError.self) {
    try validate(badInput)
}

4. Add parameterized tests

Run the same test with multiple inputs:

@Test(arguments: ["", " ", "   "])
func validate_rejectsBlankStrings(_ input: String) {
    #expect(!isValid(input))
}

@Test(arguments: [
    (input: "hello", expected: 5),
    (input: "", expected: 0)
])
func count_returnsCorrectLength(input: String, expected: Int) {
    #expect(input.count == expected)
}

5. Apply traits for test control

@Test(.enabled(if: ProcessInfo.processInfo.environment["CI"] != nil))
func onlyOnCI() { }

@Test(.disabled("Waiting for backend fix"))
func brokenEndpoint() { }

@Test(.tags(.slow, .network))
func networkHeavyTest() { }

@Test(.timeLimit(.minutes(1)))
func mustCompleteFast() { }

@Suite(.serialized)
struct DatabaseTests {
    @Test func insert() { }
    @Test func delete() { }
}

Common mistakes

Mistake	Fix
#expect(x == nil) on T??	Use try #require(x) first to unwrap outer optional
Test not running	Ensure @Test attribute is present
Shared state between tests	Use init() for setup, avoid static var
Async test not awaited	Mark test async, use await
XCTest assertions in Swift Testing	Replace with #expect / #require

Reference material

• See references/swift-testing-basics.md for core concepts and syntax.
• See references/migration-from-xctest.md for migration strategies.
• See references/advanced-patterns.md for parameterized tests, traits, and mocking.