swift-testing-pro

Swift Testing Pro

Write clear, fast tests with the Swift Testing framework. Prefer it over XCTest for new code.

When to use

• Writing new unit/integration tests.
• Migrating XCTest cases to Swift Testing.
• Adding parameterized or trait-gated tests.

Trigger: /swift-testing-pro.

Core principles

• @Test functions, not test-prefixed XCTestCase methods.
• #expect for soft assertions; #require to stop the test on failure.
• Group with @Suite (struct/actor) — a fresh instance per test gives isolation.
• Use async/throws directly; no expectation-fulfillment dance.

Basic test

❌ XCTest

import XCTest
final class MathTests: XCTestCase {
    func testAdd() { XCTAssertEqual(add(2, 3), 5) }
}

✅ Swift Testing

import Testing

@Test func add() {
    #expect(add(2, 3) == 5)
}

#expect vs #require

• #expect(x == y) records a failure but continues.
• try #require(value) unwraps/asserts and halts the test if it fails — use before dereferencing.

✅

@Test func parsesUser() throws {
    let user = try #require(User(json: sample))   // stop if nil
    #expect(user.name == "Ada")
}

Suites and state

@Suite struct CartTests {
    let cart = Cart()           // fresh per test — no shared state

    @Test func startsEmpty() { #expect(cart.items.isEmpty) }
    @Test func addsItem() {
        cart.add(.sample)
        #expect(cart.items.count == 1)
    }
}

Don't reuse mutable state across tests (no XCTest setUp shared singletons).

Parameterized tests

Replace copy-pasted near-identical tests with arguments:.

❌

@Test func evenTwo() { #expect(isEven(2)) }
@Test func evenFour() { #expect(isEven(4)) }

✅

@Test(arguments: [2, 4, 100])
func even(_ n: Int) { #expect(isEven(n)) }

Cross-product with multiple arguments: collections; use zip for paired inputs.

Async and errors

@Test func loadsFeed() async throws {
    let feed = try await api.feed()
    #expect(!feed.isEmpty)
}

@Test func throwsOnBadInput() {
    #expect(throws: ParseError.self) {
        try parse("")
    }
}

Traits

• .tags(.network) to group/filter.
• .enabled(if:) / .disabled("reason") to gate.
• .timeLimit(.minutes(1)) for runaway protection.
• .bug("JIRA-123") to link issues.

@Test(.disabled("flaky on CI — FIX-42"))
func flakyThing() { ... }

Prefer .disabled("reason") over commenting tests out — it stays visible and tracked.

Common mistakes checklist

• Still subclassing XCTestCase for new tests.
• #expect before a force-unwrap that should be try #require.
• Shared mutable state across tests in a suite.
• Copy-pasted tests that should be arguments: parameterized.
• Commented-out tests instead of .disabled("reason").
• XCTestExpectation where plain async/await works.

Output format (when reviewing)

Per issue: file:line, rule, before/after. Flag missing coverage of error paths and edge cases.