Migrate to Swift Testing

Summary

• Swift Testing simplifies migration from XCTest with interoperability.

• Parameterized tests and exit tests supercharge testing in Swift.

• Xcode 27 defaults to interoperability, easing transition to Swift Testing.

• Contribute to Swift Testing. It’s open source and community-driven.

Presenters

• Jerry Chen, Developer Tools

Swift Testing Overview

• Introduction: Swift Testing introduced in Xcode 16 as a modern library for writing tests, with an expressive interface, and built for Swift concurrency.

• Core Macros:

  • @Test for declaring tests.

  • #expect for creating expectations.

  • Issue.record replaces XCTFail for unconditional failures.

import Testing

@testable import DemoApp

@Test func `Default climate: tropical`() async throws {
    let fruit = Fruit(name: "Coconut")
    #expect(fruit.climate == .tropical)
}

Migration Strategy

• Keep Existing Tests: Leave most XCTests in place; modify a few at a time.

• Interoperability:

  • Supports calling APIs across XCTest and Swift Testing.

  • Modes:

    • Limited: Cross-framework issues as warnings.

    • Complete: Issues as errors. Recommended.

    • Strict: Fatal errors from XCTest issues.

    • None: Opt-out, but not recommended.

func testUniqueFruitNames() async throws {
    assertUnique(Market.fruits + [Fruit.lychee])
}

// TestHelpers.swift

func assertUnique(_ fruits: [Fruit], file: StaticString = #filePath, line: UInt = #line) {
    var uniqueNames = Set<String>()
    for name in fruits.map(\.name) {
        if !uniqueNames.insert(name).inserted {
            XCTFail("Duplicate name: \(name)", file: file, line: line)
        }
    }
}

import Testing

func assertUnique(_ fruits: [Fruit], sourceLocation: SourceLocation = ...) {
    var uniqueNames = Set<String>()
    for name in fruits.map(\.name) {
        if !uniqueNames.insert(name).inserted {
            Issue.record("Duplicate name: \(name)", sourceLocation: sourceLocation)
        }
    }
}

Test Framework Interoperability

• Usage: Allows seamless use of helper functions across testing frameworks.

• Modes:

  • Change modes with Test Plan Settings.

  • Use environment variable for Swift Package projects.

  SWIFT_TESTING_XCTEST_INTEROP_MODE=strict swift test

Advanced Testing Features

• Parameterized Tests: Run tests with different arguments in parallel for efficiency.

  struct BirdTests {
  
     @Test func `Birds flap wings successfully`() async throws {
        for bird in Aviary.birds {
              for count in (40...100) {
                 try await bird.flapWings(count: count)
              }
        }
     }
  
  }

  struct BirdTests {
  
     @Test(arguments: Aviary.birds, 40...100)
     func `Birds flap wings successfully`(bird: Bird, count: Int) async throws {
        try await bird.flapWings(count: count)
     }
  
  }

• Exit Tests: Test code that might crash, running in isolated processes to ensure stability.

  // In `Bird.init(...)`
  if name.isEmpty {
     preconditionFailure("Bird name cannot be empty")
  }
  
  extension BirdTests {
  
     @Test func `Bird with empty name crashes`() async throws {
        await #expect(processExitsWith: .failure) {
              _ = Bird(name: "")
        }
     }
  
  }

Tips for Migrating

• Skip Tests: Use Test.cancel in Swift Testing.

  let isFall = false
  
  // XCTest
  func testSwallowFallMigration() async throws {
     try XCTSkipIf(!isFall, "Wrong season for migration")
     // ...
  }
  
  // Test.cancel interoperability from Swift Testing
  func testSwallowFallMigration() async throws {
     if !isFall {
        try Test.cancel("Wrong season for migration")
     }
     // ...
  }
  
  // ✅ Prefer test trait in Swift Testing
  @Test(.enabled(if: isFall, "Wrong season for migration"))
  func `Swallow fall migration`() async throws {
     // ...
  }

• Halt on Failure: Use #require to stop tests on first failed assertion.

  func testExample() async throws {
     #expect(Fruit.banana.climate == .temperate)
  
     try #require(Fruit.banana == Fruit.plantain)
     XCTFail("This is never reached")
  }

Contribution and Community

• Open Source: Part of SwiftLang on GitHub; contributions are welcome.

• Governance: Managed by the Testing Workgroup and guided by Swift Evolution.

• Community Engagement: Join discussions on Swift Forums.

Recommendations

• Focus on New Tests: Use Swift Testing for new tests while maintaining existing XCTests.

• Leverage Interoperability: Replace XCTest API gradually with Swift Testing API.

• Explore New Tools: Utilize parameterized and exit tests to enhance testing capabilities.