swift-testing-agent-skill

For a hypothetical e-commerce app called 'ShopSwift' with a `PaymentProcessor` module handling credit card transactions, here's a Swift Testing framework test suite design:

**Test Doubles for PaymentProcessor:**
```swift
struct MockPaymentGateway: PaymentGatewayProtocol {
    var shouldFail: Bool = false
    var capturedAmount: Double?
    
    func processPayment(amount: Double) async throws -> PaymentResult {
        capturedAmount = amount
        if shouldFail { throw PaymentError.networkFailure }
        return PaymentResult.success
    }
}
```

**Fixtures for Checkout Scenarios:**
```swift
struct CheckoutFixtures {
    static let validCart = Cart(items: [
        CartItem(product: "iPhone 15", price: 999.99, quantity: 1),
        CartItem(product: "AirPods Pro", price: 249.00, quantity: 2)
    ])
    
    static let emptyCart = Cart(items: [])
    
    static let invalidPaymentMethod = PaymentMethod(type: .creditCard, number: "4111111111111111", expiry: "12/25")
}
```

**Async Testing for Payment Processing:**
```swift
@Test func testAsyncPaymentProcessing() async {
    let mockGateway = MockPaymentGateway()
    let processor = PaymentProcessor(gateway: mockGateway)
    
    await #expect(throws: PaymentError.self) {
        try await processor.processCheckout(
            cart: CheckoutFixtures.validCart,
            paymentMethod: CheckoutFixtures.invalidPaymentMethod
        )
    }
    
    #expect(mockGateway.capturedAmount == 1497.99)
}
```

**XCTest Migration Guide:**
For migrating from XCTest's `XCTestCase` to Swift Testing's `@Suite`:
1. Replace `XCTestCase` subclasses with `@Suite` structs
2. Convert `XCTestExpectation` to Swift Testing's `#expect`
3. Replace `setUp()`/`tearDown()` with `@Suite` initialization
4. Use `Tag` to categorize tests instead of XCTest's `XCTestCase` grouping

Testing tradeoffs considered:
- Mocking provides isolation but may hide integration issues
- Fixtures improve readability but increase test maintenance if product changes
- Async testing requires careful handling of timeouts and race conditions
- Migration path preserves existing test coverage while adopting new patterns