Swift Testing vs XCTest Comparison

// Swift Testing — parameterized async unit test
import Testing
@testable import PaymentKit

@Suite("Price calculation")
struct PriceCalculatorTests {

    @Test("Discount is calculated correctly for valid coupon codes",
          arguments: [
              ("SAVE10", 100.0, 90.0),
              ("SAVE20", 100.0, 80.0),
              ("NONE", 100.0, 100.0)
          ])
    func discountIsApplied(code: String, base: Double, expected: Double) async throws {
        let calculator = PriceCalculator()
        let result = try await calculator.applyDiscount(code: code, to: base)
        #expect(result == expected, "expected \(expected) for \(code), got \(result)")
    }

    @Test("Invalid coupon throws an error")
    func invalidCouponThrows() async throws {
        let calculator = PriceCalculator()
        await #expect(throws: CouponError.invalid) {
            try await calculator.applyDiscount(code: "XXX", to: 100.0)
        }
    }

    @Test("Cart total is required", .disabled("Cart service has not been migrated yet"))
    func cartTotalRequired() async throws {
        let calculator = PriceCalculator()
        try #require(calculator.cartTotal > 0)
    }
}

// XCTest — same scenario (price calculation) + UI test
import XCTest
@testable import PaymentKit

final class PriceCalculatorTests: XCTestCase {
    var calculator: PriceCalculator!

    override func setUpWithError() throws {
        calculator = PriceCalculator()
    }

    func testDiscountIsAppliedForKnownCoupons() async throws {
        let cases: [(String, Double, Double)] = [
            ("SAVE10", 100.0, 90.0),
            ("SAVE20", 100.0, 80.0),
            ("NONE", 100.0, 100.0)
        ]
        for (code, base, expected) in cases {
            let result = try await calculator.applyDiscount(code: code, to: base)
            XCTAssertEqual(result, expected, "coupon: \(code)")
        }
    }

    // XCTAssertThrowsError is synchronous; use do/catch + XCTFail in an async call
    func testInvalidCouponThrows() async throws {
        do {
            _ = try await calculator.applyDiscount(code: "XXX", to: 100.0)
            XCTFail("expected an error")
        } catch {
            XCTAssertEqual(error as? CouponError, .invalid)
        }
    }
}

// UI test — only possible with XCTest/XCUITest
final class CheckoutUITests: XCTestCase {
    func testApplyCouponButtonUpdatesTotal() throws {
        let app = XCUIApplication()
        app.launch()
        app.textFields["couponField"].tap()
        app.textFields["couponField"].typeText("SAVE10")
        app.buttons["applyCouponButton"].tap()
        XCTAssertEqual(app.staticTexts["totalLabel"].label, "$90.00")
    }
}