Using the SKTestSession in XCTest.

SubscriptionTests.swift

import XCTest
import StoreKitTest

@available(iOS 14.0, *)
class SubscriptionTests: XCTestCase {

    private var session: SKTestSession!
    private var subscriptionsController: SubscriptionsController!

    override func setUpWithError() throws {
        session = try SKTestSession(configurationFileNamed: "SKFooConfiguration")
        session.disableDialogs = true
        session.clearTransactions()
        subscriptionsController = SubscriptionsController()
    }

    func testIfThereIsNoUnexpectedSubscriptionProduct() {
        let productsExpectation = XCTestExpectation(description: "products")
        subscriptionsController.onProductsDidLoad = {
            productsExpectation.fulfill()
        }
        subscriptionsController.onProductsLoadingDidFailed = { error in
            XCTFail("Should not reach this block")
        }
        subscriptionsController.loadProducts()
        wait(for: [productsExpectation], timeout: 5.0)
        XCTAssertEqual(subscriptionsController.products.count, 2)
        XCTAssertTrue(subscriptionsController.products.contains(where: { $0.productIdentifier == "foo.iap.monthly" }))
        XCTAssertTrue(subscriptionsController.products.contains(where: { $0.productIdentifier == "foo.iap.annually" }))
    }

    func testPurchasingProductShouldSucceed() throws {
        let productsExpectation = XCTestExpectation(description: "products")
        subscriptionsController.onProductsDidLoad = {
            productsExpectation.fulfill()
        }
        subscriptionsController.loadProducts()
        wait(for: [productsExpectation], timeout: 5.0)
        XCTAssertEqual(subscriptionsController.products.count, 2)
        let product = try XCTUnwrap(subscriptionsController.products.first)
        let purchaseProductExpectation = XCTestExpectation(description: "purchase product")
        subscriptionsController.startPurchase(of: product) { error in
            XCTAssertNil(error)
            purchaseProductExpectation.fulfill()
        }
        wait(for: [purchaseProductExpectation], timeout: 5.0)
        let transaction = try XCTUnwrap(subscriptionsController.purchasesStore.getLatestPurchaseTransaction())
        XCTAssertEqual(transaction.productIdentifier, product.productIdentifier)
    }

}