What are mocking and stubbing techniques for StoreKit 2 in Swift?

Mocking and stubbing are essential techniques in testing, particularly when dealing with external frameworks like StoreKit 2 in Swift. These techniques allow developers to simulate the behavior of StoreKit without making actual purchases or connecting to the App Store, thus avoiding unintended costs and ensuring smoother testing processes.

Mocking refers to creating a fake version of an object that simulates the behavior of the real object. This allows you to verify interactions between your code and the mock without making real requests. Stubbing, on the other hand, involves providing a predetermined response to a method call, which is useful when you want to isolate the code and control its environment effortlessly.

Here’s an example of how you can use mocking and stubbing with StoreKit 2 in Swift:

    import XCTest
    import StoreKit

    class MockProduct: SKProduct {
        var mockProductIdentifier: String
        var mockPrice: NSDecimalNumber
        
        init(identifier: String, price: NSDecimalNumber) {
            self.mockProductIdentifier = identifier
            self.mockPrice = price
        }
        
        override var productIdentifier: String {
            return mockProductIdentifier
        }

        override var price: NSDecimalNumber {
            return mockPrice
        }
    }

    class StoreKitTests: XCTestCase {
        func testProductFetching() {
            let mockProduct = MockProduct(identifier: "com.example.app.product", price: NSDecimalNumber(string: "9.99"))
            // Here you would normally call your StoreKit code that fetches products
            // Instead, you use mockProduct to simulate fetching a product
            XCTAssertEqual(mockProduct.productIdentifier, "com.example.app.product")
            XCTAssertEqual(mockProduct.price, NSDecimalNumber(string: "9.99"))
        }
    }