ViewModelTests.swift

func test_WhenProductIsNotNilAndAddToCartIsCalled_ThenTheProductIsAdded() {
  // given
  self.viewModel.product = Product(id: 1, name: "Grey T-Shirt", pricePerUnit: 20)
  let exp = self.expectation(description: "Waiting for async operation")

  // when
  self.viewModel.addToCart { (result) in
    // then
    switch result {
    case .success:
      print("Passed")

      // Reading the products from the cart to verify the addition of the product
      self.viewModel.getProductsFromCart { (products) in

        // There should be exactly 1 product in the cart
        XCTAssertEqual(products.count, 1)

        // Comparing the product details
        let product = products.first!
        XCTAssertEqual(product.id, 1)
        XCTAssertEqual(product.name, "Grey T-Shirt")
        XCTAssertEqual(product.pricePerUnit, 20)

        exp.fulfill()
      }

    default: XCTFail()
    }
  }

  self.waitForExpectations(timeout: 10, handler: nil)
}