Testing Combine code example

Example: Testing Combine

class CarViewModelTest: XCTestCase {
  var car: Car!
  var carViewModel: CarViewModel!
  var cancellables: Set<AnyCancellable>!

  override func setUp() {
    car = Car()
    carViewModel = CarViewModel(car: car)
    cancellables = []
  }

  func testCarViewModelEmitsCorrectStrings() {
    // determine what kwhInBattery would be after driving 10km
    let newValue: Double = car.kwhInBattery - car.kwhPerKilometer * 10

    // configure an array of expected output
    var expectedValues = [car.kwhInBattery, newValue].map { doubleValue in
      return "The car now has \(doubleValue)kwh in its battery"
    }

    // expectation to be fulfilled when we've received all expected values
    let receivedAllValues = expectation(description: "all values received")

    // subscribe to the batterySubject to run the test
    carViewModel.batterySubject.sink(receiveValue: { value in
      guard  let expectedValue = expectedValues.first else {
        XCTFail("Received more values than expected.")
        return
      }

      guard expectedValue == value else {
        XCTFail("Expected received value \(value) to match first expected value \(expectedValue)")
        return
      }

      // remove the first value from the expected values because we no longer need it
      expectedValues = Array(expectedValues.dropFirst())

      if expectedValues.isEmpty {
        // the  test is completed when we've received all expected values
        receivedAllValues.fulfill()
      }
    }).store(in: &cancellables)

    // call drive to trigger a second value
    carViewModel.drive(kilometers: 10)

    // wait for receivedAllValues to be fulfilled
    waitForExpectations(timeout: 1, handler: nil)
  }
}