func testMultiply() {
  let test = RxExpect()
  let value = PublishSubject<Int>()
  let result = value.map { $0 * 2 }

  // provide inputs
  test.input(value, [
    next(100, 1),
    next(200, 2),
    next(300, 3),
    completed(400)
  ])
  
  // test output
  test.assert(result) { events in
    XCTAssertEqual(events.values, [
      next(100, 2),
      next(200, 4),
      next(300, 6),
      completed(400)
    ])
  }
}

final class ArticleDetailViewModelTests: XCTestCase {
  func testLikeButtonSelected() {
    let test = RxExpect()
    let viewModel = ArticleDetailViewModel()
    test.retain(viewModel) // IMPORTANT: prevent from being disposed while testing

    // providing an user input: user tapped like button
    test.input(viewModel.likeButtonDidTap, [
      next(100, Void()),
    ])

    // test output: like button become selected
    test.assert(viewModel.isLikeButtonSelected) { events in
      XCTAssertEqual(events.at(100...).elements, [true])
    }
  }

  func testLikeButtonUnselected() {
    let test = RxExpect()
    let viewModel = ArticleDetailViewModel()
    test.retain(viewModel) // IMPORTANT: prevent from being disposed while testing

    // providing an user input: user tapped like button
    test.input(viewModel.likeButtonDidTap, [
      next(100, Void()),
    ])

    // test output: like button become selected
    test.assert(viewModel.isLikeButtonSelected) { events in
      XCTAssertEqual(events.at(100...).elements, [false])
    }
  }
}