Swift Language Getting Started with Protocol Oriented Programming Leveraging Protocol Oriented Programming for Unit Testing
Example
Protocol Oriented Programming is a useful tool in order to easily write better unit tests for our code.
Let's say we want to test a UIViewController that relies on a ViewModel class.
The needed steps on the production code are:
- Define a protocol that exposes the public interface of the class ViewModel, with all the properties and methods needed by the UIViewController.
- Implement the real ViewModel class, conforming to that protocol.
- Use a dependency injection technique to let the view controller use the implementation we want, passing it as the protocol and not the concrete instance.
protocol ViewModelType {
var title : String {get}
func confirm()
}
class ViewModel : ViewModelType {
let title : String
init(title: String) {
self.title = title
}
func confirm() { ... }
}
class ViewController : UIViewController {
// We declare the viewModel property as an object conforming to the protocol
// so we can swap the implementations without any friction.
var viewModel : ViewModelType!
@IBOutlet var titleLabel : UILabel!
override func viewDidLoad() {
super.viewDidLoad()
titleLabel.text = viewModel.title
}
@IBAction func didTapOnButton(sender: UIButton) {
viewModel.confirm()
}
}
// With DI we setup the view controller and assign the view model.
// The view controller doesn't know the concrete class of the view model,
// but just relies on the declared interface on the protocol.
let viewController = //... Instantiate view controller
viewController.viewModel = ViewModel(title: "MyTitle")
Then, on unit test:
- Implement a mock ViewModel that conforms to the same protocol
- Pass it to the UIViewController under test using dependency injection, instead of the real instance.
- Test!
class FakeViewModel : ViewModelType {
let title : String = "FakeTitle"
var didConfirm = false
func confirm() {
didConfirm = true
}
}
class ViewControllerTest : XCTestCase {
var sut : ViewController!
var viewModel : FakeViewModel!
override func setUp() {
super.setUp()
viewModel = FakeViewModel()
sut = // ... initialization for view controller
sut.viewModel = viewModel
XCTAssertNotNil(self.sut.view) // Needed to trigger view loading
}
func testTitleLabel() {
XCTAssertEqual(self.sut.titleLabel.text, "FakeTitle")
}
func testTapOnButton() {
sut.didTapOnButton(UIButton())
XCTAssertTrue(self.viewModel.didConfirm)
}
}
