How to update variable in MVVM? - swift

Question

I am trying to use MVVM. I am going to VC2 from VC1. I am updating the viewModel.fromVC = 1, but the value is not updating in the VC2.
Here is what I mean:
There is a viewModel, in it there is a var fromVC = Int(). Now, in vc1, I am calling the viewModel as
let viewModel = viewModel().
Now, on the tap of button, I am updating the viewModel.fromVC = 8. And, moving to the next screen. In the next screen, when I print fromVC then I get the value as 0 instead of 8.
This is how the VC2 looks like
class VC2 {
let viewModel = viewModel()
func abc() {
print(viewModel.fromVC)
}
}
Now, I am calling abc() in viewDidLoad and the fromVC is printed as 0 instead of 8. Any help?

Answer 1

For the MVVM pattern you need to understand that it's a layer split in 2 different parts: Inputs & Outputs.
Int terms of inputs, your viewModel needs to catch every event from the viewController, and for the Outputs, this is the way were the viewModel will send data (correctly formatted) to the viewController.
So basically, if we have a viewController like this:
final class HomeViewController: UIViewController {
// MARK: - Outlets
#IBOutlet private weak var titleLabel: UILabel!
// MARK: - View life cycle
override func viewDidLoad() {
super.viewDidLoad()
}
// MARK: - Actions
#IBAction func buttonTouchUp(_ sender: Any) {
titleLabel.text = "toto"
}
}
We need to extract the responsibilities to a viewModel, since the viewController is handling the touchUp event, and owning the data to bring to th label.
By Extracting this, you will keep the responsibility correctly decided and after all, you'll be able to test your viewModel correctly 🙌
So how to do it? Easy, let's take a look to our futur viewModel:
final class HomeViewModel {
// MARK: - Private properties
private let title: String
// MARK: - Initializer
init(title: String) {
self.title = title
}
// MARK: - Outputs
var titleText: ((String) -> Void)?
// MARK: - Inputs
func viewDidLoad() {
titleText?("")
}
func buttonDidPress() {
titleText?(title)
}
}
So now, by doing this, you are keeping safe the different responsibilities, let's see how to bind our viewModel to our previous viewController :
final class HomeViewController: UIViewController {
// MARK: - public var
var viewModel: HomeViewModel!
// MARK: - Outlets
#IBOutlet private weak var titleLabel: UILabel!
// MARK: - View life cycle
override func viewDidLoad() {
super.viewDidLoad()
bind(to: viewModel)
viewModel.viewDidLoad()
}
// MARK: - Private func
private func bind(to viewModel: HomeViewModel) {
viewModel.titleText = { [weak self] title in
self?.titleLabel.text = title
}
}
// MARK: - Actions
#IBAction func buttonTouchUp(_ sender: Any) {
viewModel.buttonDidPress()
}
}
So one thing is missing, you'll asking me "but how to initialise our viewModel inside the viewController?"
Basically you should once again extract responsibilities, you could have a Screens layer which would have the responsibility to create the view like this:
final class Screens {
// MARK: - Properties
private let storyboard = UIStoryboard(name: StoryboardName, bundle: Bundle(for: Screens.self))
// MARK: - Home View Controller
func createHomeViewController(with title: String) -> HomeViewController {
let viewModel = HomeViewModel(title: title)
let viewController = storyboard.instantiateViewController(withIdentifier: "Home") as! HomeViewController
viewController.viewModel = viewModel
return viewController
}
}
And finally do something like this:
let screens = Screens()
let homeViewController = screens.createHomeViewController(with: "Toto")
But the main subject was to bring the possibility to test it correctly, so how to do it? very easy!
import XCTest
#testable import mvvmApp
final class HomeViewModelTests: XCTestCase {
func testGivenAHomeViewModel_WhenViewDidLoad_titleLabelTextIsEmpty() {
let viewModel = HomeViewModel(title: "toto")
let expectation = self.expectation("Returned title")
viewModel.titleText = { title in
XCTAssertEqual(title, "")
expectation.fulfill()
}
viewModel.viewDidLoad()
waitForExpectations(timeout: 1.0, handler: nil)
}
func testGivenAHomeViewModel_WhenButtonDidPress_titleLabelTextIsCorrectlyReturned() {
let viewModel = HomeViewModel(title: "toto")
let expectation = self.expectation("Returned title")
var counter = 0
viewModel.titleText = { title in
if counter == 1 {
XCTAssertEqual(title, "toto")
expectation.fulfill()
}
counter += 1
}
viewModel.viewDidLoad()
viewModel.buttonDidPress()
waitForExpectations(timeout: 1.0, handler: nil)
}
}
And that's it 💪