Swift protocol inside of protocol - swift

Question

I am running into a problem with Swift protocols. I have something like this:
protocol OnboardingPage {
var viewModel: OnboardingPageViewModel! { get }
}
protocol OnboardingPageViewModel {
func getValue() -> Bool
}
// ---
class FirstNameViewController: UIViewController, OnboardingPage {
var viewModel: FirstNameViewModel!
}
class FirstNameViewModel: OnboardingPageViewModel {
func getValue() -> Bool {
return true
}
}
class GenderViewController: UIViewController, OnboardingPage {
var viewModel: GenderViewModel!
}
class GenderViewModel: OnboardingPageViewModel {
func getValue() -> Bool {
return false
}
}
I have a bunch of view controllers that confirm to the OnboardingPage protocol, and they all have a view model that all conform to the OnboardingPageViewModel protocol.
But sadly the code above doesn't work: it says FirstNameViewController doesn’t confirm to the OnboardingPage protocol, since its viewModel property has type FirstNameViewModel instead of OnboardingPageViewModel - even though FirstNameViewModel IS a OnboardingPageViewModel.
How can I solve this?

Answer 1

Use associated type OnboardingPageViewModel type ViewModel
Edited
protocol OnboardingPage {
associatedtype ViewModel: OnboardingPageViewModel
var viewModel: ViewModel { get }
}
protocol OnboardingPageViewModel {
func getValue() -> Bool
}
class FirstNameViewController: OnboardingPage {
let viewModel: FirstNameViewModel
init(viewModel: FirstNameViewModel) {
self.viewModel = viewModel
}
}
class FirstNameViewModel: OnboardingPageViewModel {
func getValue() -> Bool {
return true
}
}
class GenderViewController: OnboardingPage {
let viewModel: GenderViewModel
init(viewModel: GenderViewModel) {
self.viewModel = viewModel
}
}
class GenderViewModel: OnboardingPageViewModel {
func getValue() -> Bool {
return false
}
}

Answer 2

You may solve your pages array issue with deletion of associatedtype:
protocol OnboardingPage {
var viewModel: OnboardingPageViewModel { get }
}
protocol OnboardingPageViewModel {
func getValue() -> Bool
}
class FirstNameViewModel: OnboardingPageViewModel {
func getValue() -> Bool {
return true
}
}
class FirstNameViewController: OnboardingPage {
let viewModel: OnboardingPageViewModel
init(viewModel: FirstNameViewModel) {
self.viewModel = viewModel
}
}
class GenderViewModel: OnboardingPageViewModel {
func getValue() -> Bool {
return false
}
}
class GenderViewController: OnboardingPage {
let viewModel: OnboardingPageViewModel
init(viewModel: GenderViewModel) {
self.viewModel = viewModel
}
}
Usage:
let nameModel = FirstNameViewModel()
let genderModel = GenderViewModel()
let array: [OnboardingPage] = [FirstNameViewController(viewModel: nameModel), GenderViewController(viewModel: genderModel)]
for page in array {
print(page.viewModel.getValue())
}
However, in such manner concrete types of your models will not be available for callers of OnboardingPage conforming classes. You may try to solve it with implementing visitor pattern within OnboardingPage or enhancing its capabilities.
Of course, you can always write let pages: [Any] = [FirstNameViewController(), GenderViewController()] using Cruz example, but it is ugly as it goes, and I strongly discourage you from using Any in your API if possible.