Workaround for simple protocol inheriting conformance

Question

In our app, we have a service that helps us decide which Modal UIVIewController should we present next. Every ModalVIewController has common function such as dismiss() but also a specific function it implements. So that's what we tried:

The base protocol that is common to all VC's base functions.

protocol ModalScreenDelegate: AnyObject {
    func modalScreenWantsToDissmiss(_ modalScreen: ModalScreen)
}

A base protocol that every UIViewController implements

protocol ModalScreen: UIViewController {
    var delegate: ModalScreenDelegate? { get set }
}

Now we create a protocol with specific-implementation of ModalScreenDelegate base protocol like so:

protocol ShareToFacebookDelegate: ModalScreenDelegate {
    func someCustomMethod()
}

And assign it to:

class ShareToFacebookViewController: UIViewController, ModalScreen {
   weak var delegate: ModalScreenDelegate? // **WORKS**
   weak var delegate: ShareToFacebookDelegate? // **DOESN'T WORKS**
}

If I'm trying to use ShareToFacebookDelegate to instead of ModalScreenDelegate the compiler throws an IDE error saying I have to change it back to ModalScreenDelegate.

Why wouldn't it work? It's ShareToFacebookDelegate conforms to ModalScreenDelegate. Any help would be highly appreciated. Thank you!

UPDATE Based on Alexandr Kolesnik:

Your method works. But when I try to "fetch" the correct VC within the service under one method like so:

func fetchModal<T: ModalScreen & UIViewController>() -> T? {
    return AddInstagramViewController.create() as? T
}

And then have a coordinator that wants to get this vc:

guard let currentModalViewController vc = modalScreenSupplierService.fetchModal() else {
    return
}

I'm getting:

Generic parameter 'T' could not be inferred

And I can't really say what T will be, all I know that it's going to conform to UIViewController & ModalScreen. Is it solvable?

don't you want to rename your delegates to understand which one you call? — Alexandr Kolesnik, Dec 05 '19 at 10:34
@AlexandrKolesnik Hey Alex, I've made a rough sketch of the issue, sorry! Feel free to edit I'm on mobile right now — Roi Mulia, Dec 05 '19 at 10:36
@giorashc If I'm trying to use `ShareToFacebookDelegate` to instead of `ModalScreenDelegate` the compiler throws an IDE error saying I have to change it back to `ModalScreenDelegate`. — Roi Mulia, Dec 05 '19 at 10:40
@giorashc I've updated the question. Pasting the code into playgrounds will result in the same compile error.. — Roi Mulia, Dec 05 '19 at 10:55
I have no any problems with ` func fetchModal() -> M? { return AddInstagramViewController.create() as? M }` — Alexandr Kolesnik, Dec 05 '19 at 12:34
@AlexandrKolesnik I'm getting error when trying to fetch it in the modalScreenSupplierService.fetchModal() method.. Can you take a look? — Roi Mulia, Dec 05 '19 at 16:58

score 1 · Answer 1 · answered Dec 05 '19 at 11:07

This solution:

   protocol ModalScreenDelegate: AnyObject {
        func modalScreenWantsToDissmiss(_ modalScreen: ModalScreen)
    }
    protocol ModalScreen: UIViewController {
        var delegate: (ModalScreenDelegate & ShareToFacebookDelegate)? { get set }
    }
    protocol ShareToFacebookDelegate: ModalScreenDelegate {
        func someCustomMethod()
    }
    class ShareToFacebookViewController: UIViewController, ModalScreen {
       weak var delegate: (ModalScreenDelegate & ShareToFacebookDelegate)? 
    }

or inheritance:

protocol ModalScreenDelegate: AnyObject {
    func modalScreenWantsToDissmiss(_ modalScreen: ModalScreen)
}
protocol ModalScreen: ShareToFacebookDelegate where Self: UIViewController {
    var delegate: ModalScreenDelegate? { get set }
}
protocol ShareToFacebookDelegate: ModalScreenDelegate {
    func someCustomMethod()
}
class ShareToFacebookViewController: UIViewController, ModalScreen {
    func someCustomMethod() {

    }

    func modalScreenWantsToDissmiss(_ modalScreen: ModalScreen) {

    }

   weak var delegate: ModalScreenDelegate? // **WORKS**
}

Hey @Vyacheslav! Thank you for your answer. I think you misunderstood me (might be my fault). I have a `Coordinator` that responsible to show those modals. I'm trying to create a base delegate that my coordinator can conform to. So basically someCustomMethod() and modalScreenWantsToDissmiss() should be implemented at the `Coordinator` and not on the actual VC. Is this still possible? I tried inside my project and seems that it's not working for that purpose :/ — Roi Mulia, Dec 05 '19 at 11:22

Alexandr Kolesnik · Accepted Answer · 2019-12-06T07:11:57.557

If I understood you correctly you can use generic types to manage the problem. Look through the code below. Hope it helps

    protocol ModalScreenDelegate: AnyObject {

    typealias T = ModalScreenDelegate

    func modalScreenWantsToDissmiss(_ modalScreen: T)
}

protocol ShareToFacebookDelegate: ModalScreenDelegate {
    func someCustomMethod()
}

protocol ModalScreen: UIViewController {

    associatedtype T

    var delegate: T? { get set }
}


class ShareToFacebookViewController: UIViewController, ModalScreen {

    typealias T = ShareToFacebookDelegate

    weak var delegate: T?

    override func viewDidLoad() {
        super.viewDidLoad()
        delegate?.someCustomMethod()
    }

}

UPDATE:

    class AddInstagramViewController: SuperVC {

    typealias T = ShareToFacebookDelegate

    private var instaDelegate: ShareToFacebookDelegate?

    override var delegate: ModalScreenDelegate? {
        set {
            instaDelegate = newValue as? ShareToFacebookDelegate
        }
        get {
            return instaDelegate
        }
    }

    static func create() -> AddInstagramViewController {
        return AddInstagramViewController()
    }

}

class SuperVC: UIViewController, ModalScreen {

    typealias T = ModalScreenDelegate

    var delegate: T?

}

class Supplier {

    func fetchModal<M: ModalScreen>() -> M? { return AddInstagramViewController.create() as? M }

}

class SupplierImpl {

    let modalScreenSupplierService: Supplier? = nil

    func goto() {
        guard
            let vc: SuperVC = modalScreenSupplierService?.fetchModal()
        else {
            return
        }
    }

}

That works! The only issue we found is that when we try to have a function that basically expected output of `ModalScreen` it doesn't work... I've updated the question. Can you take a look? — Roi Mulia, Dec 05 '19 at 12:22

Workaround for simple protocol inheriting conformance

2 Answers2