Swift transitive protocol conformance

Question

Could someone explain why won't the following example compile? Am I missing something trivial?

protocol Coordinator {}

protocol Coordinatable {
    var coordinator: Coordinator { get set }
}

class ExampleCoordinator: Coordinator {}

class ViewController: UIViewController, Coordinatable {
    var coordinator: ExampleCoordinator!
}

The error is:

Type 'ViewController' does not conform to protocol 'Coordinatable'

Thanks!

Does this answer your question? Protocol variable implementing another protocol (also note that you are providing an optional when the protocol requires a non-optional) — Alladinian
– Alladinian, Commented May 20, 2020 at 20:58

RedX · Accepted Answer · 2020-05-20 21:01:11Z

1

Replace var coordinator: ExampleCoordinator! with var coordinator: Coordinator. You are trying to conform to Coordinatable so you have to implement it's properties.

EDIT:

import UIKit
protocol Coordinator {}

protocol Coordinatable {
    var coordinator: Coordinator { get set }
}

class ExampleCoordinator: Coordinator {}

class ViewController: UIViewController, Coordinatable {
    var coordinator: Coordinator

    init(coordinator: Coordinator) {
        self.coordinator = coordinator
        super.init(nibName: nil, bundle: nil)
    }

    // ignore this, it has nothing to do with the question/answer
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

let testVC = ViewController(coordinator: ExampleCoordinator()) // works

edited May 20, 2020 at 21:01

answered May 20, 2020 at 20:45

RedX

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3 Comments

Balázs Vincze Over a year ago

But ExampleCoordinator implements Coordinator, so I am implicitly implementing the required property.

RedX Over a year ago

Well the protocol requires you to implement a Coordinator not an ExampleCoordinator. So do what I described above and then pass an ExampleCoordinator during the initialisation and set self.coordinator to this very ExampleCoordinator

RedX Over a year ago

I updated my answer a bit to make it more clear :) (Just paste it into a playground a play with it)

Community · Accepted Answer · 2020-06-20 09:12:55Z

The reason that it doesn't work

Assuming your code would work and we write additionally

class ExampleCoordinator2: Coordinator {}

val viewController = ViewController()
viewController.coordinator = ExampleCoordinator2()

The assignment to coordinator cannot work since ExampleCoordinator2 is not a subclass of ExampleCoordinator.

So, just considering the types the following code should work but the Swift compiler doesn't allow it (this might work in the future):

protocol Coordinatable {
    var coordinator: Coordinator { get } // removing the `set`
}

class ViewController: UIViewController, Coordinatable {
    var coordinator: ExampleCoordinator! // still an error
}

Possible solutions

Using `var coordinator: Coordinator` in `ViewController`

class ViewController: UIViewController, Coordinatable {

    var coordinator: Coordinator

    init(coordinator: Coordinator) {
        self.coordinator = coordinator
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

Using only `{ get }` in `Coordinatable`

protocol Coordinatable {
    var coordinator: Coordinator { get } // removing the `set`
}

class ViewController: UIViewController, Coordinatable {
    // computed property
    var coordinator: Coordinator {
        // implicit return
        exampleCoordinator
    }
    var exampleCoordinator: ExampleCoordinator!
}

Using an `associatedtype` in `Coordinatable`

protocol Coordinatable {
    associatedtype C: Coordinator
    var coordinator: C { get set }
}

class ViewController: UIViewController, Coordinatable {
    
    var coordinator: ExampleCoordinator
    
    init(coordinator: ExampleCoordinator) {
        self.coordinator = coordinator
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

This has however a big disadvantage that you cannot have a heterogenous array of type [Coordinatable] since Coordinatable has an associatedtype. And still, you cannot assign a value of type ExampleCoordinator2.

You can only write generic code over <C: Coordinatable>. E.g.

func printCoordinators<C: Coordinatable>(coordinatables: [C]) {
    
    coordinatables.forEach {
        print($0.coordinator)
    }
}

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Swift transitive protocol conformance

2 Answers 2

3 Comments

The reason that it doesn't work

Possible solutions

Using `var coordinator: Coordinator` in `ViewController`

Using only `{ get }` in `Coordinatable`

Using an `associatedtype` in `Coordinatable`

Comments

Your Answer

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Collectives™ on Stack Overflow

2 Answers 2

3 Comments

The reason that it doesn't work

Possible solutions

Using var coordinator: Coordinator in ViewController

Using only { get } in Coordinatable

Using an associatedtype in Coordinatable

Comments

Your Answer

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Using `var coordinator: Coordinator` in `ViewController`

Using only `{ get }` in `Coordinatable`

Using an `associatedtype` in `Coordinatable`