Why associatedtype protocol fails to compile with error: does not conform to protocol

Question

I try to compile Swift file with associated protocol and it fails to compile with error

import Foundation

protocol ViewProtocol {
}

protocol PresenterProtocol {
    associatedtype ViewType: ViewProtocol

    var view: ViewType {get}
}

protocol ExampleViewProtocol: ViewProtocol { 
}

class ExamplePresenter: PresenterProtocol {
    var view: ExampleViewProtocol

    init(view: ExampleViewProtocol) {
        self.view = view
    }
}

Error appears in Xcode 10.2 and XCode 10.1:

Type 'ExamplePresenter' does not conform to protocol 'PresenterProtocol'

I don't understand why. What should be done to make it compile?

Answer 1

The problem is that the ExampleViewProtocol inherits from ViewProtocol instead of conforming to it. To make your code compile, you could try adapting ExampleViewProtocol to make it a class. This would make ExampleViewProtocol a concrete type which will allow it to conform to ViewProtocol .

Answer 2

you can add generic type constraints to ExamplePresenter

class ExamplePresenter<T: ExampleViewProtocol>: PresenterProtocol {
    typealias ViewType = T

    var view: T

    init(view: T) {
        self.view = view
    }
}