AppleAuthService.swift

@available(iOS 13.0, *)
extension AppleAuthService: ASAuthorizationControllerDelegate {
    func authorizationController(controller: ASAuthorizationController, didCompleteWithAuthorization authorization: ASAuthorization) {
        if let appleIDCredential = authorization.credential as? ASAuthorizationAppleIDCredential {
            backendAuth?.authorize(appleIDCredential)
        }
    }

    func authorizationController(controller: ASAuthorizationController, didCompleteWithError error: Error) {
        switch error {
        case let error as ASAuthorizationError:
            delegate?.authService(authType, didFailToAuthorizeWith: AuthenticationError.appleError(code: error.errorCode))
        default:
            delegate?.authService(authType, didFailToAuthorizeWith: error)
        }
    }
}

@available(iOS 13.0, *)
extension AppleAuthService: ASAuthorizationControllerPresentationContextProviding {
    func presentationAnchor(for controller: ASAuthorizationController) -> ASPresentationAnchor {
        guard let window = viewController?.view.window else {
            fatalError("couldn't find window")
        }
        return window
    }
}