Sign In with Apple: initial view login check

*.swift

/// Prompts the user if an existing iCloud Keychain credential or Apple ID credential is found.
func performExistingAccountSetupFlows() {
    // Prepare requests for both Apple ID and password providers.
    let requests = [ASAuthorizationAppleIDProvider().createRequest(),
                    ASAuthorizationPasswordProvider().createRequest()]

    // Create an authorization controller with the given requests.
    let authorizationController = ASAuthorizationController(authorizationRequests: requests)
    authorizationController.delegate = self
    authorizationController.presentationContextProvider = self
    authorizationController.performRequests()
}