if let email = self.emailField.text {
    showSpinner {
        // [START signin_emaillink]
        Auth.auth().signIn(withEmail: email, link: self.link) { (user, error) in
            // [START_EXCLUDE]
            self.hideSpinner {
                if let error = error {
                    self.showMessagePrompt(error.localizedDescription)
                        return
                }
                self.navigationController!.popViewController(animated: true)
            }
            // [END_EXCLUDE]
        }
        // [END signin_emaillink]
    }
} else {
    self.showMessagePrompt("Email can't be empty")
}

//(コメント略)

#import "UIViewController+Alerts.h"

//(省略)

/*! @fn showSpinner
 @brief Shows the please wait spinner.
 @param completion Called after the spinner has been hidden.  */
- (void)showSpinner:(nullable void (^)(void))completion;

/*! @fn hideSpinner
 @brief Hides the please wait spinner.
 @param completion Called after the spinner has been hidden.  */
- (void)hideSpinner:(nullable void (^)(void))completion;

//(省略)

//(省略)

- (void)showSpinner:(nullable void (^)(void))completion {
  if ([self supportsAlertController]) {
    [self showModernSpinner:completion];
  } else {
    [self showIOS7Spinner:completion];
  }
}

//(省略)

- (void)hideSpinner:(nullable void (^)(void))completion {
  if ([self supportsAlertController]) {
    [self hideModernSpinner:completion];
  } else {
    [self hideIOS7Spinner:completion];
  }
}

//(省略)

extension UIViewController {
    //...

    func showSpinner(_ completion: (()->Void)?) {
        //...
    }

    //...

    func hideSpinner(_ completion: (()->Void)?) {
        //...
    }

    //...
}