magento2/app/code/Magento/ProductAlert/Controller/Add.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\ProductAlert\Controller;

use Magento\Framework\App\Action\Action;
use Magento\Framework\App\Action\Context;
use Magento\Customer\Model\Session as CustomerSession;
use Magento\Framework\App\RequestInterface;

/**
 * Abstract controller for notifying.
 */
abstract class Add extends Action
{
    /**
     * @var \Magento\Customer\Model\Session
     */
    protected $customerSession;

    /**
     * @param \Magento\Framework\App\Action\Context $context
     * @param \Magento\Customer\Model\Session $customerSession
     */
    public function __construct(
        Context $context,
        CustomerSession $customerSession
    ) {
        $this->customerSession = $customerSession;
        parent::__construct($context);
    }

    /**
     * Check customer authentication for some actions
     *
     * @param \Magento\Framework\App\RequestInterface $request
     * @return \Magento\Framework\App\ResponseInterface
     */
    public function dispatch(RequestInterface $request)
    {
        if (!$this->customerSession->authenticate()) {
            $this->_actionFlag->set('', 'no-dispatch', true);
            if (!$this->customerSession->getBeforeAuthUrl()) {
                $this->customerSession->setBeforeAuthUrl($this->_redirect->getRefererUrl());
            }
        }
        return parent::dispatch($request);
    }
}