magento2/app/code/Magento/Checkout/Controller/Cart.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
declare(strict_types=1);

namespace Magento\Checkout\Controller;

use Magento\Catalog\Controller\Product\View\ViewInterface;
use Magento\Checkout\Model\Cart as CustomerCart;
use Magento\Checkout\Model\Session;
use Magento\Framework\App\Action\Context;
use Magento\Framework\App\Config\ScopeConfigInterface;
use Magento\Framework\Controller\Result\Redirect;
use Magento\Framework\Data\Form\FormKey\Validator;
use Magento\Framework\UrlInterface;
use Magento\Store\Model\ScopeInterface;
use Magento\Store\Model\Store;
use Magento\Store\Model\StoreManagerInterface;

/**
 * Shopping cart controller
 *
 * @api
 */
abstract class Cart extends \Magento\Framework\App\Action\Action implements ViewInterface
{
    /**
     * @var ScopeConfigInterface
     */
    protected $_scopeConfig;

    /**
     * @var Session
     */
    protected $_checkoutSession;

    /**
     * @var StoreManagerInterface
     */
    protected $_storeManager;

    /**
     * @var Validator
     */
    protected $_formKeyValidator;

    /**
     * @var CustomerCart
     */
    protected $cart;

    /**
     * @param Context $context
     * @param ScopeConfigInterface $scopeConfig
     * @param Session $checkoutSession
     * @param StoreManagerInterface $storeManager
     * @param Validator $formKeyValidator
     * @param CustomerCart $cart
     * @codeCoverageIgnore
     */
    public function __construct(
        Context $context,
        ScopeConfigInterface $scopeConfig,
        Session $checkoutSession,
        StoreManagerInterface $storeManager,
        Validator $formKeyValidator,
        CustomerCart $cart
    ) {
        $this->_formKeyValidator = $formKeyValidator;
        $this->_scopeConfig = $scopeConfig;
        $this->_checkoutSession = $checkoutSession;
        $this->_storeManager = $storeManager;
        $this->cart = $cart;
        parent::__construct($context);
    }

    /**
     * Set back redirect url to response
     *
     * @param null|string $backUrl
     *
     * @return Redirect
     */
    protected function _goBack($backUrl = null)
    {
        $resultRedirect = $this->resultRedirectFactory->create();

        if ($backUrl || $backUrl = $this->getBackUrl($this->_redirect->getRefererUrl())) {
            $resultRedirect->setUrl($backUrl);
        }

        return $resultRedirect;
    }

    /**
     * Check if URL corresponds store
     *
     * @param string $url
     * @return bool
     */
    protected function _isInternalUrl($url)
    {
        if (strpos($url, 'http') === false) {
            return false;
        }

        /**
         * Url must start from base secure or base unsecure url
         */
        /** @var $store Store */
        $store = $this->_storeManager->getStore();
        $unsecure = strpos($url, (string) $store->getBaseUrl()) === 0;
        $secure = strpos($url, (string) $store->getBaseUrl(UrlInterface::URL_TYPE_LINK, true)) === 0;
        return $unsecure || $secure;
    }

    /**
     * Get resolved back url
     *
     * @param string|null $defaultUrl
     * @return mixed|null|string
     */
    protected function getBackUrl($defaultUrl = null)
    {
        $returnUrl = $this->getRequest()->getParam('return_url');
        if ($returnUrl && $this->_isInternalUrl($returnUrl)) {
            $this->messageManager->getMessages()->clear();
            return $returnUrl;
        }

        if ($this->shouldRedirectToCart() || $this->getRequest()->getParam('in_cart')) {
            if ($this->getRequest()->getActionName() == 'add' && !$this->getRequest()->getParam('in_cart')) {
                $this->_checkoutSession->setContinueShoppingUrl($this->_redirect->getRefererUrl());
            }
            return $this->_url->getUrl('checkout/cart');
        }

        return $defaultUrl;
    }

    /**
     * Is redirect should be performed after the product was added to cart.
     *
     * @return bool
     */
    private function shouldRedirectToCart()
    {
        return $this->_scopeConfig->isSetFlag(
            'checkout/cart/redirect_to_cart',
            ScopeInterface::SCOPE_STORE
        );
    }
}