Add.php

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

use Magento\Framework\App\Action\HttpPostActionInterface as HttpPostActionInterface;
use Magento\Catalog\Api\ProductRepositoryInterface;
use Magento\Checkout\Model\Cart as CustomerCart;
use Magento\Framework\Exception\NoSuchEntityException;

/**
 * Controller for processing add to cart action.
 *
 * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
 */
class Add extends \Magento\Checkout\Controller\Cart implements HttpPostActionInterface
{
    /**
     * @var ProductRepositoryInterface
     */
    protected $productRepository;

    /**
     * @param \Magento\Framework\App\Action\Context $context
     * @param \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
     * @param \Magento\Checkout\Model\Session $checkoutSession
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param \Magento\Framework\Data\Form\FormKey\Validator $formKeyValidator
     * @param CustomerCart $cart
     * @param ProductRepositoryInterface $productRepository
     * @codeCoverageIgnore
     */
    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig,
        \Magento\Checkout\Model\Session $checkoutSession,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Framework\Data\Form\FormKey\Validator $formKeyValidator,
        CustomerCart $cart,
        ProductRepositoryInterface $productRepository
    ) {
        parent::__construct(
            $context,
            $scopeConfig,
            $checkoutSession,
            $storeManager,
            $formKeyValidator,
            $cart
        );
        $this->productRepository = $productRepository;
    }

    /**
     * Initialize product instance from request data
     *
     * @return \Magento\Catalog\Model\Product|false
     */
    protected function _initProduct()
    {
        $productId = (int)$this->getRequest()->getParam('product');
        if ($productId) {
            $storeId = $this->_objectManager->get(
                \Magento\Store\Model\StoreManagerInterface::class
            )->getStore()->getId();
            try {
                return $this->productRepository->getById($productId, false, $storeId);
            } catch (NoSuchEntityException $e) {
                return false;
            }
        }
        return false;
    }

    /**
     * Add product to shopping cart action
     *
     * @return \Magento\Framework\Controller\Result\Redirect
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     */
    public function execute()
    {
        if (!$this->_formKeyValidator->validate($this->getRequest())) {
            $this->messageManager->addErrorMessage(
                __('Your session has expired')
            );
            return $this->resultRedirectFactory->create()->setPath('*/*/');
        }

        $params = $this->getRequest()->getParams();

        try {
            if (isset($params['qty'])) {
                $filter = new \Zend_Filter_LocalizedToNormalized(
                    ['locale' => $this->_objectManager->get(
                        \Magento\Framework\Locale\ResolverInterface::class
                    )->getLocale()]
                );
                $params['qty'] = $filter->filter($params['qty']);
            }

            $product = $this->_initProduct();
            $related = $this->getRequest()->getParam('related_product');

            /**
             * Check product availability
             */
            if (!$product) {
                return $this->goBack();
            }

            $this->cart->addProduct($product, $params);
            if (!empty($related)) {
                $this->cart->addProductsByIds(explode(',', $related));
            }

            $this->cart->save();

            /**
             * @todo remove wishlist observer \Magento\Wishlist\Observer\AddToCart
             */
            $this->_eventManager->dispatch(
                'checkout_cart_add_product_complete',
                ['product' => $product, 'request' => $this->getRequest(), 'response' => $this->getResponse()]
            );

            if (!$this->_checkoutSession->getNoCartRedirect(true)) {
                if (!$this->cart->getQuote()->getHasError()) {
                    if ($this->shouldRedirectToCart()) {
                        $message = __(
                            'You added %1 to your shopping cart.',
                            $product->getName()
                        );
                        $this->messageManager->addSuccessMessage($message);
                    } else {
                        $this->messageManager->addComplexSuccessMessage(
                            'addCartSuccessMessage',
                            [
                                'product_name' => $product->getName(),
                                'cart_url' => $this->getCartUrl(),
                            ]
                        );
                    }
                }
                return $this->goBack(null, $product);
            }
        } catch (\Magento\Framework\Exception\LocalizedException $e) {
            if ($this->_checkoutSession->getUseNotice(true)) {
                $this->messageManager->addNoticeMessage(
                    $this->_objectManager->get(\Magento\Framework\Escaper::class)->escapeHtml($e->getMessage())
                );
            } else {
                $messages = array_unique(explode("\n", $e->getMessage()));
                foreach ($messages as $message) {
                    $this->messageManager->addErrorMessage(
                        $this->_objectManager->get(\Magento\Framework\Escaper::class)->escapeHtml($message)
                    );
                }
            }

            $url = $this->_checkoutSession->getRedirectUrl(true);

            if (!$url) {
                $url = $this->_redirect->getRedirectUrl($this->getCartUrl());
            }

            return $this->goBack($url);
        } catch (\Exception $e) {
            $this->messageManager->addExceptionMessage(
                $e,
                __('We can\'t add this item to your shopping cart right now.')
            );
            $this->_objectManager->get(\Psr\Log\LoggerInterface::class)->critical($e);
            return $this->goBack();
        }
    }

    /**
     * Resolve response
     *
     * @param string $backUrl
     * @param \Magento\Catalog\Model\Product $product
     * @return $this|\Magento\Framework\Controller\Result\Redirect
     */
    protected function goBack($backUrl = null, $product = null)
    {
        if (!$this->getRequest()->isAjax()) {
            return parent::_goBack($backUrl);
        }

        $result = [];

        if ($backUrl || $backUrl = $this->getBackUrl()) {
            $result['backUrl'] = $backUrl;
        } else {
            if ($product && !$product->getIsSalable()) {
                $result['product'] = [
                    'statusText' => __('Out of stock')
                ];
            }
        }

        $this->getResponse()->representJson(
            $this->_objectManager->get(\Magento\Framework\Json\Helper\Data::class)->jsonEncode($result)
        );
    }

    /**
     * Returns cart url
     *
     * @return string
     */
    private function getCartUrl()
    {
        return $this->_url->getUrl('checkout/cart', ['_secure' => true]);
    }

    /**
     * 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',
            \Magento\Store\Model\ScopeInterface::SCOPE_STORE
        );
    }
}