How to multiply the quantity of the added product by 2 by clicking the add to cart button through the plugin?

Question

How to multiply the quantity of the added product by 2 by clicking the add to cart button through the before plugin?

Answer 1

this can be achieved with a Plugin:

In your module

on app\code\vendor\module\etc\events.xml

put this

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="controller_action_predispatch_checkout_cart_add">
        <observer name="restrict_sales_model_cart_add_before" instance="vendor\module\Observer\CartAddingConditionCheck" />
    </event>  
</config>

In your app\code\vendor\module\Observer create the file CartAddingConditionCheck.php and place the following code.

<?php
namespace vendor\module\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\App\Response\RedirectInterface;
use Magento\Checkout\Model\Cart;
use Magento\Framework\Message\ManagerInterface;
use Magento\Framework\App\RequestInterface;
use Magento\Catalog\Model\Product;
use \Magento\Customer\Model\Session as customerSession;

class CartAddingConditionCheck implements ObserverInterface
{
    protected $cart;
    protected $messageManager;
    protected $redirect;
    protected $request;
    protected $product;
    protected $customerSession;
    protected $productloader;
    protected $quoteRepository;
    protected $customerRepositoryInterface;
    protected $productRepository;
    protected $storeManager;
    
    public function __construct(
       RedirectInterface $redirect,
       Cart $cart,
       ManagerInterface $messageManager,
       RequestInterface $request,
       Product $product,
       customerSession $session,
       \Magento\Catalog\Model\ProductFactory      $productloader,
       \Magento\Quote\Api\CartRepositoryInterface $quoteRepository,
       \Magento\Customer\Api\CustomerRepositoryInterface $customerRepositoryInterface,
       \Magento\Catalog\Model\ProductRepository         $productRepository,
       \Magento\Store\Model\StoreManagerInterface $storeManager
       
    )
    {
        $this->redirect        = $redirect;
        $this->cart            = $cart;
        $this->messageManager  = $messageManager;
        $this->request         = $request;
        $this->product         = $product;
        $this->customerSession = $session;
        $this->productloader   = $productloader;
        $this->quoteRepository = $quoteRepository;
        $this->customerRepositoryInterface = $customerRepositoryInterface;
        $this->productRepository  = $productRepository;
        $this->storeManager    = $storeManager;
    }
    
    public function execute(
        \Magento\Framework\Event\Observer $observer
    )
    {   
        $postValues = $this->request->getPostValue();
        $item = $this->product;   
        if ($item->getParentItem()) 
        {
            $item = $item->getParentItem();
        }
        $quantity         = isset($postValues['qty']) ? $postValues['qty'] : 1;
        $item->setQty($quantity * 2);
    }
}
?>