Magento – Magento2: Programatically update cart via event

Please use this even checkout_cart_product_add_after, and also I suggest you that

ObserverInterface has only one method which is execute, so write your code in this method.

And do not use parent Item because while you are adding to cart any item then price depend upon child item so the best way is to use directly that item object that you are getting. So best practice is to use that item itself.

And also return $this object at the end of your code inside execute method.

For eg: namespace Navin\Testcart\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\App\RequestInterface;

class CustomPrice implements ObserverInterface
{
    public function execute(\Magento\Framework\Event\Observer $observer) {

        $item=$observer->getEvent()->getData('quote_item');
        $product=$observer->getEvent()->getData('product');
        // here i am using item's product final price
        $price = $item->getProduct()->getFinalPrice()+10; // 10 is custom price. It will increase in product price.
        // Set the custom price
        $item->setCustomPrice($price);
        $item->setOriginalCustomPrice($price);
        // Enable super mode on the product.
        $item->getProduct()->setIsSuperMode(true);
        return $this;
    }

}

Hope this will help you. :)

I ended up using plugin di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Checkout\Model\Cart">
        <plugin name="interceptUpdateProductToCart" type="Mycompany\Inventory\Plugin\Rationingcheck"/>
    </type>
</config>

Rationingcheck.php

<?php
namespace Mycompany\Inventoryrationing\Plugin;
use \Magento\Framework\Message\ManagerInterface ;
use Myvendor\Inventoryrationing\Helper\Rationinglogic;


class Rationingcheck
{
    /**
     * @var Rationinglogic|Data
     */
    protected $_helperData;

    /**
     * @var ManagerInterface
     */
    protected $_messageManager;

    /**
     * @var \Magento\Quote\Model\Quote
     */
    protected $quote;

    /**
     * Plugin constructor.
     *
     * @param \Magento\Checkout\Model\Session $checkoutSession
     */
    public function __construct(
        \Magento\Checkout\Model\Session $checkoutSession,
        Rationinglogic $helperData,
        ManagerInterface $messageManager
    ) {
        $this->quote = $checkoutSession->getQuote();
        $this->_helperData = $helperData;
        $this->_messageManager = $messageManager;
    }

    /**
     * @param \Magento\Checkout\Model\Cart $subject
     * @param $data
     * @return array
     */
    public function beforeupdateItems(\Magento\Checkout\Model\Cart $subject,$data)
    {
        $quote = $subject->getQuote();
        $mappedProductSkus= $this->_helperData->getSku();
        foreach($data as $key=>$value){
            $item = $quote->getItemById($key);
             $productSku= $item->getSku();
              $itemQty= $value['qty'];
                if(Condition ))
                {
                    $data[$itemId]['qty'] = 1;
                    if($item->getQty()>1){
                        $this->_messageManager->addNoticeMessage('Only one item can be bought at a time');
                    }
                }

        }
        return [$data];

    }
}