Magento 2 : Update quantity of cart item in observer

Question

I am observing controller_action_postdispatch_checkout_cart_add, in some cases if customer adds a product we have to reset to quantity 1

$productId= $this->_request->getParam('product');
        $productAdded = $this->_productRepository->getById($productId);
$item= $quote->getItemByProduct($productAdded);
            $item_id= $item->getItemId();

            $params[$item_id]['qty'] = 1;
            $params = array(
                'product' => $productId,
                'qty'     => 1,
            );
            echo $item_id;
            $quote->updateItem($item_id,$params);

But updateItem expects 3 parameters

public function updateItem($itemId, $buyRequest, $params = null)

What would be $buyRequest in my case

or there is another way to update quantity in cart

Answer 1

used controller_action_postdispatch_checkout_cart_add event

<?php

namespace Mycompany\Inventoryrationing\Observer;

use \Magento\Framework\Event\Observer;
use \Magento\Framework\Event\ObserverInterface;
use \Magento\Catalog\Model\ProductFactory;
use \Magento\Framework\App\RequestInterface;
use Magento\Catalog\Model\ProductRepository;
use \Magento\Checkout\Model\Cart;
use \Magento\Framework\Message\ManagerInterface ;

use \Magento\Checkout\Model\Session as CheckoutSession;

class Beforeaddtocart implements ObserverInterface {
    /** @var CheckoutSession */
    protected $checkoutSession;
    /**
     * @var ProductFactory
     */
    protected $_productloader;
    /**
     * @var ProductRepository
     */
    protected $_productRepository;
    /**
     * @var RequestInterface
     */
    protected $_request;
    /**
     * @var Rationinglogic|Data
     */
    protected $_helperData;
    /**
     * @var Cart
     */
    protected $_cart;
    /**
     * @var ManagerInterface
     */
    protected $_messageManager;

    /**
     * @param CheckoutSession $checkoutSession
     * @param ProductFactory $productLoader
     * @param RequestInterface $request
     * @param ProductRepository $productRepository
     * @param Cart $cart
     * @param ManagerInterface $messageManager
     */
    public function __construct(
        CheckoutSession $checkoutSession,
        ProductFactory $productLoader,
        RequestInterface $request,
        ProductRepository $productRepository,
        cart $cart,
        ManagerInterface $messageManager

    ) {
        $this->checkoutSession = $checkoutSession;
        $this->_productloader = $productLoader;
        $this->_request = $request;
        $this->_productRepository = $productRepository;
        $this->_helperData = $helperData;
        $this->_cart = $cart;
        $this->_messageManager = $messageManager;
    }

    /**
     * After cart observer
     * @param Observer $observer
     * @return Cart
     * @throws \Magento\Framework\Exception\LocalizedException
     * @throws \Magento\Framework\Exception\NoSuchEntityException
     */
    public function execute(Observer $observer) {
        $productId= $this->_request->getParam('product');
        $productAdded = $this->_productRepository->getById($productId);
        $productSku = $productAdded->getSku();
        $mappedProductSkus= $this->_helperData->getSku();
        $quote = $this->checkoutSession->getQuote();
        if(Condition)
        {
            $item= $quote->getItemByProduct($productAdded);
            $itemId= $item->getItemId();
            $params[$itemId]['qty'] = 1;
            $this->_cart->updateItems($params);
            $this->_cart->saveQuote();
            $this->_cart->save();
            $this->_messageManager->addNoticeMessage('Only one item can be bought at a time');
            return true;
        }
    }

}

Answer 2

You may alter qty on cart_product_add_after observer

$quoteItem = $observer->getEvent()->getQuoteItem();
$avl_qty = 1; 
if ($avl_qty == '0') {
  $quoteItem->getQuote()->removeItem($quoteItem->getItemId());
  throw new LocalizedException(__("This product is currently out of stock."));                    
} 
elseif ($order_qty > $avl_qty) {                   
  $quoteItem->setData('qty', $avl_qty);
  $this->_cart->save();
  $this->messageManager->addNoticeMessage('Sorry we have only '.$avl_qty.' qty of this product available');                    
} 
else {                   
}