**File Paths:**

namespace EwallCustomPriceObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkAppRequestInterface;

class SalesQuoteSaveAfterCustom
{
    protected $checkoutSession;

        public function __construct(MagentoCheckoutModelSession $checkoutSession)
        {
        $this->checkoutSession = $checkoutSession;
        }
    public function invoke($observer)
    {
        $writer = new ZendLogWriterStream(BP . '/var/log/test.log');
        $logger = new ZendLogLogger();
        $logger->addWriter($writer);
        $logger->info('invoke function is calling');

        $quote = $observer->getEvent()->getQuote();

        /* @var $quote MagentoQuoteModelQuote */

        if($quote->getIsCheckoutCart())
        {
        $this->checkoutSession->getQuoteId($quote->getId());
        }
     }
}

<?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="sales_quote_save_after">
<observer name="Ewall_CustomPrice_Custom" instance="EwallCustomPriceObserverSalesQuoteSaveAfterCustom" />
</event>