<?php
namespace VendorNameModuleNameControllerIndex;

use MagentoFrameworkAppActionContext;
use MagentoFrameworkViewResultPageFactory;

class Addtocart extends MagentoFrameworkAppActionAction {

protected $_messageManager;

public function __construct(
    Context $context,
    MagentoFrameworkMessageManagerInterface $messageManager,
) {
    parent::__construct($context);
    $this->_messageManager = $messageManager;
}

public function execute() {
    // your logic code
    $this->_messageManager->addSuccess('Shopping cart updated succesfully.');
}