【EC-CUBE3】カートページで数量を増やした後の在庫数を取得するイベント

SampleEvent.php

<?php
/*
 * This file is part of the Sample
 *
 * Copyright (C) 2017 サンプルプラグイン for EC-CUBE3
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */
namespace Plugin\Sample;
use Eccube\Application;
use Eccube\Event\EventArgs;
class SampleEvent
{
    /** @var  \Eccube\Application $app */
    private $app;
    public function __construct(Application $app)
    {
        $this->app = $app;
    }
  
    /**
     * 数量を増やした時
     * 
     * @param EventArgs $event
     */
    public function onFrontCartUpComplete(EventArgs $event)
    {
        $productClassId = $event->getArgument('productClassId');
        $quantity = $this->app['eccube.service.cart']->getProductQuantity($productClassId);

        echo $quantity;
    }
}