text: <?php
  /**
   * Copyright © Magento, Inc. All rights reserved.
   * See COPYING.txt for license details.
   */
  namespace Magento\Framework\Indexer;
  
  use Magento\Framework\Indexer\Handler\DefaultHandler;
  use Magento\Framework\ObjectManagerInterface;
  
  /**
   * @api Instantiate save handler when implementing custom Indexer\Action
   */
  class HandlerPool
  {
      /**
       * @var ObjectManagerInterface
       */
      protected $objectManager;
  
      /**
       * @var HandlerInterface
       */
      protected $defaultHandler;
  
      /**
       * @param ObjectManagerInterface $objectManager
       * @param DefaultHandler $defaultHandler
       */
      public function __construct(
          ObjectManagerInterface $objectManager,
          DefaultHandler $defaultHandler
      ) {
          $this->defaultHandler = $defaultHandler;
          $this->objectManager = $objectManager;
      }
  
      /**
       * Get handler class instance
       *
       * @param string $handlerClass
       * @throws \InvalidArgumentException
       * @return HandlerInterface
       */
      public function get($handlerClass = null)
      {
          if ($handlerClass === null) {
              return $this->defaultHandler;
          }
  
          $handler = $this->objectManager->get($handlerClass);
          if (!$handler instanceof HandlerInterface) {
              throw new \InvalidArgumentException(
                  $handlerClass . ' doesn\'t implement \Magento\Framework\Indexer\HandlerInterface'
              );
          }
  
          return $handler;
      }
  }
  