Drupal 8: how to create a sub-request

SubRequestController.php

<?php

namespace Drupal\content_helper\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpKernel\HttpKernelInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * Class SubRequest.
 *
 * @package Drupal\content_helper\Controller
 */
class SubRequestController extends ControllerBase implements ContainerInjectionInterface {

  /**
   * Symfony\Component\HttpKernel\HttpKernelInterface definition.
   *
   * @var Symfony\Component\HttpKernel\HttpKernelInterface
   */
  protected $httpKernel;

  /**
   * {@inheritdoc}
   */
  public function __construct(HttpKernelInterface $http_kernel) {
    $this->httpKernel = $http_kernel;
  }

  /**
   * Performs a subrequest.
   *
   * @param string $path
   *   Path to use for subrequest.
   * @param array $parameters
   *   The query parameters.
   *
   * @return string
   *   The response String.
   *
   * @throws \Exception
   */
  public function subRequest($path, array $parameters = []) {
    $sub_request = Request::create($path, 'GET', $parameters);

    $subResponse = $this->httpKernel->handle($sub_request, HttpKernelInterface::SUB_REQUEST, FALSE);

    return $subResponse->getContent();
  }
  
}

Thanks for sharing this, it was very helpful in working out some issues I had with my own subrequest implementation.