RouteProcessorSubscriber.php

<?php

/**
 * @file
 * Definition of Drupal\Core\EventSubscriber\RouteProcessorSubscriber.
 */
namespace Drupal\Core\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\HttpKernelInterface;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
use Symfony\Component\HttpKernel\Exception\MethodNotAllowedHttpException;
use Symfony\Component\HttpKernel\Log\LoggerInterface;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\Routing\Matcher\UrlMatcherInterface;
use Symfony\Component\Routing\Exception\ResourceNotFoundException;

/**
 * Listener to process request controller information.
 */
class RouteProcessorSubscriber implements EventSubscriberInterface {

  /**
   * Sets a default controller for a route if one was not specified.
   *
   * @param Symfony\Component\HttpKernel\Event\GetResponseEvent $event
   *   Event that is created to create a response for a request.
   */
  public function onRequestSetController(GetResponseEvent $event) {
    $request = $event
      ->getRequest();
    if (!$request->attributes
      ->has('_controller') && $request->attributes
      ->has('_content')) {
      $request->attributes
        ->set('_controller', '\\Drupal\\Core\\HtmlPageController::content');
    }
  }

  /**
   * Registers the methods in this class that should be listeners.
   *
   * @return array
   *   An array of event listener definitions.
   */
  static function getSubscribedEvents() {

    // The RouterListener has priority 32, and we need to run after that.
    $events[KernelEvents::REQUEST][] = array(
      'onRequestSetController',
      30,
    );
    return $events;
  }

}