Amazon CloudFront with Drupal 8

use Drupal\Component\Utility\UrlHelper;
 
/**
 * Implements hook_file_url_alter().
 */
function mt_custom_file_url_alter(&$uri) {
 
  // Route static files to Amazon CloudFront, for anonymous users only.
  if (\Drupal::request()->server->get('HTTP_HOST') == 'www.metaltoad.com' &&
    \Drupal::currentUser()->isAnonymous() &&
    !\Drupal::request()->isSecure()) {
 
    // Multiple hostnames to parallelize downloads.
    $shard = crc32($uri) % 4 + 1;
    $cdn = "http://static$shard.metaltoad.com";
 
    $scheme = file_uri_scheme($uri);
    if ($scheme == 'public') {
      $wrapper = file_stream_wrapper_get_instance_by_scheme('public');
      $path = $wrapper->getDirectoryPath() . '/' . file_uri_target($uri);
      $uri = "$cdn/" . UrlHelper::encodePath($path);
    }
    else if (!$scheme && strpos($uri, '//') !== 0) {
      $uri = "$cdn/" . UrlHelper::encodePath($uri);
    }
  }
}
 
/**
 * Implements hook_css_alter().
 */
function mt_custom_css_alter(&$css) {
  // Mangle the paths slightly so that Drupal\Core\Asset\AssetDumper will generate
  // different keys on HTTPS.  Necessary because CDN URL varies by protocol.
  if (\Drupal::request()->isSecure()) {
    foreach ($css as $key => $file) {
      if ($file['type'] === 'file') {
        $css[$key]['data'] = './' . $css[$key]['data'];
      }
    }
  }
}

namespace Drupal\mt_custom\EventSubscriber;
 
use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Drupal\Core\Session\AccountInterface;
 
class MTCustomSubscriber implements EventSubscriberInterface {
 
  protected $account;
 
  public function checkForCloudFront(GetResponseEvent $event) {
    $req = $event->getRequest();
 
    /*
     * Make sure Amazon CloudFront doesn't serve dynamic content
     * from static*.metaltoad.com
     */
    if (strstr($req->server->get('HTTP_HOST'), 'static')) {
      if (!strstr($req->getPathInfo(), 'files/styles')) {
        header("HTTP/1.0 404 Not Found");
        print '404 Not Found';
        exit();
      }
    }
  }
 
  /**
   * {@inheritdoc}
   */
  static function getSubscribedEvents() {
    $events[KernelEvents::REQUEST][] = array('checkForCloudFront');
    return $events;
  }
 
  public function __construct(AccountInterface $account) {
    $this->account = $account;
  }
 
}