class ParamConversionEnhancer

class ParamConversionEnhancer implements EnhancerInterface, EventSubscriberInterface {
    
    /**
     * The parameter conversion manager.
     *
     * @var \Drupal\Core\ParamConverter\ParamConverterManagerInterface
     */
    protected $paramConverterManager;
    
    /**
     * Constructs a new ParamConversionEnhancer.
     *
     * @param \Drupal\Core\ParamConverter\ParamConverterManagerInterface $param_converter_manager
     *   The parameter conversion manager.
     */
    public function __construct(ParamConverterManagerInterface $param_converter_manager) {
        $this->paramConverterManager = $param_converter_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public function enhance(array $defaults, Request $request) {
        // Just run the parameter conversion once per request.
        if (!isset($defaults['_raw_variables'])) {
            $defaults['_raw_variables'] = $this->copyRawVariables($defaults);
            $defaults = $this->paramConverterManager
                ->convert($defaults);
        }
        return $defaults;
    }
    
    /**
     * Store a backup of the raw values that corresponding to the route pattern.
     *
     * @param array $defaults
     *   The route defaults array.
     *
     * @return \Symfony\Component\HttpFoundation\ParameterBag
     */
    protected function copyRawVariables(array $defaults) {
        
        /** @var \Symfony\Component\Routing\Route $route */
        $route = $defaults[RouteObjectInterface::ROUTE_OBJECT];
        $variables = array_flip($route->compile()
            ->getVariables());
        // Foreach will copy the values from the array it iterates. Even if they
        // are references, use it to break them. This avoids any scenarios where raw
        // variables also get replaced with converted values.
        $raw_variables = [];
        foreach (array_intersect_key($defaults, $variables) as $key => $value) {
            $raw_variables[$key] = $value;
        }
        // Route defaults that do not start with a leading "_" are also
        // parameters, even if they are not included in path or host patterns.
        foreach ($route->getDefaults() as $name => $value) {
            if (!isset($raw_variables[$name]) && substr($name, 0, 1) !== '_') {
                $raw_variables[$name] = $value;
            }
        }
        return new ParameterBag($raw_variables);
    }
    
    /**
     * Catches failed parameter conversions and throw a 404 instead.
     *
     * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
     *   The event.
     */
    public function onException(ExceptionEvent $event) {
        $exception = $event->getThrowable();
        if ($exception instanceof ParamNotConvertedException) {
            $event->setThrowable(new NotFoundHttpException($exception->getMessage(), $exception));
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() {
        $events[KernelEvents::EXCEPTION][] = [
            'onException',
            75,
        ];
        return $events;
    }

}