class HelpTopicTwigLoader

class HelpTopicTwigLoader extends FilesystemLoader {
  
  /**
   * {@inheritdoc}
   */
  const MAIN_NAMESPACE = 'help_topics';
  
  /**
   * Constructs a new HelpTopicTwigLoader object.
   *
   * @param string $root_path
   *   The root path.
   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
   *   The module handler service.
   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
   *   The theme handler service.
   */
  public function __construct($root_path, ModuleHandlerInterface $module_handler, ThemeHandlerInterface $theme_handler) {
    parent::__construct([], $root_path);
    // Add help_topics directories for modules and themes in the 'help_topic'
    // namespace, plus core.
    $this->addExtension($root_path . '/core');
    array_map([
      $this,
      'addExtension',
    ], $module_handler->getModuleDirectories());
    array_map([
      $this,
      'addExtension',
    ], $theme_handler->getThemeDirectories());
  }
  
  /**
   * Adds an extensions help_topics directory to the Twig loader.
   *
   * @param $path
   *   The path to the extension.
   */
  protected function addExtension($path) {
    $path .= DIRECTORY_SEPARATOR . 'help_topics';
    if (is_dir($path)) {
      $this->cache = $this->errorCache = [];
      $this->paths[self::MAIN_NAMESPACE][] = rtrim($path, '/\\');
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function getSourceContext(string $name) : Source {
    $path = $this->findTemplate($name);
    $contents = file_get_contents($path);
    try {
      // Note: always use \Drupal\Core\Serialization\Yaml here instead of the
      // "serializer.yaml" service. This allows the core serializer to utilize
      // core related functionality which isn't available as the standalone
      // component based serializer.
      $front_matter = new FrontMatter($contents, Yaml::class);
      // Reconstruct the content if there is front matter data detected. Prepend
      // the source with {% line \d+ %} to inform Twig that the source code
      // actually starts on a different line past the front matter data. This is
      // particularly useful when used in error reporting.
      if ($front_matter->getData() && ($line = $front_matter->getLine())) {
        $contents = "{% line {$line} %}" . $front_matter->getContent();
      }
    } catch (InvalidDataTypeException $e) {
      throw new LoaderError(sprintf('Malformed YAML in help topic "%s": %s.', $path, $e->getMessage()));
    }
    return new Source($contents, $name, $path);
  }
  
  /**
   * {@inheritdoc}
   */
  protected function findTemplate($name, $throw = TRUE) {
    if (!str_ends_with($name, '.html.twig')) {
      if (!$throw) {
        return NULL;
      }
      $extension = pathinfo($name, PATHINFO_EXTENSION);
      throw new LoaderError(sprintf("Help topic %s has an invalid file extension (%s). Only help topics ending .html.twig are allowed.", $name, $extension));
    }
    return parent::findTemplate($name, $throw);
  }

}