class HelpTopicPluginManager

class HelpTopicPluginManager extends DefaultPluginManager implements HelpTopicPluginManagerInterface {
  
  /**
   * Provides default values for all help topic plugins.
   *
   * @var array
   */
  protected $defaults = [
    // The plugin ID.
'id' => '',
    // The title of the help topic plugin.
'label' => '',
    // Whether or not the topic should appear on the help topics list.
'top_level' => '',
    // List of related topic machine names.
'related' => [],
    // The class used to instantiate the plugin.
'class' => '',
  ];
  
  /**
   * The theme handler.
   *
   * @var \Drupal\Core\Extension\ThemeHandlerInterface
   */
  protected $themeHandler;
  
  /**
   * The app root.
   *
   * @var string
   */
  protected $root;
  
  /**
   * Constructs a new HelpTopicManager object.
   *
   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
   *   The module handler.
   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
   *   The theme handler.
   * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
   *   Cache backend instance to use.
   * @param string $root
   *   The app root.
   */
  public function __construct(ModuleHandlerInterface $module_handler, ThemeHandlerInterface $theme_handler, CacheBackendInterface $cache_backend, $root) {
    // Note that the parent construct is not called because this not use
    // annotated class discovery.
    $this->moduleHandler = $module_handler;
    $this->themeHandler = $theme_handler;
    $this->alterInfo('help_topics_info');
    // Use the 'config:core.extension' cache tag so the plugin cache is
    // invalidated on theme install and uninstall.
    $this->setCacheBackend($cache_backend, 'help_topics', [
      'config:core.extension',
    ]);
    $this->root = (string) $root;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getDiscovery() {
    if (!isset($this->discovery)) {
      $module_directories = $this->moduleHandler
        ->getModuleDirectories();
      $all_directories = array_merge([
        'core' => $this->root . '/core',
      ], $module_directories, $this->themeHandler
        ->getThemeDirectories());
      // Search for Twig help topics in subdirectory help_topics, under
      // modules/profiles, themes, and the core directory.
      $all_directories = array_map(function ($dir) {
        return [
          $dir . '/help_topics',
        ];
      }, $all_directories);
      $discovery = new HelpTopicDiscovery($all_directories);
      // Also allow modules/profiles to extend help topic discovery to their
      // own plugins and derivers, in mymodule.help_topics.yml files.
      $discovery = new YamlDiscoveryDecorator($discovery, 'help_topics', $module_directories);
      $discovery = new ContainerDerivativeDiscoveryDecorator($discovery);
      $this->discovery = $discovery;
    }
    return $this->discovery;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function providerExists($provider) {
    return $this->moduleHandler
      ->moduleExists($provider) || $this->themeHandler
      ->themeExists($provider);
  }
  
  /**
   * {@inheritdoc}
   */
  protected function findDefinitions() {
    $definitions = parent::findDefinitions();
    // At this point the plugin list only contains valid plugins. Ensure all
    // related plugins exist and the relationship is bi-directional. This
    // ensures topics are listed on their related topics.
    foreach ($definitions as $plugin_id => $plugin_definition) {
      foreach ($plugin_definition['related'] as $key => $related_id) {
        // If the related help topic does not exist it might be for a module
        // that is not installed. Remove it.
        // @todo Discuss this more as this could cause silent errors but it
        //   offers useful functionality to relate to help topic provided by
        //   extensions that are yet to be installed.
        if (!isset($definitions[$related_id])) {
          unset($definitions[$plugin_id]['related'][$key]);
          continue;
        }
        // Make the related relationship bi-directional.
        if (isset($definitions[$related_id]) && !in_array($plugin_id, $definitions[$related_id]['related'], TRUE)) {
          $definitions[$related_id]['related'][] = $plugin_id;
        }
      }
    }
    return $definitions;
  }

}