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;
    }

}