class HelpTopicPluginController

class HelpTopicPluginController extends ControllerBase {
    
    /**
     * The renderer service.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected $renderer;
    
    /**
     * The Help Topic plugin manager.
     *
     * @var \Drupal\help_topics\HelpTopicPluginManagerInterface
     */
    protected $helpTopicPluginManager;
    
    /**
     * Constructs a HelpTopicPluginController object.
     *
     * @param \Drupal\help_topics\HelpTopicPluginManagerInterface $help_topic_plugin_manager
     *   The help topic plugin manager service.
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer service.
     */
    public function __construct(HelpTopicPluginManagerInterface $help_topic_plugin_manager, RendererInterface $renderer) {
        $this->helpTopicPluginManager = $help_topic_plugin_manager;
        $this->renderer = $renderer;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('plugin.manager.help_topic'), $container->get('renderer'));
    }
    
    /**
     * Displays a help topic page.
     *
     * @param string $id
     *   The plugin ID. Maps to the {id} placeholder in the
     *   help.help_topic route.
     *
     * @return array
     *   A render array with the contents of a help topic page.
     */
    public function viewHelpTopic($id) {
        $build = [];
        if (!$this->helpTopicPluginManager
            ->hasDefinition($id)) {
            throw new NotFoundHttpException();
        }
        
        /* @var \Drupal\help_topics\HelpTopicPluginInterface $help_topic */
        $help_topic = $this->helpTopicPluginManager
            ->createInstance($id);
        $build['#body'] = $help_topic->getBody();
        $this->renderer
            ->addCacheableDependency($build, $help_topic);
        // Build the related topics section, starting with the list this topic
        // says are related.
        $links = [];
        $related = $help_topic->getRelated();
        foreach ($related as $other_id) {
            if ($other_id !== $id) {
                
                /** @var \Drupal\help_topics\HelpTopicPluginInterface $topic */
                $topic = $this->helpTopicPluginManager
                    ->createInstance($other_id);
                $links[$other_id] = [
                    'title' => $topic->getLabel(),
                    'url' => Url::fromRoute('help.help_topic', [
                        'id' => $other_id,
                    ]),
                ];
                $this->renderer
                    ->addCacheableDependency($build, $topic);
            }
        }
        if (count($links)) {
            uasort($links, [
                SortArray::class,
                'sortByTitleElement',
            ]);
            $build['#related'] = [
                '#theme' => 'links__related',
                '#heading' => [
                    'text' => $this->t('Related topics'),
                    'level' => 'h2',
                ],
                '#links' => $links,
            ];
        }
        $build['#theme'] = 'help_topic';
        $build['#title'] = $help_topic->getLabel();
        return $build;
    }

}