class BlockContentController

class BlockContentController extends ControllerBase {
    
    /**
     * The custom block storage.
     *
     * @var \Drupal\Core\Entity\EntityStorageInterface
     */
    protected $blockContentStorage;
    
    /**
     * The custom block type storage.
     *
     * @var \Drupal\Core\Entity\EntityStorageInterface
     */
    protected $blockContentTypeStorage;
    
    /**
     * The theme handler.
     *
     * @var \Drupal\Core\Extension\ThemeHandlerInterface
     */
    protected $themeHandler;
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        $entity_type_manager = $container->get('entity_type.manager');
        return new static($entity_type_manager->getStorage('block_content'), $entity_type_manager->getStorage('block_content_type'), $container->get('theme_handler'));
    }
    
    /**
     * Constructs a BlockContent object.
     *
     * @param \Drupal\Core\Entity\EntityStorageInterface $block_content_storage
     *   The custom block storage.
     * @param \Drupal\Core\Entity\EntityStorageInterface $block_content_type_storage
     *   The custom block type storage.
     * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
     *   The theme handler.
     */
    public function __construct(EntityStorageInterface $block_content_storage, EntityStorageInterface $block_content_type_storage, ThemeHandlerInterface $theme_handler) {
        $this->blockContentStorage = $block_content_storage;
        $this->blockContentTypeStorage = $block_content_type_storage;
        $this->themeHandler = $theme_handler;
    }
    
    /**
     * Displays add custom block links for available types.
     *
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The current request object.
     *
     * @return array
     *   A render array for a list of the custom block types that can be added or
     *   if there is only one custom block type defined for the site, the function
     *   returns the custom block add page for that custom block type.
     */
    public function add(Request $request) {
        $types = $this->blockContentTypeStorage
            ->loadMultiple();
        if ($types && count($types) == 1) {
            $type = reset($types);
            return $this->addForm($type, $request);
        }
        if (count($types) === 0) {
            return [
                '#markup' => $this->t('You have not created any block types yet. Go to the <a href=":url">block type creation page</a> to add a new block type.', [
                    ':url' => Url::fromRoute('block_content.type_add')->toString(),
                ]),
            ];
        }
        return [
            '#theme' => 'block_content_add_list',
            '#content' => $types,
        ];
    }
    
    /**
     * Presents the custom block creation form.
     *
     * @param \Drupal\block_content\BlockContentTypeInterface $block_content_type
     *   The custom block type to add.
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The current request object.
     *
     * @return array
     *   A form array as expected by
     *   \Drupal\Core\Render\RendererInterface::render().
     */
    public function addForm(BlockContentTypeInterface $block_content_type, Request $request) {
        $block = $this->blockContentStorage
            ->create([
            'type' => $block_content_type->id(),
        ]);
        if (($theme = $request->query
            ->get('theme')) && in_array($theme, array_keys($this->themeHandler
            ->listInfo()))) {
            // We have navigated to this page from the block library and will keep track
            // of the theme for redirecting the user to the configuration page for the
            // newly created block in the given theme.
            $block->setTheme($theme);
        }
        return $this->entityFormBuilder()
            ->getForm($block);
    }
    
    /**
     * Provides the page title for this controller.
     *
     * @param \Drupal\block_content\BlockContentTypeInterface $block_content_type
     *   The custom block type being added.
     *
     * @return string
     *   The page title.
     */
    public function getAddFormTitle(BlockContentTypeInterface $block_content_type) {
        return $this->t('Add %type custom block', [
            '%type' => $block_content_type->label(),
        ]);
    }

}