BlockContentViewBuilder.php

<?php

namespace Drupal\block_content;

use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Entity\EntityViewBuilder;

/**
 * View builder handler for custom blocks.
 */
class BlockContentViewBuilder extends EntityViewBuilder {
    
    /**
     * {@inheritdoc}
     */
    public function view(EntityInterface $entity, $view_mode = 'full', $langcode = NULL) {
        return $this->viewMultiple([
            $entity,
        ], $view_mode, $langcode)[0];
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewMultiple(array $entities = [], $view_mode = 'full', $langcode = NULL) {
        $build_list = parent::viewMultiple($entities, $view_mode, $langcode);
        // Apply the buildMultiple() #pre_render callback immediately, to make
        // bubbling of attributes and contextual links to the actual block work.
        // @see \Drupal\block\BlockViewBuilder::buildBlock()
        unset($build_list['#pre_render'][0]);
        return $this->buildMultiple($build_list);
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getBuildDefaults(EntityInterface $entity, $view_mode) {
        $build = parent::getBuildDefaults($entity, $view_mode);
        // The custom block will be rendered in the wrapped block template already
        // and thus has no entity template itself.
        unset($build['#theme']);
        return $build;
    }

}