AuthorFormatter.php

<?php

namespace Drupal\user\Plugin\Field\FieldFormatter;

use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase;

/**
 * Plugin implementation of the 'author' formatter.
 *
 * @FieldFormatter(
 *   id = "author",
 *   label = @Translation("Author"),
 *   description = @Translation("Display the referenced author user entity."),
 *   field_types = {
 *     "entity_reference"
 *   }
 * )
 */
class AuthorFormatter extends EntityReferenceFormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($this->getEntitiesToView($items, $langcode) as $delta => $entity) {
            
            /** @var $referenced_user \Drupal\user\UserInterface */
            $elements[$delta] = [
                '#theme' => 'username',
                '#account' => $entity,
                '#link_options' => [
                    'attributes' => [
                        'rel' => 'author',
                    ],
                ],
                '#cache' => [
                    'tags' => $entity->getCacheTags(),
                ],
            ];
        }
        return $elements;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function isApplicable(FieldDefinitionInterface $field_definition) {
        return $field_definition->getFieldStorageDefinition()
            ->getSetting('target_type') == 'user';
    }
    
    /**
     * {@inheritdoc}
     */
    protected function checkAccess(EntityInterface $entity) {
        return $entity->access('view label', NULL, TRUE);
    }

}