AuthorFormatter.php

<?php

namespace Drupal\user\Plugin\Field\FieldFormatter;

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

/**
 * Plugin implementation of the 'author' formatter.
 */
class AuthorFormatter extends EntityReferenceFormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    foreach ($this->getEntitiesToView($items, $langcode) as $delta => $entity) {
      $elements[$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);
  }

}