UserNameFormatter.php

<?php

namespace Drupal\user\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\Attribute\FieldFormatter;
use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\FormatterBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Plugin implementation of the 'user_name' formatter.
 */
class UserNameFormatter extends FormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    $options = parent::defaultSettings();
    $options['link_to_entity'] = TRUE;
    return $options;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $form = parent::settingsForm($form, $form_state);
    $form['link_to_entity'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Link to the user'),
      '#default_value' => $this->getSetting('link_to_entity'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    foreach ($items as $delta => $item) {
      /** @var \Drupal\user\UserInterface $user */
      if ($user = $item->getEntity()) {
        if ($this->getSetting('link_to_entity')) {
          $elements[$delta] = [
            '#theme' => 'username',
            '#account' => $user,
            '#link_options' => [
              'attributes' => [
                'rel' => 'user',
              ],
            ],
            '#cache' => [
              'tags' => $user->getCacheTags(),
            ],
          ];
        }
        else {
          $elements[$delta] = [
            '#markup' => $user->getDisplayName(),
            '#cache' => [
              'tags' => $user->getCacheTags(),
            ],
          ];
        }
      }
    }
    return $elements;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    return $field_definition->getTargetEntityTypeId() === 'user' && $field_definition->getName() === 'name';
  }

}