class StringFormatter

class StringFormatter extends FormatterBase {
  
  /**
   * The entity type manager.
   *
   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  protected $entityTypeManager;
  
  /**
   * Constructs a StringFormatter instance.
   *
   * @param string $plugin_id
   *   The plugin_id for the formatter.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
   *   The definition of the field to which the formatter is associated.
   * @param array $settings
   *   The formatter settings.
   * @param string $label
   *   The formatter label display setting.
   * @param string $view_mode
   *   The view mode.
   * @param array $third_party_settings
   *   Any third party settings.
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager.
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, EntityTypeManagerInterface $entity_type_manager) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings);
    $this->entityTypeManager = $entity_type_manager;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($plugin_id, $plugin_definition, $configuration['field_definition'], $configuration['settings'], $configuration['label'], $configuration['view_mode'], $configuration['third_party_settings'], $container->get('entity_type.manager'));
  }
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    $options = parent::defaultSettings();
    $options['link_to_entity'] = FALSE;
    return $options;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $form = parent::settingsForm($form, $form_state);
    $entity_type = $this->entityTypeManager
      ->getDefinition($this->fieldDefinition
      ->getTargetEntityTypeId());
    if ($entity_type->hasLinkTemplate('canonical')) {
      $form['link_to_entity'] = [
        '#type' => 'checkbox',
        '#title' => $this->t('Link to the @entity_label', [
          '@entity_label' => $entity_type->getLabel(),
        ]),
        '#default_value' => $this->getSetting('link_to_entity'),
      ];
    }
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [];
    if ($this->getSetting('link_to_entity')) {
      $entity_type = $this->entityTypeManager
        ->getDefinition($this->fieldDefinition
        ->getTargetEntityTypeId());
      if ($entity_type->hasLinkTemplate('canonical')) {
        $summary[] = $this->t('Linked to the @entity_label', [
          '@entity_label' => $entity_type->getLabel(),
        ]);
      }
    }
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    $url = NULL;
    $entity = $items->getEntity();
    $entity_type = $entity->getEntityType();
    if ($this->getSetting('link_to_entity') && !$entity->isNew() && $entity_type->hasLinkTemplate('canonical')) {
      $url = $this->getEntityUrl($entity);
    }
    foreach ($items as $delta => $item) {
      $view_value = $this->viewValue($item);
      if ($url) {
        $elements[$delta] = [
          '#type' => 'link',
          '#title' => $view_value,
          '#url' => $url,
        ];
      }
      else {
        $elements[$delta] = $view_value;
      }
    }
    return $elements;
  }
  
  /**
   * Generate the output appropriate for one field item.
   *
   * @param \Drupal\Core\Field\FieldItemInterface $item
   *   One field item.
   *
   * @return array
   *   The textual output generated as a render array.
   */
  protected function viewValue(FieldItemInterface $item) {
    // The text value has no text format assigned to it, so the user input
    // should equal the output, including newlines.
    return [
      '#type' => 'inline_template',
      '#template' => '{{ value|nl2br }}',
      '#context' => [
        'value' => $item->value,
      ],
    ];
  }
  
  /**
   * Gets the URI elements of the entity.
   *
   * @param \Drupal\Core\Entity\EntityInterface $entity
   *   The entity object.
   *
   * @return \Drupal\Core\Url
   *   The URI elements of the entity.
   */
  protected function getEntityUrl(EntityInterface $entity) {
    // For the default revision, the 'revision' link template falls back to
    // 'canonical'.
    // @see \Drupal\Core\Entity\Entity::toUrl()
    $rel = $entity->getEntityType()
      ->hasLinkTemplate('revision') ? 'revision' : 'canonical';
    return $entity->toUrl($rel);
  }

}