class BaseFieldFileFormatterBase

abstract class BaseFieldFileFormatterBase extends FormatterBase {
  
  /**
   * The file URL generator.
   *
   * @var \Drupal\Core\File\FileUrlGeneratorInterface
   */
  protected $fileUrlGenerator;
  
  /**
   * Constructs a BaseFieldFileFormatterBase object.
   *
   * @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\File\FileUrlGeneratorInterface $file_url_generator
   *   The file URL generator.
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, FileUrlGeneratorInterface $file_url_generator) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings);
    $this->fileUrlGenerator = $file_url_generator;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    $settings['link_to_file'] = FALSE;
    return $settings;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $form = parent::settingsForm($form, $form_state);
    $form['link_to_file'] = [
      '#title' => $this->t('Link this field to the file download URL'),
      '#type' => 'checkbox',
      '#default_value' => $this->getSetting('link_to_file'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    $url = NULL;
    // Add support to link to the entity itself.
    if ($this->getSetting('link_to_file')) {
      $url = $this->fileUrlGenerator
        ->generate($items->getEntity()
        ->getFileUri());
    }
    foreach ($items as $delta => $item) {
      $view_value = $this->viewValue($item);
      if ($url) {
        $elements[$delta] = [
          '#type' => 'link',
          '#title' => $view_value,
          '#url' => $url,
        ];
      }
      else {
        $elements[$delta] = is_array($view_value) ? $view_value : [
          '#markup' => $view_value,
        ];
      }
    }
    return $elements;
  }
  
  /**
   * Generate the output appropriate for one field item.
   *
   * @param \Drupal\Core\Field\FieldItemInterface $item
   *   One field item.
   *
   * @return mixed
   *   The textual output generated.
   */
  abstract protected function viewValue(FieldItemInterface $item);
  
  /**
   * {@inheritdoc}
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    return $field_definition->getTargetEntityTypeId() === 'file';
  }

}