class TestFieldEmptyFormatter

class TestFieldEmptyFormatter extends FormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'test_empty_string' => '**EMPTY FIELD**',
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    if ($items->isEmpty()) {
      // For fields with no value, just add the configured "empty" value.
      $elements[0] = [
        '#markup' => $this->getSetting('test_empty_string'),
      ];
    }
    else {
      foreach ($items as $delta => $item) {
        // This formatter only needs to output raw for testing.
        $elements[$delta] = [
          '#markup' => $item->value,
        ];
      }
    }
    return $elements;
  }

}