class TestFieldPrepareViewFormatter

class TestFieldPrepareViewFormatter extends FormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'test_formatter_setting_additional' => 'dummy test string',
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element['test_formatter_setting_additional'] = [
      '#title' => $this->t('Setting'),
      '#type' => 'textfield',
      '#size' => 20,
      '#default_value' => $this->getSetting('test_formatter_setting_additional'),
      '#required' => TRUE,
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [];
    $summary[] = $this->t('@setting: @value', [
      '@setting' => 'test_formatter_setting_additional',
      '@value' => $this->getSetting('test_formatter_setting_additional'),
    ]);
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function prepareView(array $entities_items) {
    foreach ($entities_items as $items) {
      foreach ($items as $item) {
        // Don't add anything on empty values.
        if (!$item->isEmpty()) {
          $item->additional_formatter_value = $item->value + 1;
        }
      }
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    foreach ($items as $delta => $item) {
      $elements[$delta] = [
        '#markup' => $this->getSetting('test_formatter_setting_additional') . '|' . $item->value . '|' . $item->additional_formatter_value,
      ];
    }
    return $elements;
  }

}