class TestFieldDefaultFormatter

class TestFieldDefaultFormatter extends FormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'test_formatter_setting' => 'dummy test string',
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element['test_formatter_setting'] = [
      '#title' => $this->t('Setting'),
      '#type' => 'textfield',
      '#size' => 20,
      '#default_value' => $this->getSetting('test_formatter_setting'),
      '#required' => TRUE,
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [];
    $summary[] = $this->t('@setting: @value', [
      '@setting' => 'test_formatter_setting',
      '@value' => $this->getSetting('test_formatter_setting'),
    ]);
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    foreach ($items as $delta => $item) {
      $elements[$delta] = [
        '#markup' => $this->getSetting('test_formatter_setting') . '|' . $item->value,
      ];
    }
    return $elements;
  }

}