class TestFieldEmptySettingFormatter

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

}