Implements Drupal\field\Plugin\Type\Formatter\FormatterInterface::settingsForm().
Overrides FormatterBase::settingsForm
public function settingsForm(array $form, array &$form_state) {
$element['test_formatter_setting_additional'] = array(
'#title' => t('Setting'),
'#type' => 'textfield',
'#size' => 20,
'#default_value' => $this
->getSetting('test_formatter_setting_additional'),
'#required' => TRUE,
);
return $element;
}