View source  
  <?php
namespace Drupal\Core\Field\Plugin\Field\FieldWidget;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;
class StringTextfieldWidget extends WidgetBase {
  
  public static function defaultSettings() {
    return array(
      'size' => 60,
      'placeholder' => '',
    ) + parent::defaultSettings();
  }
  
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element['size'] = array(
      '#type' => 'number',
      '#title' => t('Size of textfield'),
      '#default_value' => $this
        ->getSetting('size'),
      '#required' => TRUE,
      '#min' => 1,
    );
    $element['placeholder'] = array(
      '#type' => 'textfield',
      '#title' => t('Placeholder'),
      '#default_value' => $this
        ->getSetting('placeholder'),
      '#description' => t('Text that will be shown inside the field until a value is entered. This hint is usually a sample value or a brief description of the expected format.'),
    );
    return $element;
  }
  
  public function settingsSummary() {
    $summary = array();
    $summary[] = t('Textfield size: @size', array(
      '@size' => $this
        ->getSetting('size'),
    ));
    $placeholder = $this
      ->getSetting('placeholder');
    if (!empty($placeholder)) {
      $summary[] = t('Placeholder: @placeholder', array(
        '@placeholder' => $placeholder,
      ));
    }
    return $summary;
  }
  
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element['value'] = $element + array(
      '#type' => 'textfield',
      '#default_value' => isset($items[$delta]->value) ? $items[$delta]->value : NULL,
      '#size' => $this
        ->getSetting('size'),
      '#placeholder' => $this
        ->getSetting('placeholder'),
      '#maxlength' => $this
        ->getFieldSetting('max_length'),
      '#attributes' => array(
        'class' => array(
          'js-text-full',
          'text-full',
        ),
      ),
    );
    return $element;
  }
}