class TestFieldWidgetMultiple

class TestFieldWidgetMultiple extends WidgetBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'test_widget_setting_multiple' => 'dummy test string',
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element['test_widget_setting_multiple'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Field test field widget setting'),
      '#description' => $this->t('A dummy form element to simulate field widget setting.'),
      '#default_value' => $this->getSetting('test_widget_setting_multiple'),
      '#required' => FALSE,
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [];
    $summary[] = $this->t('@setting: @value', [
      '@setting' => 'test_widget_setting_multiple',
      '@value' => $this->getSetting('test_widget_setting_multiple'),
    ]);
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $values = [];
    foreach ($items as $item) {
      $values[] = $item->value;
    }
    $element += [
      '#type' => 'textfield',
      '#default_value' => implode(', ', $values),
      '#element_validate' => [
        [
          static::class,
          'multipleValidate',
        ],
      ],
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function errorElement(array $element, ConstraintViolationInterface $violation, array $form, FormStateInterface $form_state) {
    return $element;
  }
  
  /**
   * Element validation helper.
   */
  public static function multipleValidate($element, FormStateInterface $form_state) {
    $values = array_map('trim', explode(',', $element['#value']));
    $items = [];
    foreach ($values as $value) {
      $items[] = [
        'value' => $value,
      ];
    }
    $form_state->setValueForElement($element, $items);
  }
  
  /**
   * Test is the widget is applicable to the field definition.
   *
   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
   *   The field definition that should be checked.
   *
   * @return bool
   *   TRUE if the machine name of the field is not equals to
   *   field_onewidgetfield, FALSE otherwise.
   *
   * @see \Drupal\Tests\field\Functional\EntityReference\EntityReferenceAdminTest::testAvailableFormatters
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    // Returns FALSE if machine name of the field equals field_onewidgetfield.
    return $field_definition->getName() != "field_onewidgetfield";
  }

}