class BooleanCheckboxWidget

class BooleanCheckboxWidget extends WidgetBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'display_label' => TRUE,
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element['display_label'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Use field label instead of the "On" label as the label.'),
      '#default_value' => $this->getSetting('display_label'),
      '#weight' => -1,
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [];
    $display_label = $this->getSetting('display_label');
    $summary[] = $this->t('Use field label: @display_label', [
      '@display_label' => $display_label ? $this->t('Yes') : $this->t('No'),
    ]);
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element['value'] = $element + [
      '#type' => 'checkbox',
      '#default_value' => !empty($items[0]->value),
    ];
    // Override the title from the incoming $element.
    if ($this->getSetting('display_label')) {
      $element['value']['#title'] = $this->fieldDefinition
        ->getLabel();
    }
    else {
      $element['value']['#title'] = $this->fieldDefinition
        ->getSetting('on_label');
    }
    return $element;
  }

}