code: public function generate()
      {
          if (empty($this->_items)) {
              return '';
          }
          $result = [];
          $formOptions = [
              'options' => ['class' => ''], //form-horizontal
              'enableClientValidation' => false,
          ];
          if (Yii::$app->request->isAjax) {
              Html::addCssClass($formOptions['options'], 'ajax');
          }
  
          if ($this->hasFile() && !isset($formOptions['options']['enctype'])) {
              $formOptions['options']['enctype'] = 'multipart/form-data';
          }
  
          list($this->form, $formStartRow) = ActiveForm::begin($formOptions, false);
          $result[] = $formStartRow;
          // $result[] = Html::beginForm('', 'post', array('class' => $this->class));
          $result[] = Html::beginTag('div', ['class' => '']);
          foreach ($this->_items as $item) {
              $result[] = $item->generate();
          }
          //if (!Yii::$app->request->isAjax) {
              $result[] = Html::beginTag('div', ['class' => 'row form-group submit-group']);
          $result[] = Html::beginTag('div', ['class' => 'col-sm-12']);
          $result[] = Html::submitButton('Save', ['class' => 'btn btn-primary']);
          $result[] = Html::endTag('div');
          $result[] = Html::endTag('div');
          //}
          $result[] = Html::endTag('div');
          $result[] = ActiveForm::end(false);
  
          return implode("\n", $result);
      }
docs: [[@doctodo method_description:generate]].
  
  @return unknown