code: public function render(ElementInterface $element, $buttonContent = null)
      {
          if (null === $buttonContent) {
              $buttonContent = $element->getLabel();
              if (null === $buttonContent) {
                  throw new Exception\DomainException(
                      sprintf(
                          '%s expects either button content as the second argument, ' .
                          'or that the element provided has a label value; neither found',
                          __METHOD__
                      )
                  );
              }
  
              if (null !== ($translator = $this->getTranslator())) {
                  $buttonContent = $translator->translate(
                      $buttonContent,
                      $this->getTranslatorTextDomain()
                  );
              }
              $element->setLabel('');
          }
  
          $value          = $element->getValue();
          $checkedBoole   = ($value == 1 || $value == 'on');
          
  
          $checkedClass   = $checkedBoole?'active"':'';
  
          $hiddenElement = '';
          if ($element->useHiddenElement()) {
              $hiddenAttributes = [
                  'name'     => $element->getName(),
                  'value'    => $element->getUncheckedValue(),
              ];
  
              $hiddenElement = sprintf(
                          '<input type="hidden" %s%s',
                          $this->createAttributesString($hiddenAttributes),
                          '>'
                      );
              $element->setUseHiddenElement(false);
          }
  
          $buttonContent = $hiddenElement . PHP_EOL
                  . '<div class="btn-group" data-toggle="buttons">' . PHP_EOL
                  . '<label class="btn btn-default ' . $checkedClass . '">' . PHP_EOL
                  . parent::render($element) . $buttonContent . PHP_EOL . '</label>' . PHP_EOL
                  . '</div>' . PHP_EOL;
  
          return $buttonContent;
      }
docs: Render a form <input> element from the provided $element
  
  @param  ElementInterface $element
  @param String $buttonContent
  @throws Exception\DomainException
  @return string