code: public function FieldHolder($attributes = array ())
      {
          if ($this->shouldDisplay()) {
              $this->setContent($this->getButtonHTML());
              return parent::FieldHolder($attributes);
          }
      }
docs: Generates the button. Updates the literal field with the correct HTML
  based on any post-contruct updates
  
  @param array $attributes
  @return SSViewer