code: public function ajaxTreeView($strFolder, $level)
  	{
  		if (!Environment::get('isAjaxRequest'))
  		{
  			return '';
  		}
  
  		/** @var Session $objSession */
  		$objSession = System::getContainer()->get('session');
  
  		$blnClipboard = false;
  		$arrClipboard = $objSession->get('CLIPBOARD');
  
  		// Check clipboard
  		if (!empty($arrClipboard[$this->strTable]))
  		{
  			$blnClipboard = true;
  			$arrClipboard = $arrClipboard[$this->strTable];
  		}
  
  		$this->import(Files::class, 'Files');
  		$this->import(BackendUser::class, 'User');
  
  		return $this->generateTree($this->strRootDir.'/'.$strFolder, ($level * 20), false, $this->isProtectedPath($strFolder), ($blnClipboard ? $arrClipboard : false));
  	}
docs: Generate a particular subpart of the tree and return it as HTML string
  
  @param string  $strFolder
  @param integer $level
  
  @return string