protected function addClass($addClass) {
        $targetClass = $this->getAttrib('class');
        if(!empty($targetClass) && (strpos($targetClass, $addClass) === false)) {
            $targetClass .= " {$addClass}";
        } else {
            $targetClass = $addClass;
        }
        $this->setAttrib('class', $targetClass);
        return $this;
    }