public function performReadonlyTransformation()
    {
        /** @var TreeMultiselectField_Readonly $copy */
        $copy = $this->castedCopy(TreeMultiselectField_Readonly::class);
        $copy->setKeyField($this->getKeyField());
        $copy->setLabelField($this->getLabelField());
        $copy->setSourceObject($this->getSourceObject());
        $copy->setTitleField($this->getTitleField());
        return $copy;
    }