public function getVersion($of = 'atk')
    {
        // TODO: get version of add-on
        if (!$this->version_cache) {
            $f = $this->app->pathfinder->atk_location->base_path.DIRECTORY_SEPARATOR.'VERSION';
            if (file_exists($f)) {
                $this->version_cache = trim(file_get_contents($f));
            } else {
                $this->version_cache = '4.0.1';
            }
        }

        return $this->version_cache;
    }