protected function validateConfig()
    {
        $defaults = [
            'tpl_folder',
            'cache_folder',
            'tpl_file_format',
            //'compress',
            //'event_emitter',
        ];

        foreach ($defaults as $key) {
            if (! isset($this->config[$key])) {
                throw new InvalidConfigException(sprintf("Missing a configuration key (%s)", $key));
            }
        }
    }