public function convert($mode, $type, $data, $column = [], $options = [])
    {
        $formatter = null;
        $type = $data === null ? 'null' : $type;
        if (isset($this->_formatters[$mode][$type])) {
            $formatter = $this->_formatters[$mode][$type];
        } elseif (isset($this->_formatters[$mode]['_default_'])) {
            $formatter = $this->_formatters[$mode]['_default_'];
        }
        return $formatter ? $formatter($data, $column, $options) : $data;
    }