private function resolveMessageKey($key, $keyIndex, $callback, $prefix="")
    {
        if (is_array($key)) {
            $_prefix = $prefix ? $prefix.$keyIndex."." : $keyIndex.".";

            foreach ($key as $_index=>$_key) {
                $this->resolveMessageKey($_key, $_index, $callback, $_prefix);
            }

        } else {
            $callback($prefix.$key);
        }
    }