https://blog.wongcw.com/2021/09/27/signal%e5%8a%a0%e5%af%86%e6%b6%88%e6%81%af%e5%82%b3%e9%81%9e%e6%9c%8d%e5%8b%99%e9%81%ad%e9%81%87%e7%aa%81%e7%99%bc%e6%8a%80%e8%a1%93%e6%95%85%e9%9a%9c-%e5%ae%98%e6%96%b9%e6%ad%a3%e5%9c%a8%e5%84%98/