https://purengom.com/2011/06/15/ios-%ec%95%b1%ea%b3%bc-%ec%95%88%eb%93%9c%eb%a1%9c%ec%9d%b4%eb%93%9c-%ec%95%b1%ec%9d%84-%ec%93%b0%eb%8b%a4%ea%b0%80-%eb%8a%90%eb%82%80%ec%a0%90%ec%97%90-%ea%b4%80%ed%95%98%ec%97%ac/