https://soooprmx.com/cocoa-%ed%82%a4-%eb%b0%b8%eb%a5%98-%ec%98%b5%ec%a0%b8%eb%b9%99-%ea%b0%84%eb%8b%a8-%ec%98%88%ec%a0%9c/
[cocoa] 키-밸류 옵져빙 간단 예제