KVO：
https://blog.csdn.net/yahibo/article/details/93861479
KVC：
https://www.jianshu.com/p/3b3f206275a0?utm_campaign=maleskine&utm_content=note&utm_medium=seo_notes&utm_source=recommendation
https://blog.csdn.net/qq_18505715/article/details/80205796
https://www.imooc.com/article/285662
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