1.iOS 7 出来的者三个属性
文档是这么描述它的作用的：
If the table contains variable height rows, it might be expensive to calculate
all their heights when the table loads. Using estimation allows you to defer
some of the cost of geometry calculation from load time to scrolling time.
设计的初衷是好的,UItableView继承UIScrollview, UIScrollView加载的时候contentSize
http://blog.sunnyxx.com/2015/05/17/cell-height-calculation/
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