A -> B | B -> A
在A中hidesBottomBarWhenPushed = YES，到B了TabBar就隐藏了。
当我从B Pop回来的时候，在ViewWillDisappear中hidesBottomBarWhenPushed = YES，Tabbar还是隐藏了的，怎么破？怎么让回来后Tabbar显示出来？
经摸索和总结，出了一篇文，供参考：http://blog..com/isaced/1190000000327529
在viewWillAppear和viewWillDisappear的时候设置
呃..竟然是这么久的坑了..今天自己遇到了就来填一下好了, 根据文档的介绍, hidesBottomBarWhenPushed
就是 A Boolean value indicating whether the toolbar at the bottom of the screen is hidden when the view controller is pushed on to a navigation controller. The value of this property on the topmost view controller determines whether the toolbar is visible. If the value of this property is true, the toolbar is hidden. If the value of this property is false, the bar is visible. 重点在于topmost, 当你A压B的时候, B将成为topmost view controller, 将B的hidesBottomBarWhenPushed设为true才是正解.