UITabBarController在我們的開發中經常被用到。關於它的使用，有幾個須要注意的地方：
1、UITabBarController會抗拒和別的視圖控制器組合，所以最好只將UITabBarController做爲整個App的根視圖。
When deploying a tab bar interface, you must install this view as the root of your window. Unlike other view controllers, a tab bar interface should never be installed as a child of another view controller.
[Apple]
蘋果的官方文檔裏要求我們將UITabBarController實例做爲整個應用的“根”，這是最爲被蘋果接受的用法。
參考:http://blog.zhaojie.me/2010/12/iphone-composition-resistant-uitabbarcontroller.html
http://stackoverflow.com/questions/8929462/uinavigationcontroller-pushing-uitabbarcontroller-only-displays-one-tab
2、最好不要去子類化UITabBarController。
This class is not intended for subclassing. Instead, you use instances of it as-is to present an interface that allows the user to choose between different modes of operation. [Apple]
蘋果建議我們最好不要去子類化它，這個類主要被設計用來管理多個view controller並且展示多個view。
3、UITabBarController的viewControllers數組中不要出現相同的指針，標籤欄控制器不會爲相同的指針創建有效的標籤。
Because selecting a tab replaces the contents of the interface, the type of interface managed in each tab need not be similar in any way. [Apple]
