View Controllers 是APP内在架构的基础，总体来讲一个APP的View Controllers 主要由以下两种设计而成:
There are two types of view controllers:
Content view controllers manage a discrete piece of your app’s content and are the main type of view controller that you create.
Container view controllers* collect information from other view controllers (known as child view controllers) and present it in a way that facilitates navigation or presents the content of those view controllers differently.
Most apps are a mixture of both types of view controllers.
第一种Content view controllers，上下文控制器，我喜欢叫导航控制器，它维护着一个视图控制器栈，通过push或者pop进行管理。
第二种Container view controllers，容器控制器，如tabbar view controller，本身作为parent view controller，拥有一个childViewControllers属性（NSArray* & read-only）。可以在 View Controller Programming Guide for iOS 了解详情。
Container view controllers功能很强大，我们可以利用它来自定义控制器，制作出如下所示的控制器效果：
