View Basics
iOS app的UI由View对象组成,一个对象:
1. 是UIView或其子类的一个实例
2. 知道如何画出本身
3. 可以处理事件,例如触摸
4. 存在于一组视图,这些视图的根视图是app的window
View drawing occurs on an as-needed basis. When a view is first shown, or when all or part of it becomes visible due to layout changes, the system asks the view to draw its contents.