View Basics
iOS app的UI由View對象組成将塑，一個對象：
1. 是UIView或其子類的一個實(shí)例
2. 知道如何畫出本身
3. 可以處理事件棺禾，例如觸摸
4. 存在于一組視圖锐借，這些視圖的根視圖是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.
