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.
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