概述
在iOS 6之前，可以使用UIView的autoresizingMask属性进行自动适配，autoresizingMask是个枚举UIViewAutoresizing 。
typedef NS_OPTIONS(NSUInteger, UIViewAutoresizing) {
UIViewAutoresizingNone = 0,
UIViewAutoresizingFlexibleLeftMargin = 1 << 0,
UIViewAutoresizingFlexibleWidth = 1 << 1,
UIViewAutoresizingFlexibleRightMa...