iOS 开发 -《Effective Objective-C 2

枚举NS_OPTIONS和NS_ENUM的区别
代码的表现形式上

typedef NS_ENUM(NSInteger, UIViewAnimationTransition) {
    UIViewAnimationTransitionNone,
    UIViewAnimationTransitionFlipFromLeft,
    UIViewAnimationTransitionFlipFromRight,
    UIViewAnimationTransitionCurlUp,
    UIViewAnimationTransitionCurlDown,
};

typedef NS_OPTIONS(NSUInteger, UISwipeGestureRecognizerDirection) {
    UISwipeGestureRecognizerDirectionNone = 0,  //值为0
    UISwipeGestureRecognizerDirectionRight = 1 << 0,  //值为2的0次方
    UISwipeGestureRecognizerDirectionLeft = 1 << 1,  //值为2的1次方
    UISwipeGestureRecognizerDirectionUp = 1 << 2,  //值为2的2次方
    UISwipeGestureRecognizerDirectionDown = 1 << 3  //值为2的3次方
};

NS_ENUM这种声明出来的东西大部分是单选. NS_OPTIONS声明出来的大部分是多选.

像UIViewAnimationTransition这种在用的时候肯定是只能选一种效果,你要么从左翻到右,要么从右翻到左,你做动画的时候总不能同一时刻让他同时从左到右,又从右到左翻,对吧.

而UIViewAutosizing就不一样了.我要是让子view的宽高和父View一样,那么autoviewsizing的选项肯定是类似于这种.UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight