iOS 结构体快速转为NSValue对象

objc_boxable
OC可能你经常会看到@(100)等用法。不用奇怪，就是这个Function attributes
使用示例：

struct __attribute__((objc_boxable)) some_struct {
  int i;
};
union __attribute__((objc_boxable)) some_union {
  int i;
  float f;
};
typedef struct __attribute__((objc_boxable)) _some_struct some_struct;

some_struct ss;
NSValue *boxed = @(ss);

NSValue转为自定义结构体：

struct __attribute__((objc_boxable)) MapPoint {
    int x;
    int y;
};

@interface NSValue(MapPoint)

- (struct MapPoint)mappointValue;

@end

@implementation NSValue(MapPoint)

- (struct MapPoint)mappointValue {
    struct MapPoint point;
    [self getValue:&point];
    return point;
}

@end