Category中的属性

#import <objc/runtime.h>

static char UIScrollViewSpringHeadView;

@implementation UIScrollView (SpringHeadView)

- (void)setTopView:(UIView *)topView{

    objc_setAssociatedObject(self, &UIScrollViewSpringHeadView,

                             topView,

                             OBJC_ASSOCIATION_ASSIGN);

}

- (UIView *)topView{

    returnobjc_getAssociatedObject(self, &UIScrollViewSpringHeadView);

}