运行时机制之为分类扩充成员变量

#import "UIImageView+Extersion.h"
#import <objc/message.h>

@interface UIImageView (_Extersion)
@property (nonatomic, assign) double height;
@end

@implementation UIImageView (Extersion)

//分类扩充成员变量
static double HeightKey; //用来保存和取东西的

- (void)setHeight:(double)height
{
    //将变量存起来
    objc_setAssociatedObject(self, &height, @(height), OBJC_ASSOCIATION_ASSIGN);
}

- (double)height
{
    //取出变量
    return [objc_getAssociatedObject(self, &HeightKey) doubleValue];
}

@end