测试给UIlabel添加一个str属性
分类的.h文件

#import <UIKit/UIKit.h>

#import <objc/runtime.h>

@interface UILabel (NSObject)

@property (nonatomic,copy) NSString *str;

@end

分类的.m文件

#import "UILabel+NSObject.h"

static const NSString *strKey = @"strKey";

@implementation UILabel (NSObject)


- (void)setStr:(NSString *)str {
    
    self.text = str;
    objc_setAssociatedObject(self, &strKey, str, OBJC_ASSOCIATION_COPY);
}
- (NSString *)str {
    return objc_getAssociatedObject(self, &strKey);
}

@end

在VC中测试：

label.str = @"sss";