一 定义一个类

Student.h文件

@interface Student : NSObject
@property (nonatomic,assign) NSInteger age;
@end

Student.m文件

@implementation Student
@end

二 定义分类

import "Student.h"
@interface Student (Extention)
@property (nonatomic,copy) NSString *name;
@property (nonatomic,assign) NSInteger height;
@end

import "Student+Extention.h"
import <objc/runtime.h>

static char *nameKey = "nameKey";
static char *heightKey = "heightKey";

@implementation Student (Extention)

- (void)setName:(NSString*)name{
    objc_setAssociatedObject(self, nameKey, name, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

- (NSString *)name{
    return objc_getAssociatedObject(self, nameKey);
}

- (void)setHeight:(NSInteger)height{
  objc_setAssociatedObject(self, heightKey, @(height), OBJC_ASSOCIATION_ASSIGN);
}

- (NSInteger)height{
  return [objc_getAssociatedObject(self, heightKey) integerValue];
}

@end

三 说明

/*

OBJC_ASSOCIATION_ASSIGN;            //assign策略

OBJC_ASSOCIATION_COPY_NONATOMIC;    //copy策略

OBJC_ASSOCIATION_RETAIN_NONATOMIC;  // retain策略

OBJC_ASSOCIATION_RETAIN;

OBJC_ASSOCIATION_COPY;

*/

/*

* id object 给哪个对象的属性赋值

const void *key 属性对应的key

id value  设置属性值为value

objc_AssociationPolicy policy  使用的策略，是一个枚举值，和copy，retain，assign是一样的，手机开发一般都选择NONATOMIC

objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy);

*/

四 注意事项

需包含 #import <objc/runtime.h> 头文件