Student

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

@implementation Student
@end

Student + Category

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>

////定义成常量  用C语言
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

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);