面试时碰到几次，类别中如何添加属性。说实话，平时这个几乎不用，也没怎么查过相关文档，对此不甚了解，也就知道匿名类别（即扩展）支持添加属性。

看了下别人写的关于类别中添加属性的博客，好多是利用runtime实现的，个人感觉这种方法也挺简单暴力的。

基本照着别人来的,有些地方稍微改了下：

1.新建一个Person类：

.h文件：

#import <Foundation/Foundation.h>

@interface Person : NSObject

@property (nonatomic, assign) NSInteger age;

- (void)test;

@end

.m文件:

#import "Person.h"

@implementation Person

- (void)test
{
    NSLog(@"----");
    NSLog(@"%s", __func__);
    NSLog(@"----");
    
}

@end

Person类的类别：

.h文件:

#import "Person.h"

@interface Person (Category)

@property (nonatomic, copy) NSString *name;

- (void)printInfo;

@end

.m文件:

#import "Person+Category.h"
#import <objc/runtime.h>


@implementation Person (Category)

//定义常量 必须是C语言字符串
static char *PersonNameKey = "PersonNameKey";

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

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

-(void)printInfo{
    NSLog(@"%s----%@-----%@",__func__,self, self.name);
    
    NSLog(@"-------------");
    
    NSLog(@"The file is %s\nThe date is %s\nThe time is %s\nThis is line %d\nThis function is %s", __FILE__, __DATE__, __TIME__, __LINE__, __func__);
}

控制器中调用：

#import "ViewController.h"
#import "Person.h"
#import "Person+Category.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    Person *person = [[Person alloc] init];
    person.age = 24;
    
    [person test];
    
    person.name = @"PersonTest";
    
    [person printInfo];
}

打印结果:

2016-06-23 10:40:28.794 Hello[1791:84826] ----
2016-06-23 10:40:28.795 Hello[1791:84826] -[Person test]
2016-06-23 10:40:28.795 Hello[1791:84826] ----
2016-06-23 10:40:28.795 Hello[1791:84826] -[Person(Category) printInfo]----<Person: 0x7ff128f202d0>-----PersonTest
2016-06-23 10:40:28.795 Hello[1791:84826] -------------
2016-06-23 10:40:28.795 Hello[1791:84826] The file is /Users/Desktop/Hello/Hello/Person+Category.m
The date is Jun 23 2016
The time is 10:40:24
This is line 47
This function is -[Person(Category) printInfo]

原文：http://www.jianshu.com/p/0ca0bbb0b200