使用运行时我们需要导入#import <objc/runtime.h>
我们看下面代码，也就是用C实现简单的set和get 方法

//
//  NSObject+CategoryWithProperty.h
//  RunTimeTest
//
//  Created by plee on 15/10/8.
//  Copyright © 2015年 franklee. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <objc/runtime.h>
@interface NSObject (CategoryWithProperty)
@property (nonatomic,strong) NSObject * property;

@end

//
//  NSObject+CategoryWithProperty.m
//  RunTimeTest
//
//  Created by plee on 15/10/8.
//  Copyright © 2015年 franklee. All rights reserved.
//

#import "NSObject+CategoryWithProperty.h"

@implementation NSObject (CategoryWithProperty)

- (NSObject *)property{
    return objc_getAssociatedObject(self, @selector(property));
}
- (void)setProperty:(NSObject *)property{
    objc_setAssociatedObject(self, @selector(property), property, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
}
@end

//
//  ViewController.m
//  RunTimeTest
//
//  Created by plee on 15/10/8.
//  Copyright © 2015年 franklee. All rights reserved.
//

#import "ViewController.h"
#import "NSObject+CategoryWithProperty.h"
@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSObject * object = [[NSObject alloc] init];
    object.property = @"123";
    NSLog(@"%@",object.property);
    
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

• objc_getAssociatedObject 相当于iOS内部的get方法
  OBJC_EXPORT id objc_getAssociatedObject(id object, const void *key)
  __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

1. id object : 关联对象
2. const void *key: 我们需要定义一个常量void * 类型的key

• objc_setAssociatedObject 相当于iOS的属性的set方法

OBJC_EXPORT void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
__OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

1.id object :被关联的对象
2.const void *key ：char 类型的key
3.id value :要被赋值的值
4.objc_AssociationPolicy policy :runtime内存管理有一下集几种

  OBJC_ASSOCIATION_ASSIGN = 0,           /**< Specifies a weak reference to the associated object. */
  OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object. 
                                          *   The association is not made atomically. */
  OBJC_ASSOCIATION_COPY_NONATOMIC = 3,   /**< Specifies that the associated object is copied. 
                                          *   The association is not made atomically. */
  OBJC_ASSOCIATION_RETAIN = 01401,       /**< Specifies a strong reference to the associated object.
                                          *   The association is made atomically. */
  OBJC_ASSOCIATION_COPY = 01403     

• OBJC_EXPORT void objc_removeAssociatedObjects(id object)
  __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);
  id object: 移除关联对象，用法不是很难，但是在使用中最好是合理使用，