利用运行时给分类添加属性

1、在.h文件里：

#import <Foundation/Foundation.h>

@interface NSObject (Property)

@property (nonatomic, strong) id propretyName;

@end

2、在.m文件里

#import "NSObject+Property.h"
#import <objc/runtime.h>

@implementation NSObject (Property)

- (id)propretyName
{
    return objc_getAssociatedObject(self, @selector(propretyName));
}

- (void)setPropretyName:(id)propretyName
{
    objc_setAssociatedObject(self, @selector(propretyName), propretyName, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}


@end