关联引用（Associative References）为现有类添加变量

能够模拟添加一个实例变量到一个已有的类中，能够添加存储到一个对象中而不需要改变类的定义。

1.通过分类给已有类添加属性

 @interface Person:NSObject
 @end

 @implementation Person
 @end

@interface Person(EmailAddress)
@property (nonatomic,readwrite,copy)NSString *emailAddress;
@end

@implement Person(EmailAddress)

static char emailAddressKey;

-(NSString*)emailAddress{
    return obj_getAssociatedObject(self,&emailAddressKey);//取出关联引用
}

-(void)setEmailAddress:(NSString*)emailAddress{
    objc_setAssociatedObject(self,&emailAddressKey,emailAddress,OBJC_ASSOCIATION_COPY);//设置关联引用
}

@end

2.利用关联引用，给UIAlertView添加一个变量。

static char kRepresentedObject;

NSString *addThing = "添加的东西";
UIAlertView *alert = [UIAlertView alloc]initWithTitile:@"Alert" message:nil delegate:self cancelButtonTitle:@"ok" otherButtonTitle:nil];

obj_setAssociatedObject(alert,&kRepresentedObject,addThing,OBJC_ASSOCIATION_RETAIN_NONATOMIC);//将addThing添加到alert

[alert show];


- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex{
    NSString *addThing = objc_getAssociatedObject(alertView,&kRepresentedObject);  //取出
}