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

1  @interface Person:NSObject

2  @end

3 

4  @implementation Person

5  @end

 1 @interface Person(EmailAddress)

 2 @property (nonatomic,readwrite,copy)NSString *emailAddress;

 3 @end

 4 

 5 @implement Person(EmailAddress)

 6 

 7 static char emailAddressKey;

 8 

 9 -(NSString*)emailAddress{

10     return obj_getAssociatedObject(self,&emailAddressKey);//取出关联引用

11 }

12 

13 -(void)setEmailAddress:(NSString*)emailAddress{

14     objc_setAssociatedObject(self,&emailAddressKey,emailAddress,OBJC_ASSOCIATION_COPY);//设置关联引用

15 }

16 

17 @end

2.利用关联引用，给UIAlertView添加一个变量。

 1 static char kRepresentedObject;

 2 

 3 NSString *addThing = "添加的东西";

 4 UIAlertView *alert = [UIAlertView alloc]initWithTitile:@"Alert" message:nil delegate:self cancelButtonTitle:@"ok" otherButtonTitle:nil];

 5 

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

 7 

 8 [alert show];

 9 

10 

11 - (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex{

12     NSString *addThing = objc_getAssociatedObject(alertView,&kRepresentedObject);  //取出

13 }