为类添加实例变量 (Associative References)

在object-c中我们知道可以使用categories来为扩展类方法(比如我们可以为系统的类添加自己的方法)
例如：我们要想在每个NSString前面添加一个

@interface NSString ( CategoryName )

// method declarations - (NSString *) getNSString;

@end

@implementation NSString ( CategoryName )

// method definitions - (NSString *)getNSString { return [NSString stringWithFormat:@"hello+%@", self]; }

@end

调用方法如下：
NSString *str =
@"world";
NSLog(@"str == [%@]",
[str getNSString]);
打印出来的结果：str == [hello+world]

我们知道Categories可以为类扩展自己的方法，但是如何添加属性呢？
例如我们如何为NSString添加一个tag的属性(我们可以用Associative)
********************************************************************
Associative references are available only in iOS and in Mac OS X v10.6 and later
********************************************************************
#import
<objc/runtime.h>

@interface NSString(categories)
@property(nonatomic,retain)
id objectTag;
- (NSString *)getNSString;
@end

static
const char *ObjectTagKey =
"ObjectTag";
@implementation NSString(categories)
@dynamic objectTag;
- (id)objectTag {

return
objc_getAssociatedObject(self,
ObjectTagKey);
}

- (void)setObjectTag:(id)newObjectTag {

objc_setAssociatedObject(self,
ObjectTagKey, newObjectTag,
OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSString *)getNSString
{

return [NSString
stringWithFormat:@"%@+hello",
self];
}
@end

NSString *str =
@"world";
NSLog(@"str == [%@]",
[str getNSString]);

str.objectTag = [NSNumber
numberWithInt:7];//对objectTag设置NSMunber类型的值
NSLog(@"str.objectTag===[%@],object class
=== [%@]", str.objectTag, [str.objectTag
class]);

str.objectTag =
@"5";//对objectTag设置NSString类型的值
NSLog(@"str.objectTag===[%@],object class
=== [%@]", str.objectTag, [str.objectTag
class]);

str.objectTag =
nil;//对objectTag设置为nil,当然我们也可以用objc_removeAssociatedObjects
NSLog(@"str.objectTag===[%@],object class
=== [%@]", str.objectTag, [str.objectTag
class]);

打印结果如图



详见官方文档 http://developer.apple.com/library/mac/#documentation/cocoa/conceptual/ObjectiveC/Chapters/ocAssociativeReferences.html%23//apple_ref/doc/uid/TP30001163-CH24-SW1