為類添加執行個體變數 (Associative References)

在object-c中我們知道可以使用categories來為擴充類方法(比如我們可以為系統的類添加自己的方法)

例如：我們要想在每個NSString前面添加一個

@interface NSString ( CategoryName )

// method declarations - (NSString *) getNSString;

@end

@implementation NSString ( CategoryName )

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

@end

 調用方法如下：

NSString *str =
@"world";

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

列印出來的結果：str == [hello+world]

我們知道Categories可以為類擴充自己的方法，但是如何添加屬性呢？

例如我們如何為NSString添加一個tag的屬性(我們可以用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];//對objectTag設定NSMunber類型的值

NSLog(@"str.objectTag===[%@],object class
=== [%@]", str.objectTag, [str.objectTag
class]);

str.objectTag =
@"5";//對objectTag設定NSString類型的值

NSLog(@"str.objectTag===[%@],object class
=== [%@]", str.objectTag, [str.objectTag
class]);

str.objectTag =
nil;//對objectTag設定為nil,當然我們也可以用objc_removeAssociatedObjects

NSLog(@"str.objectTag===[%@],object class
=== [%@]", str.objectTag, [str.objectTag
class]);

列印結果

詳見官方文檔

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