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