iOS扩展属性：category可以扩展类的方法，但是不能扩张类的属性。如果要扩展类的属性，可以用associative，这个相对于category来说用的比较少，而且他还用到运行时编程，必须使用<obj/runtime.h>,使用objc_getAssociatedObject,objc_setAssociatedObject以及objc_removeAssociatedObjects.这几个方法的生命如下：

/** 
 * Sets an associated value for a given object using a given key and association policy.
 * 
 * @param object The source object for the association.
 * @param key The key for the association.
 * @param value The value to associate with the key key for object. Pass nil to clear an existing association.
 * @param policy The policy for the association. For possible values, see “Associative Object Behaviors.”
 * 
 * @see objc_setAssociatedObject
 * @see objc_removeAssociatedObjects
 */
OBJC_EXPORT void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

/** 
 * Returns the value associated with a given object for a given key.
 * 
 * @param object The source object for the association.
 * @param key The key for the association.
 * 
 * @return The value associated with the key \e key for \e object.
 * 
 * @see objc_setAssociatedObject
 */
OBJC_EXPORT id objc_getAssociatedObject(id object, const void *key)
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

/** 
 * Removes all associations for a given object.
 * 
 * @param object An object that maintains associated objects.
 * 
 * @note The main purpose of this function is to make it easy to return an object 
 *  to a "pristine state”. You should not use this function for general removal of
 *  associations from objects, since it also removes associations that other clients
 *  may have added to the object. Typically you should use \c objc_setAssociatedObject 
 *  with a nil value to clear an association.
 * 
 * @see objc_setAssociatedObject
 * @see objc_getAssociatedObject
 */
OBJC_EXPORT void objc_removeAssociatedObjects(id object)
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

示例代码如下：

1、先创建一个person类

#import <Foundation/Foundation.h>

@interface Person : NSObject

@property (nonatomic, copy) NSString *name;

@end

#import "Person.h"

@implementation Person

@end

2、新建一个分类，运用associative添加属性

#import "Person.h"

@interface Person (addproty)

@property(nonatomic, copy)NSString *addr;

@end

#import "Person+addproty.h"
#import <objc/runtime.h>

@implementation Person (addproty)

static char strAddrKey = 'a';

- (NSString *)addr
{
    return objc_getAssociatedObject(self, &strAddrKey);
}

- (void)setAddr:(NSString *)addr
{
    objc_setAssociatedObject(self, &strAddrKey, addr, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

@end