Instance variables may is placed in categories

Avoid Properties in Categories

The OBJECTIVE-C classification is not allowed to increase the member variable (Instance variables May is not is placed in categories), we can use the runtime function objc_setAssociatedObject and objc_getAssociatedObject to let the classification support to save and get some data, This supports attributes.

eocperson+friendship.h@InterfaceEocperson (Friendship) @property (nonatomic, strong) nsarray *friends;@end//eocperson+ Friendship.mstatic const char* Kfriendspropertykey =  "Kfriendspropertykey"; @ Implementation eocperson (friendship)-(NSArray*) Friends {return objc_getassociatedobject (self, kfriendspropertykey);} -(void) setfriends: (nsarray*) Friends {objc_setassociatedobject ( span class= "keyword" >self, kfriendspropertykey, Friends, objc_association_retain_nonatomic);} @end              

Instance variables may is placed in categories