Objective-C : adding attribute to a category

Question

I have built a category for NSDate and I would like to encapsulate an attribute in this category to hold some data. But I can't achieve adding this attribute, only methods.

Is there any way to achieve this ?

Thank you.

Answer 1

Here some Code:

Filename: NSObject+dictionary.h

#import <Foundation/Foundation.h>
#import <objc/runtime.h>
@interface NSObject (dictionary)
- (NSMutableDictionary*) getDictionary;
@end

Filename: NSObject+dictionary.m

#import "NSObject+dictionary.h"
@implementation NSObject (dictionary)
- (NSMutableDictionary*) getDictionary
{
  if (objc_getAssociatedObject(self, @"dictionary")==nil) 
  {
    objc_setAssociatedObject(self,@"dictionary",[[NSMutableDictionary alloc] init],OBJC_ASSOCIATION_RETAIN);
  }
  return (NSMutableDictionary *)objc_getAssociatedObject(self, @"dictionary");
}

Now every instance (of every class) has a dictionary, where you can store your custom attributes. With Key-Value Coding you can set a value like this:

[myObject setValue:attributeValue forKeyPath:@"dictionary.attributeName"]

And you can get the value like this:

[myObject valueForKeyPath:@"dictionary.attributeName"]

That even works great with the Interface Builder and User Defined Runtime Attributes.

Key Path                   Type                     Value
dictionary.attributeName   String(or other Type)    attributeValue