Instance Variables for Objective C Categories

Question

I have a situation where it seems like I need to add instance variables to a category, but I know from Apple\'s docs that I can\'t do that. So I\'m wondering what the best a

Answer 1

It mentioned in many document's online that you can't create create new variable in category but I found a very simple way to achieve that. Here is the way that let declare new variable in category.

In Your .h file

@interface UIButton (Default)

   @property(nonatomic) UIColor *borderColor;

@end  

In your .m file

#import <objc/runtime.h>
static char borderColorKey;

@implementation UIButton (Default)

- (UIColor *)borderColor
{
    return objc_getAssociatedObject(self, &borderColorKey);
}

- (void)setBorderColor:(UIColor *)borderColor
{
    objc_setAssociatedObject(self, &borderColorKey,
                             borderColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    self.layer.borderColor=borderColor.CGColor;
}

@end

That's it now you have the new variable.

Answer 2

Depending on what you're doing, you may want to use Static Category Methods.

So, I assume you've got this kind of problem:

ScrollView has a couple of textedits in them. User types on text edit, you want to scroll the scroll view so the text edit is visible above the keyboard.

+ (void) staticScrollView: (ScrollView*)sv scrollsTo:(id)someView
{
  // scroll view to someviews's position or some such.
}

returning from this wouldn't necessarily require the view to move back, and so it doesn't need to store anything.

But that's all I can thinkof without code examples, sorry.

Answer 3

This is best achieved using the built-in ObjC feature Associated Objects (aka Associated References), in the example below just change to your category and replace associatedObject with your variable name.

NSObject+AssociatedObject.h

@interface NSObject (AssociatedObject)
@property (nonatomic, strong) id associatedObject;
@end

NSObject+AssociatedObject.m

#import <objc/runtime.h>

@implementation NSObject (AssociatedObject)
@dynamic associatedObject;

- (void)setAssociatedObject:(id)object {
     objc_setAssociatedObject(self, @selector(associatedObject), object, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (id)associatedObject {
    return objc_getAssociatedObject(self, @selector(associatedObject));
}

See here for the full tutorial:

http://nshipster.com/associated-objects/

Answer 4

I believe it is possible to add variables to a class using the Obj-C runtime.
I found this discussion also.

Answer 5

Recently, I needed to do this (add state to a Category). @Dave DeLong has the correct perspective on this. In researching the best approach, I found a great blog post by Tom Harrington. I like @JeremyP's idea of using @property declarations on the Category, but not his particular implementation (not a fan of the global singleton or holding global references). Associative References are the way to go.

Here's code to add (what appear to be) ivars to your Category. I've blogged about this in detail here.

In File.h, the caller only sees the clean, high-level abstraction:

@interface UIViewController (MyCategory)
@property (retain,nonatomic) NSUInteger someObject;
@end

In File.m, we can implement the @property (NOTE: These cannot be @synthesize'd):

@implementation UIViewController (MyCategory)

- (NSUInteger)someObject
{
  return [MyCategoryIVars fetch:self].someObject;
}

- (void)setSomeObject:(NSUInteger)obj
{
  [MyCategoryIVars fetch:self].someObject = obj;
}

We also need to declare and define the class MyCategoryIVars. For ease of understanding, I've explained this out of proper compilation order. The @interface needs to be placed before the Category @implementation.

@interface MyCategoryIVars : NSObject
@property (retain,nonatomic) NSUInteger someObject;
+ (MyCategoryIVars*)fetch:(id)targetInstance;
@end

@implementation MyCategoryIVars

@synthesize someObject;

+ (MyCategoryIVars*)fetch:(id)targetInstance
{
  static void *compactFetchIVarKey = &compactFetchIVarKey;
  MyCategoryIVars *ivars = objc_getAssociatedObject(targetInstance, &compactFetchIVarKey);
  if (ivars == nil) {
    ivars = [[MyCategoryIVars alloc] init];
    objc_setAssociatedObject(targetInstance, &compactFetchIVarKey, ivars, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    [ivars release];
  } 
  return ivars;
}

- (id)init
{
  self = [super init];
  return self;
}

- (void)dealloc
{
  self.someObject = nil;
  [super dealloc];
}

@end

The above code declares and implements the class which holds our ivars (someObject). As we cannot really extend UIViewController, this will have to do.

Answer 6

Yes you can do this, but since you're asking, I have to ask: Are you absolutely sure that you need to? (If you say "yes", then go back, figure out what you want to do, and see if there's a different way to do it)

However, if you really want to inject storage into a class you don't control, use an associative reference.