Ole Begemann

iOS Development

Faking instance variables in Objective-C categories with Associative References

In OS X 10.6 and iOS 3.1, Apple added Associative References to the Objective-C runtime. Essentially, this means that each and every object has an optional dictionary you can add arbitrary key/value pairs to.

This is a great feature, especially considering that Objective-C has forever had a feature to add methods to existing classes: categories. Categories, however, do not permit you to add instance variables. Using associative references, it’s easy to fake ivars.

In the C API of the Objective-C runtime, you add a key/value pair to an object and read it again with these two functions:

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void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
id objc_getAssociatedObject(id object, const void *key)

If we wrap these calls in a property’s custom getter and setter, we can make the implementation of our fake “ivar” totally opaque to the user of our API.

Using objects to tag UIViews

As an example, say we want to add the ability to add an arbitrary object as a tag to a UIView (UIView’s existing tag property only takes integers, which can be limiting at times). The interface of our “object tag” category could look like this:

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@interface UIView (ObjectTagAdditions)

@property (nonatomic, retain) id objectTag;

- (UIView *)viewWithObjectTag:(id)object;

@end

Using associative references, the implementation of the property is straightforward:

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#import <objc/runtime.h>

static char const * const ObjectTagKey = "ObjectTag";

@implementation UIView (ObjectTagAdditions)
@dynamic objectTag;

- (id)objectTag {
    return objc_getAssociatedObject(self, ObjectTagKey);
}

- (void)setObjectTag:(id)newObjectTag {
    objc_setAssociatedObject(self, ObjectTagKey, newObjectTag, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

...

By specifying OBJC_ASSOCIATION_RETAIN_NONATOMIC, we tell the runtime to retain the value for us. Other possible values are OBJC_ASSOCIATION_ASSIGN, OBJC_ASSOCIATION_COPY_NONATOMIC, OBJC_ASSOCIATION_RETAIN, OBJC_ASSOCIATION_COPY, corresponding to the familiar property declaration attributes.

Update December 22, 2011: It’s important to note that the key for the association is a void pointer void *key, not a string. That means that when retrieving an associated reference, you have to pass the exact same pointer to the runtime. It would not work as intended if you used a C string as the key, then copied the string to another place in memory and tried to access the associated reference by passing the pointer to the copied string as a key.

Finally, here is the code for the recursive -viewWithObjectTag: method:

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- (UIView *)viewWithObjectTag:(id)object {
    // Raise an exception if object is nil
    if (object == nil) {
        [NSException raise:NSInternalInconsistencyException format:@"Argument to -viewWithObjectTag: must not be nil"];
    }

    // Recursively search the view hierarchy for the specified objectTag
    if ([self.objectTag isEqual:object]) {
        return self;
    }
    for (UIView *subview in self.subviews) {
        UIView *resultView = [subview viewWithObjectTag:object];
        if (resultView != nil) {
            return resultView;
        }
    }
    return nil;
}

Update May 16, 2011: Vadim Shpakovski asked on Twitter why, when the argument to viewWithObjectTag: is nil, I chose to generate an exception over returning nil to the caller. The reason is that returning nil would be ambiguous since it means that no view with the specified objectTag could be found. Since nil is the default value of objectTag, this is highly unlikely.

A good alternative to raising an exception is to return the first view whose objectTag actually is nil, just like viewWithTag: does. I chose not to do that because I imagine it’s more likely that calling -viewWithObjectTag: with a nil argument is a programmer error than intended usage. It’s just a matter of preference, though.

Update December 22, 2011: Improved the declaration of the ObjectTagKey pointer and added a missing #import statement to the code sample.