Add additional information to a UIImage

Question

I'm trying to add some information to a UIImage I am doing some experimentation with to add details like the number of UILabel's present on the screen and their frames.

I tried using setValue:forKeyPath: but this does not work since UIImage is not key-value compliant. What is an efficient way to do this?

Adding metadata using the CGImageMetadata API's seems to need me to convert the UIImage to NSData. Also, I would like to access the data without saving it every time. Can anyone point me to a good way to do this?

Answer 1

You can get use of associated objects to complement objects of classes source of which you don't have access to with some additional data (with use of objc_setAssociatedObject to set and objc_getAssociatedObject to read the data):

UIImage *image = [UIImage new];
NSString *extraData = @"extra info";

objc_setAssociatedObject(image, @"example", extraData, OBJC_ASSOCIATION_COPY_NONATOMIC);
NSLog(@"%@", objc_getAssociatedObject(image, @"example"));

For convenience it can be effectively wrapped with a property by extending the class with a category:

//  UIImage+TDWExtraData.h

#import <UIKit/UIKit.h>

NS_ASSUME_NONNULL_BEGIN

@interface UIImage (TDWExtraData)

@property (nonatomic, copy) NSString *tdw_extraData;

@end

NS_ASSUME_NONNULL_END

//  UIImage+TDWExtraData.m

#import "UIImage+TDWExtraData.h"
#import <objc/runtime.h>

static void const *const kExtraDataKey = &kExtraDataKey;

@implementation UIImage (TDWExtraData)

- (NSString *)tdw_extraData {
    return objc_getAssociatedObject(self, kExtraDataKey);
}

- (void)setTdw_extraData:(NSString *)data {
    objc_setAssociatedObject(self, kExtraDataKey, data, OBJC_ASSOCIATION_COPY_NONATOMIC);
}


@end

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P.S. It's not correct to say that UIImage is not KVC compliant only because it doesn't support setValue:forKeyPath: for non-existing key paths. It's perfectly compliant for key-paths the class has defined:

UIImage *obj = [UIImage new];
NSLog(@"%@", [obj valueForKey:@"size"]); // prints size