Using UIImageView as key in NSMutableDictionary to look up boolean

The title pretty much says it all. I want to create an NSMutableDictionary where I use UIImageViews to look up boolean values. Here's what it looks like:

My .h file:

@interface ViewController : UIViewController{
    UIImageView *image1;
    UIImageView *image2;
    NSMutableDictionary *isUseable;
}

@property (nonatomic, retain) IBOutlet UIImageView *image1;
@property (nonatomic, retain) IBOutlet UIImageView *image2;
@property (nonatomic, retain) NSMutableDictionary *isUseable;

My .m file:

@synthesize image1, image2;
@synthesize isUseable

- (void)viewDidLoad
{
    isUseable = [NSMutableDictionary dictionaryWithObjectsAndKeys:
                 @NO, image1,
                 @NO, image2,nil];
}

-(void)runEvents{
    if(some condition){
        [isUseable setObject:@YES forKey:(id)image1];
    }

    //Use it later:
    if(isUseable[image1]){
        //Do stuff
    }
}

It compiles but when I run it I get an uncaught exception 'NSInvalidArgumentException', reason: '-[UIImageView copyWithZone:]: unrecognized selector sent to instance.

I'm guessing that the problem lies with the fact that the NSDictionary class copies its keys. Is there a way to get a dictionary working in this case? If not, how should I set up a lookup like the one I want? Any ideas / suggestions?

Yes, the problem is that keys in an NSDictionary must conform to the NSCopying protocol and UIImage does not.

One solution would be to give each image a unique tag. Then use the image's tag as the key (after wrapping it in an NSNumber).

- (void)viewDidLoad {
    isUseable = [ @{ @(image1.tag) : @NO, @(image2.tag) : @NO } mutableCopy];
}

-(void)runEvents {
    if(some condition) {
        [isUseable setObject:@YES forKey:@(image1.tag)];
    }

    //Use it later:
    if(isUseable[@(image1.tag)]) {
        //Do stuff
    }
}

Just add the code to see each image's tag property.