Changing NSValueTransformer to NSSecureUnarchiveFromDataTransformer for Core Data warning

Hi,
In my Core Data schema, I have a 'transformable' attribute in an entity, which is using a custom NSValueTransformer, the purpose of which is to convert a UIImage into NSData with some compression (basically to save a small thumbnail from the image). From this attribute, I had recently started getting these warnings about using NSKeyedUnarchiveFromData:

'NSKeyedUnarchiveFromData' should not be used to for un-archiving and will be removed in a future release

So I read about this, and figured I'd need to change the NSValueTransformer to using NSSecureUnarchiveFromDataTransformer ... but after making the changes (as best as I understood them) I can't get it to work, and now the app crashes when the attribute is accessed.


This is the existing NSValueTransformer that I was using:

Code Block 
@interface ImageToDataTransformer : NSValueTransformer {
}
@implementation ImageToDataTransformer
+ (BOOL)allowsReverseTransformation {
    return YES;
}
+ (Class)transformedValueClass {
    return [NSData class];
}
- (id)transformedValue:(id)value {
    // for our smaller views this uses much less data and makes for faster syncing
    NSData *compressedData = UIImageJPEGRepresentation(value, 0.2);
    return compressedData;
}
- (id)reverseTransformedValue:(id)value {
    UIImage *uiImage = [[UIImage alloc] initWithData:value];
    NSData *data = (NSData *) value;
    //NSLog(@"reverseTransformedValue: image size: %@", [NSByteCountFormatter stringFromByteCount:data.length countStyle:NSByteCountFormatterCountStyleFile]);
    return uiImage;
}

So I changed it to subclass from NSSecureUnarchiveFromDataTransformer instead, and added the following to the implementation:

Code Block 
+ (NSArray<Class> *)allowedTopLevelClasses {
    return @[[ImageToDataTransformer class]];
}
+ (void)setValueTransformer:(nullable NSValueTransformer *)transformer forName:(NSValueTransformerName)name {
    NSLog(@"ImageToDataTransfer: calling setValueTransformer");
    [NSValueTransformer setValueTransformer:transformer forName:name];
}
+ (NSArray<NSValueTransformerName> *)valueTransformerNames {
    return @[@"ImageToDataTransformerName"];
}

Before the Core Data persistent store is accessed, I 'register' the transformer:

Code Block 
[ImageToDataTransformer setValueTransformer: [[ImageToDataTransformer alloc] init] forName:@"ImageToDataTransformerName"];

Now the app crashes when the image is read:

[__NSCFData _rasterizedImage]: unrecognized selector sent to instance 0x7ff53c108800

If I change the transformer back to NSValueTransformer, it works fine. So I'm not sure if I'm missing something from the implementation, or I have misunderstood the premise of 'NSSecureUnarchiveFromDataTransformer'. Would love to know what I can do to fix this.