Why is my transformable Core Data attribute not using my custom NSValueTransformer?

Question

I have a Core Data app with a fairly simple data model. I want to be able to store instances of NSImage in the persistent store as PNG Bitmap NSData objects, to save space.<

Answer 1

If I'm not mistaken, your value transformer has a reversed direction. I do the same thing in my application, using code like the following:

+ (Class)transformedValueClass 
{
 return [NSData class]; 
}

+ (BOOL)allowsReverseTransformation 
{
 return YES; 
}

- (id)transformedValue:(id)value 
{
 if (value == nil)
  return nil;

 // I pass in raw data when generating the image, save that directly to the database
 if ([value isKindOfClass:[NSData class]])
  return value;

 return UIImagePNGRepresentation((UIImage *)value);
}

- (id)reverseTransformedValue:(id)value
{
 return [UIImage imageWithData:(NSData *)value];
}

While this is for the iPhone, you should be able to swap in your NSImage code at the appropriate locations. I simply haven't tested my Mac implementation yet.

All I did to enable this was to set my image property to be transformable within the data model, and specify the name of the transformer. I didn't need to manually register the value transformer, as you do in your +initialize method.