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 nothing in your code elsewhere explicity uses the PNGDataValueTransformer class, then the +initialize method for that class will never be called. Specifying the name in your Core Data model will not trigger it either - it will simply try looking up a value transformer for that name, which will return nil, since no transformer instance has yet been registered under that name.

If this is indeed what's happening in your case, simply add a call to [PNGDataValueTransformer initialize] somewhere in your code before your data model gets accessed, e.g. in the +initialize method of whatever class it is that's using this data model. That should trigger the value transformer instance being created and registered so that Core Data can access it when it needs to.