What can I do with an Transformable attribute type in Core Data on the iPhone?
Solution 1:
Transformable attributes are useful for storing nonstandard object types within Core Data. For example, I provide code in this answer that lets you store UIImages as an attribute within Core Data. The image data is converted to and from an NSData instance that contains the image's PNG representation. This is all handled transparently for you by a custom NSValueTransformer.
You may also wish to encrypt individual attributes within your Core Data model, as I describe here. Using a transformable attribute for this makes this trivial to code.