Re: Value Transformer Name
Re: Value Transformer Name
- Subject: Re: Value Transformer Name
- From: Quincey Morris <email@hidden>
- Date: Fri, 10 Apr 2009 09:41:28 -0700
On Apr 10, 2009, at 08:07, Keary Suska wrote:
I would file a bug against the documentation for clarification. I
would hazard the guess that Core Data handles transformations
differently when you specify a transformer name and when you don't.
Hence the different errors.
That's exactly right. This is a beautifully engineered gotcha for the
unwary programmer. The Core Data documentation says:
If you specify a custom transformer, it must transform an instance
of the non-standard data type into an instance of NSData and support
reverse transformation.
However, the NSKeyedUnarchiveFromDataTransformerName transformer
transforms an instance of NSData to an instance of whatever was
archived in it.
So, the default transformer goes *from* NSData. Custom transformers go
*to* NSData.
So, you cannot specify the default transformer explicitly, because
that would make Core Data use it in the wrong direction.
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