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Re: Value Transformer Name
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Re: Value Transformer Name


  • Subject: Re: Value Transformer Name
  • From: Quincey Morris <email@hidden>
  • Date: Fri, 10 Apr 2009 11:56:07 -0700

On Apr 10, 2009, at 11:27, Richard Somers wrote:

On Apr 10, 2009, at 11:54AM, Quincey Morris wrote:

Core Data runs a custom transformer in the forward direction, but it runs the default transformer in the reverse direction.

The documentation states "Core Data uses an instance of NSValueTransformer to convert the attribute to and from an instance of NSData. By default, Core Data uses the NSKeyedUnarchiveFromDataTransformerName transformer."


So a single instance of NSValueTransformer is used in both directions and the default is an instance of NSKeyedUnarchiveFromDataTransformerName transformer.

OK, one more try before I give up.

What I actually said was:

To archive the value of a transformable attribute (i.e. to create its NSData representation), Core Data runs a custom transformer in the forward direction, but it runs the default transformer in the reverse direction.

The first part of that was important. Of course Core Data runs the transformers in different directions at different times. It runs:


	NSKeyedUnarchiveFromData in reverse, to get a NSData from an attribute
	NSKeyedUnarchiveFromData forward, to get an attribute from a NSData
	a custom transformer forward, to get a NSData from an attribute
	a custom transformer in reverse, to get an attribute from a NSData

Therefore, specifying NSKeyedUnarchiveFromData as a custom transformer doesn't work. Compare line #1 with line #3, or line #2 with line #4 to see why it fails.

And, again, your testing already demonstrated that it fails.


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References: 
 >Value Transformer Name (From: Richard Somers <email@hidden>)
 >Re: Value Transformer Name (From: Keary Suska <email@hidden>)
 >Re: Value Transformer Name (From: Quincey Morris <email@hidden>)
 >Re: Value Transformer Name (From: Richard Somers <email@hidden>)
 >Re: Value Transformer Name (From: Quincey Morris <email@hidden>)
 >Re: Value Transformer Name (From: Richard Somers <email@hidden>)

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