Does Core Data automatically validate new values when they are set?

Question

In this question, someone asked how to write a validation method for Core Data. I did that, and it looks cool. But one thing doesn't happen: The validation. I can easily set any "bad" value and this method doesn't get called automatically. What's the concept behind this? Must I always first call the validation method before setting any value? So would I write setter methods which call the appropriate validation method first?

And if yes, what's the point of following a strict convention in how to write the validation method signature? I guess there's also some automatic way of validation, then. How to activate this?

Answer 1

Validation is not "automatic" especially on iOS. On the desktop you will have the UI elements handling the call to validation. On iOS you should be calling validateValue:forKey:error: with the appropriate key and dealing with the error should there be one. The reason for this is the lack of a standard error display on iOS and the overhead of validating all values.

Note this comment in the documentation:

If you do implement custom validation methods, you should typically not invoke them directly. Instead you should call validateValue:forKey:error: with the appropriate key. This ensures that any constraints defined in the managed object model are also applied.