[Dbix-class] How to set callback constraint by yaml file ?

[chylli]

Hi. I strugglle with this problem for a long time.
I want to set a callback constraint on a field, for example, check
whether the username exists in db when user register. The framework is
Catalyst. Now I use HTML::FormFu directly to create a form and set
callback by hand. 
Now I want to switch to Catalyst::Controller::HTML::FormFu. But how
can I set that constraint in the configure yaml file ? Is it possible
? the main reason to set it in yml file is that if i use 
sub bar : Local : FormConfig
 or 
sub baz : Local : FormConfig('my_config') 
the form has been processed before I can set constraint.

thanks for your help

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Thanks & Regards

Chylli