[Html-widget] Behavior of Callback

[Ted Carnahan]

Sometimes you want to validate a single field, but do it in different
ways depending on the values of other fields.  This behavior doesn't
exactly fit under Callback, because it only takes one parameter, but
CallbackOnce issues an error message for each of the fields in the
validation, which isn't what we want.  Two questions:

1) Which is better - a new option for CallbackOnce to only fail
validation on the first argument in the list (I favor this), or a new
Constraint which encompasses this behavior?

2) Would anyone else want it?  If so, we'll send an appropriate patch.

Ted Carnahan
ted at tedcarnahan.com