validate_holder_change

Description:

public virtual signal Error validate_holder_change (Holder holder, Value new_value)

Gets emitted when a Holder's in set is going to change its value.

One can connect to this signal to control which values holder can have (for example to implement some business rules)

holder	the Holder which is going to change
new_value	the proposed new value for `holder`

NULL if holder is allowed to change its value to new_value, or a Error otherwise.