IntroduceValidityChangeEvent

[Matti Tahvonen]

Form needs to update its errors based on validity of contained fields. To do this it currently listens value change events of its fields and does the housekeeping task there.

Problems with this approach:

• excess form repaints. E.g. valid to valid value change in a contained field causes unnecessary tasks to be executed in the Form
• customizing isValid() method may brake behavior in Form. See DateFields handling of a string that couldn't be parsed in to a Date. An extra hack is needed for cases where value remains null but invalid text content appears or disappears.