Binder should always validate after field's internal validation

[pekam]

Currently Binder validates fields on ValueChangeEvent, because the valid/invalid -state is based only on the field's server-side value.

However, some fields should be also validated when the field is blurred and only the presentation value has changed. For example, when entering "asdf" to an empty DatePicker, the server-side value remains null, but the validation-state should change from valid to invalid. Same applies to eg. non-parseable numbers entered in number fields. This kind of validation should be implemented by the components themselves, but Binder should also re-validate, so that all validators are effective.

For more details, see: https://github.com/vaadin/vaadin-date-picker-flow/issues/223#issuecomment-623386532

Possible solutions:

• Make Binder validate also on blur, if field implements BlurNotifier (although academically incorrect)
• Add an event fired by HasValidator fields to notify Binder that it should validate

This issue would be also good to get fixed: https://github.com/vaadin/flow/issues/8241

One more thing to consider is the inconsistency between Java and web component users:
The UX for web components has been designed so that "visiting" (focus and blur even without changing value) is considered "enough" to trigger validation (https://github.com/vaadin/vaadin-text-field-flow/issues/193#issuecomment-502631352).
Java forms and fields on the other hand validate only on value changes, which can mean many situations based on the ValueChangeMode, but it never means visiting a field without changing anything.
This inconsistency might become more disturbing with Vaadin 15+, where a single application may contain views written in Java, and others created with TypeScript.

[Legioth]

To my understanding, this is only relevant in the case when a field component has a "default validator" which would always be used by Binder in addition to the user-configured validators.

Rather than reacting to every blur event, it might be more appropriate to introduce a new event that is fired by components with a default validator whenever their internal validation status changes.

[pekam]

I've changed the ticket title and description from "Binder should validate on blur" to be more generic, focusing on the problem and leaving the solution to be decided.

[mvysny]

Add an event fired by HasValidator fields to notify Binder that it should validate

👍 . Having HasValidator.addValidityChangedListener() would simplify the solution at vaadin/flow-components#2176 (comment) a lot - I would be able to remove the validationChangedListener (given that there's a good way to fire those listeners).

[vaadin-bot]

This ticket/PR has been released with Vaadin 23.1.2.