I'm thinking of validation not as a stand alone component but as part of a solution set. For AJAX form submission, I guess we will usually need at a minimum the following:

1. Server side (PHP): Designate forms that should submit via AJAX.
2. Client side (Javascript): Attach ajax submit behavior to form.
3. Client side: Add information to ajax form submission that informs server that this is an AJAX requrest.
4. Client side: On submission, remove any error messages and classes.
5. Server side: Validate form submission, returning data on error--error messages and enough data for the client to identify the elements to attach error classes to.
6. Client side: On validation failure, attach error classes to appropriate form elements, display error messages, scroll to error messages.
7. Server side: On validation success, complete form submission, return data to client. Data may include:
   • Location to forward to.
   • Data on object or objects created or modified through form submit. E.g., ID and title of a node.
8. Client side: respond in a number of possible ways to server form submission data. Options might include:
   • Forward to a new page.
   • Remove form (close dialogue that form was in).
   • Reset form in preparation for new submission.
   • Copy data from form submission result to existing or new form elements (e.g., update an autocomplete element with the ID of a node created through form submission).

Because these components will be needed for most or all AJAX form submission, I think they're what we should abstract into an API, probably for core.

I don't mean that a generic API needs to do all of the specific things in the last two points--just that it should be flexible enough to enable them.

This needn't be a lot of code. For instance the ajaxsubmit.js file covers most of these client tasks in under 90 lines.

It's not about validation, its about rebuilding the form, submit the form and close the popup where the form was, check if problems had occurred, and tells the user then, and lots of things like that.

Your form builder project is nice, but not for developers, it add lots of extra stuff, jqueryUI, jquery Update, that you'll probably don't want, but is required to make the module works.

And the AHAH form is not enough powerful or its API is not designed to handle with form submission, etc.. You have to write 10 or more line function to submit, rebuild, get data from cache, and say "Well done!" to the user. Then you have to write your custom javascript event listener to close the form the user just submit! And that work is exactly the same for each of your js handler (server side).

That function has been ported to core in D7 (according to this article:http://drupal.org/node/331941) and I agree that it should have a unified API instead of letting peoples and modules implements it on their own.