Asynchronous validation for ValidationTextBox

[William Riley]

In a lot of cases, you might want to validate a textbox against a datastore or some sort of backend, eg to make sure a username isn't taken. Currently ValidationTextBox only supports synchronous validation, which limits what you can do.

The only problem with implementing this sort of thing is that validation when submitting a form can't be done asynchronously, but I think in most cases, the async validation call will be fast enough that the validation can be done before the user has the chance to submit the form (or even have an argument specifically for doing synchronously). Another option is to just assume the field is invalid internally until the async validation has finished.

[Douglas Hays]

Waiting for a patch...

[bill]

Note that FilteringSelect already does asynchronous validation, although as you mentioned there's an issue with submitting forms if the validation hasn't yet finished.