`validator` function isn't called when typing?

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I want to restrict a field to use only alphabetic characters. I tried to configure my field like the following:

``` js
nga.field('service').validation({
required: true,
maxLength: 255,
validator: function(name) {
return !name.match(/^[A-Za-z]+$/g);
}
})
```

I may misunderstand how this validator works. I am expecting it to be called after each key, updating the field feedback icon. Yet, it seems to be called only when sending the form. And, even in this case, I have to throw an error, as returning `false` doesn't work.

``` js
nga.field('service').validation({
required: true,
maxLength: 255,
validator: function(name) {
if (!name.match(/^[A-Za-z]+$/g)) {
throw new Error('Service name should contain only alphabetic characters (no punctuation or numbers).');
}
}
})
```

But in this case, the feedback icon on the field is not updated.

Start at the nga.field('service').validation(...) entry point and reproduce the alphabetic-only validator example. Trace when the validator runs and how the field feedback icon is updated; done means the validator behavior and feedback are consistent for typing and form submission.

Written by the indexing model from the issue text.