asyncValidation on key strokes and throttled (not on blur)

Is it possible? The short answer is yes.

Is it tricky? Well, as you mentioned, theres the fact that the asyncValidation for redux-form option only works for the onBlur and you'd instead want it to work onChange.

So you could fork and add this feature into redux-form, so you can do this:

@reduxForm({
  form: 'login',
  asyncValidate: validator,
  //asyncBlurFields: ['username','password'],
  asyncChangeFields: ['username','password'], /* add this new option */
})

For the deboucing part, you'd want to somehow debounce the onChange handler rather than the validate function itself, which brings up another option...

redux-form exports its internal action creators which may be enough to hack it together. Particularly, there's the stopAsyncValidation action creator that lets you pass field-level async errors directly into a form. Pair that with the onChange prop for Field, and you actually have the right pieces to get it done like this:

import React from 'react'
import { Field, reduxForm, stopAsyncValidation } from 'redux-form'
import _debounce from 'lodash.debounce'

import renderField from './renderField'

@reduxForm({form: 'debouncedOnChangeValidation'})
class DebouncedOnChangeValidationForm extends React.Component {

  customValidate = () => {
    const { form, dispatch } = this.props
    dispatch(stopAsyncValidation(form, { username: "thats wrong..." })) /* pass in async error directly! */
  }

  debounceValidate = _debounce(this.customValidate, 1000) /* decorate with debounce! */

  render() {
    const { handleSubmit, pristine, reset, submitting} = this.props
    return (
      <form onSubmit={handleSubmit}>
        <Field name="username" type="text"
          component={renderField} label="Username"
          onChange={this.debounceValidate} /* bind validation to onChange! */
        />
        <div>
          <button type="submit" disabled={submitting}>Submit</button>
          <button type="button" disabled={pristine || submitting} onClick={reset}>Clear Values</button>
        </div>
      </form>
    )
  }
}

Additionally, to access the form values for performing validation, you'd need to use the getFormValues selector.

Of course this won't be as robust as a more built-in solution, but it may work well enough for some use cases.