field-render-prop.jsx

render() {
  return (
    <Form onSubmit={form => this.handleSubmit(form)}>
      <Field validators={[minLength(3)]}>
        {state => 
          <div>
            <input name="username"/>
            <span>{state.message}</span>
          </div>
        }
      </Field>
      <button>Submit</button>
    </Form>
  );
}