Your first form

A controller holds the fields and registers them; a widget binds to each field. The form below is the code on this page, compiled and running — submit it empty to watch the errors appear:

class SignupFormController extends AdvancedFormController {
  SignupFormController() {
    registerFields([firstName, lastName]);
  }

  final firstName = AdvancedTextFieldController(
    validator: filled('First name is required'),
  );
  final lastName = AdvancedTextFieldController(
    validator: filled('Last name is required'),
  );

  Future<void> submit() async {
    if (await validate()) {
      // send it
    }
  }
}

class SignupForm extends StatefulWidget {
  const SignupForm({super.key});

  @override
  State<SignupForm> createState() => _SignupFormState();
}

class _SignupFormState extends State<SignupForm> {
  final _form = SignupFormController();

  @override
  void dispose() {
    _form.dispose(); // disposes the registered fields too
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        _SignupTextField(field: _form.firstName, label: 'First name'),
        _SignupTextField(field: _form.lastName, label: 'Last name'),
        ElevatedButton(onPressed: _form.submit, child: const Text('Submit')),
      ],
    );
  }
}

class _SignupTextField extends StatelessWidget {
  const _SignupTextField({required this.field, required this.label});

  final AdvancedTextFieldController<String> field; // <String> is the error type
  final String label;

  @override
  Widget build(BuildContext context) {
    return AdvancedFieldBuilder<String, String>(
      field: field,
      builder: (context, state, _) => TextFormField(
        controller: field.textController,
        decoration: InputDecoration(labelText: label, errorText: state.error),
      ),
    );
  }
}

Fields stay quiet until the first validate(), then give live feedback.

Two rules to remember:

Call registerFields() once, with every field. The form then owns their lifecycle — it disposes them, tracks whether anything was modified, and includes them in validate(), resetAll(), and the other form-wide operations. Calling it a second time replaces the field list, so the earlier batch stops participating while still being disposed at teardown.
Bind widgets to field.textController, not to a controller of your own. See Rendering fields.

Own the controller wherever you like — it's a ChangeNotifier, so any DI package works as well as the StatefulWidget above. ChangeNotifierProvider from provider is one widget, and it disposes the controller for you (provider is used in the snippets below; it is not a dependency of this package). Whoever owns the form controller disposes it, and that one call disposes the registered fields and the attached subforms.