Rendering fields

AdvancedFieldBuilder rebuilds a subtree whenever its field notifies:

AdvancedFieldBuilder<String, MyError>(
  field: firstName,
  builder: (context, state, _) => TextFormField(
    controller: firstName.textController,
    decoration: InputDecoration(
      errorText: state.error != null ? translate(state.error!) : null,
    ),
  ),
);

It wraps ValueListenableBuilder, so the SDK widget works too if you'd rather spell out ValueListenableBuilder<AdvancedFieldState<String, MyError>> — the wrapper exists to hide that type argument.

Note what's not in the snippet: no TextEditingController allocation, no onChanged, no initialValue seeding. AdvancedTextFieldController owns its TextEditingController (field.textController) and keeps it in two-way sync — user input flows into the field state, and programmatic changes (setValue, reset) flow back into the visible text. It also owns a FocusNode (field.focusNode), so "jump to the first invalid field" is field.focus(). Working example: the "Scroll Form" screen in the example app (example/lib/screens/scroll_form.dart).

Rebuilds are granular by construction: each builder subscribes to one field, so a keystroke rebuilds that field's subtree and nothing else. AdvancedFieldState is value-equal, so setting a field to the value it already holds notifies nobody.

builder's third parameter is a child: you can pass for a subtree that doesn't depend on field state — built once and reused on every rebuild. See example/lib/widgets/form_text_field_with_icon.dart and the "Optimized Rendering" screen in the example app.

For parent widgets, grab the controller with context.read<SignupFormController>() — no subscription, no parent rebuilds — and let each field widget subscribe to its own field.