AdvancedFieldBuilder

AdvancedFieldBuilder<T, E> rebuilds a subtree whenever its field notifies. It wraps ValueListenableBuilder, so the SDK widget works too if you would rather spell out ValueListenableBuilder<AdvancedFieldState<String, MyError>> — the wrapper exists to hide that type argument.

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

Note what is not in the snippet: no TextEditingController allocation, no onChanged, no initialValue seeding. The field owns both controllers and keeps them in sync. Rebuilds are granular by construction — each builder subscribes to one field, so a keystroke rebuilds that field's subtree and nothing else — and AdvancedFieldState is value-equal, so setting a field to the value it already holds notifies nobody.

Type arguments follow the controller

Two habits

Null the callback while read-only. field.getValueSetter() returns null while the field is read-only, and a null callback is exactly what disables a Material control. Where the callback is not a ValueSetter<T>?, write state.readOnly ? null : ….

Translate E at the edge. ErrorTranslator<E> is String Function(E). It takes a non-null E while state.error is E?, so promote first: final error = state.error; errorText: error == null ? null : translate(error).

The pages in this section bind each kind of field to hand-written Material widgets — this section is about the wiring, so it uses none of the docs' shorthand widgets.