Forms and input

Forms and input

Text input is where most disposal leaks, un-localized strings, and jank enter a Flutter app. A form is a small state machine: fields hold text, a FormState validates them, and a ViewModel owns anything that touches the network or the clock. Keep those three responsibilities separate.

Read the reference for the task at hand:

• references/validation-sync-and-async.md — sync validator returning localized String?, AutovalidateMode choice, and the debounced-async-in-a-Notifier pattern (why async must NOT live in the sync validator).
• references/focus-and-keyboard.md — FocusNode lifecycle, traversal order, autofocus, FocusTraversalGroup, TextInputAction, onFieldSubmitted/onEditingComplete, keyboardType, autofillHints, TextInputFormatter, keyboard-avoidance.

Run scripts/check_forms.sh before a PR.

Non-negotiable rules

1. Every TextEditingController and FocusNode created in a State is disposed in dispose(). They hold native resources and listeners; a leak survives the widget and fires callbacks against a dead tree. If the value must outlive the widget, hold it in a Notifier instead — see state-management-riverpod.
2. Validator messages are localized, never hardcoded. A validator returns AppLocalizations.of(context).fieldRequired, not 'Required'. Error text is user-facing UI copy and is owned by i18n-rtl-l10n. The check_forms.sh grep fails on string literals returned from a validator.
3. The sync validator is pure and instant — no await, no network, no Future. FormFieldValidator<T> is String? Function(T?); it cannot be async and Flutter calls it synchronously during layout. Availability/uniqueness checks belong in a Notifier (rule 4).
4. Async validation lives in a debounced Riverpod Notifier and surfaces through state. Debounce with a dart:async Timer (kept deterministic in tests via fakeAsync, not by the clock), run the check, and expose AsyncValue/a sealed status the field reads via InputDecoration.errorText. Any timestamp the check records comes from ref.read(clockProvider).now(), never DateTime.now() — the Clock seam is owned by service-boundary-and-native. Never block a keystroke on I/O. See async-safety for cancel-on-dispose.
5. Submit-enabled is DERIVED from validity, never stored as a separate bool. A stored _isValid flag drifts out of sync with the fields. Compute it from FormState/Notifier state at build time. See flutter-performance (derive-don't-store).
6. A keystroke rebuilds one field, not the whole form. Give each field its own controller/FormField; do not lift raw text into a top-level setState/watch that rebuilds every sibling. Scope rebuilds with small widgets and ref.watch(....select(...)). See widget-composition and flutter-performance.
7. Choose AutovalidateMode deliberately. Default to AutovalidateMode.onUserInteraction: silent until the user touches a field, then live. Never always (screams before the user types). Validate-on-submit only for short forms where per-field feedback is noise.
8. Keyboard type, capitalization, and autofill are declared per field. keyboardType, textCapitalization, autofillHints, and TextInputFormatters are structural input contracts, not decoration. A missing autofillHints breaks OS autofill and password managers.
9. Errors are announced, not just colored. InputDecoration.labelText/errorText carry semantics that screen readers read on change; never signal an error with color alone. See accessibility-as-code.

Form skeleton

Form + a GlobalKey<FormState> is the coordination point. The key lets the submit handler call validate()/save() across all fields at once.

class TaskForm extends StatefulWidget {
  const TaskForm({super.key, required this.onSubmit});
  final void Function(String title) onSubmit;

  @override
  State<TaskForm> createState() => _TaskFormState();
}

class _TaskFormState extends State<TaskForm> {
  final _formKey = GlobalKey<FormState>();
  final _titleController = TextEditingController();
  final _titleFocus = FocusNode();

  @override
  void dispose() {
    _titleController.dispose(); // rule 1: always dispose
    _titleFocus.dispose();
    super.dispose();
  }

  void _submit() {
    if (_formKey.currentState?.validate() ?? false) {
      widget.onSubmit(_titleController.text.trim());
    }
  }

  @override
  Widget build(BuildContext context) {
    final l10n = AppLocalizations.of(context);
    return Form(
      key: _formKey,
      autovalidateMode: AutovalidateMode.onUserInteraction, // rule 7
      child: Column(
        children: [
          TextFormField(
            controller: _titleController,
            focusNode: _titleFocus,
            autofocus: true,
            textInputAction: TextInputAction.done,
            keyboardType: TextInputType.text,
            textCapitalization: TextCapitalization.sentences,
            decoration: InputDecoration(labelText: l10n.taskTitleLabel),
            validator: (value) => // rule 2 + 3: localized, pure
                (value == null || value.trim().isEmpty) ? l10n.fieldRequired : null,
            onFieldSubmitted: (_) => _submit(),
          ),
        ],
      ),
    );
  }
}

Sync validation

The validator is a total, synchronous function of the field value. Return null for valid, a localized message otherwise. Compose small checks; keep the closure short.

String? validateTitle(String? value, AppLocalizations l10n) {
  final text = value?.trim() ?? '';
  if (text.isEmpty) return l10n.fieldRequired;
  if (text.length > 120) return l10n.fieldTooLong; // structural bound, not design
  return null;
}

Async validation (out of the validator)

An availability check (is this account name taken?) is I/O. It runs in a Notifier, debounced against clockProvider, and the field reads the result through errorText. The sync validator stays pure and only guards the shape of the input. Full pattern in references/validation-sync-and-async.md and examples/async_field_notifier.dart.

// The field is driven by Notifier state, not by an async validator.
final status = ref.watch(nameAvailabilityNotifierProvider);
TextFormField(
  controller: _nameController,
  onChanged: ref.read(nameAvailabilityNotifierProvider.notifier).onNameChanged,
  decoration: InputDecoration(
    labelText: l10n.accountNameLabel,
    errorText: switch (status) {
      AsyncData(:final value) when value == NameCheck.taken => l10n.nameTaken,
      AsyncError() => l10n.nameCheckFailed,
      _ => null, // idle / loading / available: no error
    },
  ),
);

Focus and keyboard flow

TextInputAction.next moves to the next field; .done submits. Advance focus in onFieldSubmitted with FocusScope.of(context).nextFocus() or by requesting a specific node. Group related fields with FocusTraversalGroup to control tab order. Details in references/focus-and-keyboard.md.

TextFormField(
  focusNode: _titleFocus,
  textInputAction: TextInputAction.next,
  onFieldSubmitted: (_) => _dueDateFocus.requestFocus(),
),

Submit button derived from validity

Do not store an _isFormValid bool. Derive enablement each build; disable while an async submit is in flight (from Notifier state).

final submitting = ref.watch(taskFormNotifierProvider).isLoading;
FilledButton(
  onPressed: submitting ? null : _submit, // rule 5
  child: Text(l10n.saveAction),
);

Keyboard avoidance

Wrap long forms so the focused field scrolls above the keyboard: a SingleChildScrollView inside the body lets Scaffold (with resizeToAvoidBottomInset: true, the default) push content up. For last-field submit, ensure the submit button is reachable — put it in the scroll view or a bottomNavigationBar.

Anti-patterns

• validator: (v) async => await repo.isTaken(v) — a validator cannot be async; the Future is truthy so it always "passes." Move to a Notifier (rule 4).
• Returning 'Required' / 'Invalid email' from a validator — un-localized; breaks every non-English locale. Use AppLocalizations.
• Creating a TextEditingController/FocusNode in build() — a fresh one every rebuild, losing text and cursor. Create in State, dispose in dispose().
• bool _isValid toggled in onChanged to enable submit — drifts from real validity. Derive it.
• autovalidateMode: AutovalidateMode.always — errors shout before the user types a character.
• One TextEditingController listener that calls setState on the whole form — every keystroke rebuilds every field. Scope the rebuild.
• debounce timing rolled by hand with DateTime.now() diffs — use a dart:async Timer (deterministic under fakeAsync); and any timestamp the check records comes from ref.read(clockProvider).now(), never DateTime.now().

Definition of done

• Every controller/FocusNode disposed (or state lives in a Notifier); check_forms.sh clean.
• No string literal returned from any validator; all messages via AppLocalizations.
• No await/Future inside a sync validator; async checks in a debounced Notifier surfaced through errorText.
• AutovalidateMode.onUserInteraction (or an intentional submit-only choice).
• Submit enablement derived, not stored; disabled during in-flight submit.
• Each field declares keyboardType, textInputAction, and autofillHints where applicable; focus advances correctly.
• Errors announced via InputDecoration semantics, never color-only.

Related skills

• state-management-riverpod — the Notifier that owns async validation and submit state.
• async-safety — cancel debounce timers/subscriptions on dispose; mounted guards after await.
• i18n-rtl-l10n — localized validator messages and labels; the non-null AppLocalizations.of(context) getter.
• accessibility-as-code — field labels, error announcement, never-color-alone, target sizes.
• flutter-performance — scoped rebuilds and derive-don't-store.
• widget-composition — small const field widgets over _buildField methods.
• navigation-and-routing — PopScope for unsaved-changes confirmation when leaving a dirty form.
• service-boundary-and-native — the Clock seam (clockProvider) any async check reads timestamps from.

References

• Form: https://api.flutter.dev/flutter/widgets/Form-class.html
• TextFormField: https://api.flutter.dev/flutter/material/TextFormField-class.html
• FocusNode: https://api.flutter.dev/flutter/widgets/FocusNode-class.html
• TextInputAction: https://api.flutter.dev/flutter/services/TextInputAction.html
• Autofill: https://api.flutter.dev/flutter/services/AutofillHints-class.html
• Forms cookbook: https://docs.flutter.dev/cookbook/forms/validation