collection_notifiers

Reactive List / Set / Map / Queue for Flutter. Mutate in place, rebuild on real change only. Ships matching flutter_hooks hooks for one-line widget integration.

final todos = useListNotifier<String>(['buy milk']);
todos.add('walk dog');   // rebuilds — new element
todos[0] = 'buy milk';   // silent — same value
todos[0] = 'buy eggs';   // rebuilds — value changed
todos.clear();           // rebuilds — was non-empty
todos.clear();           // silent — already empty

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What it's for

• In-place collection state in a widget: selection toggles, todo lists, key/value settings, FIFO/LIFO queues.
• Anywhere ValueNotifier<List<T>> would force a [...state, x] copy per mutation.

What it isn't

• Not a state container. Wrap with ChangeNotifierProvider (Provider) or expose via a Riverpod notifier when you need DI.
• Not deep-reactive. Mutating an element in place (list[0].field = x) bypasses the equality check — use freezed or equatable for element types.
• Not for a single value. Reach for stdlib ValueNotifier<T>.

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Install

flutter pub add collection_notifiers

import 'package:collection_notifiers/collection_notifiers.dart';

flutter_hooks is a runtime dependency — the hook variants ship with the package.

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Pick the notifier

Need	Class	Hook
Ordered, indexable, reorderable	ListNotifier	useListNotifier
Unique elements, selection toggles	SetNotifier	useSetNotifier
Key → value lookup	MapNotifier	useMapNotifier
FIFO/LIFO head-or-tail mutation	QueueNotifier	useQueueNotifier

Each class extends package:collection's DelegatingX and mixes in ChangeNotifier — drop-in replacement for the matching dart:core collection, plus ValueListenable<List<E>> / <Set<E>> / <Map<K, V>> / <Queue<E>>.

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Hooks — recommended

The hook owns the lifecycle: creates the notifier on first build, disposes on unmount, rebuilds the host widget on every mutation. Zero boilerplate.

class TodoList extends HookWidget {
  const TodoList({super.key});

  @override
  Widget build(BuildContext context) {
    final todos = useListNotifier<String>(['buy milk']);
    return Column(
      children: [
        FilledButton(
          onPressed: () => todos.add('walk dog'),
          child: const Text('Add'),
        ),
        for (final t in todos) Text(t),
      ],
    );
  }
}

initial is consumed once. To reset on a dependency change, scope the host widget under a different key so the hook re-mounts.

If the notifier is owned upstream (Riverpod, parent widget), subscribe without recreating it:

useListenable(notifier);

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Without hooks

If flutter_hooks is not on the project, fall back to ValueListenableBuilder — the notifier exposes itself as the value:

final todos = ListNotifier<String>(['buy milk']);

ValueListenableBuilder<List<String>>(
  valueListenable: todos,
  builder: (context, items, _) => Column(
    children: [for (final t in items) Text(t)],
  ),
);

Dispose todos yourself — ChangeNotifierProvider / State.dispose / ref.onDispose.

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Notification contract

Mutating methods call notifyListeners() only when the underlying collection actually changes.

final tags = SetNotifier<String>({'flutter', 'dart'});
tags.add('rust');     // notifies — new element
tags.add('rust');     // silent — already present
tags.remove('rust');  // notifies — element removed
tags.remove('rust');  // silent — wasn't there
tags.clear();         // notifies — set drained
tags.clear();         // silent — already empty

final config = MapNotifier<String, int>({'volume': 50});
config['volume'] = 75;  // notifies — value changed
config['volume'] = 75;  // silent — same value
config['bass'] = 30;    // notifies — new key

The per-method strategy — length-delta diff, equality-guarded single-slot, containsKey disambiguation for nullable values — is documented in each method's dartdoc.

Exceptions

• ListNotifier.sort / shuffle on length > 1 always notify. Verifying order-preservation costs O(n) per call and defeats the optimisation budget. Lists of length 0 or 1 short-circuit silently.
• MapNotifier.addEntries uses a length-only check: re-inserting an existing key with a different value mutates the map but does not notify. Use operator []= or addAll when per-key value-diff matters.

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Patterns

Multi-select with SetNotifier

final selected = useSetNotifier<int>();

CheckboxListTile(
  value: selected.contains(id),
  onChanged: (_) => selected.invert(id),
  title: Text('Item $id'),
);

invert(e) toggles presence — returns true if added, false if removed.

Settings with MapNotifier

final settings = useMapNotifier<String, Object>({
  'darkMode': false,
  'fontSize': 14,
});

settings['darkMode'] = !(settings['darkMode']! as bool);
settings['fontSize'] = 14;   // silent — already 14

Todos with ListNotifier

final todos = useListNotifier<Todo>();

todos.add(Todo(title: 'learn Flutter'));
todos.removeWhere((t) => t.completed);

// reorder
final item = todos.removeAt(oldIndex);
todos.insert(newIndex, item);

// sort always notifies on length > 1 — see "Exceptions"
todos.sort((a, b) => a.priority.compareTo(b.priority));

Riverpod

final todosProvider = ChangeNotifierProvider((ref) {
  return ListNotifier<String>(['initial']);
});

class TodoList extends ConsumerWidget {
  const TodoList({super.key});

  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final todos = ref.watch(todosProvider);
    return Column(children: [for (final t in todos) Text(t)]);
  }
}

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Agent setup

A bundled skill ships at skills/flutter-collection-notifiers/SKILL.md. Vendor it into your agent's skill directory (~/.claude/skills/flutter-collection-notifiers/, or the Cursor / AntiGravity equivalent). It teaches the agent to pick the right notifier, wire the matching hook, respect dispose discipline, and stop replacing the collection instead of mutating it.

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Pitfalls

❌	✅
notifier = ListNotifier([...]) — reassigning kills listeners	notifier..clear()..addAll([...]) — mutate in place
Custom element types with default == / hashCode	freezed / equatable so equality checks can see real changes
useListNotifier(seed) with a fresh seed per rebuild expecting a reset	initial is consumed once — change the host widget's key to reset
StatefulWidget holding a notifier without disposing	Use the matching hook, or override dispose and call notifier.dispose()
Expecting addEntries to fire on value change for an existing key	Use operator []= / addAll — addEntries is length-only

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Migration 1.x → 2.x

Breaking: SetNotifier.invert(e) return value:

// v1.x — returned whichever of add()/remove() ran
// v2.x — true if element was added, false if removed
selected.invert(1);

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License

MIT.