Signals Dart

Signals.dart

Reactive state management using fine-grained reactive primitives. Works across Dart VM, WASM, Flutter, and server.

Installation

# Flutter
flutter pub add signals

# Dart
dart pub add signals

// Dart projects
import 'package:signals/signals.dart';

// Flutter projects (includes ValueNotifier compatibility)
import 'package:signals/signals_flutter.dart';

Core Primitives

Signal - Reactive State

final counter = signal(0);
print(counter.value);  // Read
counter.value = 1;     // Write (notifies dependents)
counter.peek();        // Read without tracking
counter.set(value, force: true);  // Force update

Computed - Derived Values

final name = signal("Jane");
final surname = signal("Doe");
final fullName = computed(() => "${name.value} ${surname.value}");

Effect - Side Effects

final dispose = effect(() => print(counter.value));
dispose();  // Stop effect

// With cleanup
effect(() {
  print(s.value);
  return () => print('Cleanup');
});

Warning: Never mutate signals inside effects without untracked() - causes infinite loops.

Untracked and Batch

// Read without creating dependency
untracked(() => counter.value);

// Batch multiple updates
batch(() {
  name.value = "Foo";
  surname.value = "Bar";
});  // Single notification

Flutter Integration

Watch Widget (Recommended)

Watch((context) => Text('Count: ${counter.value}'));

// With child optimization
Watch.builder(
  builder: (context, child) => Column(children: [
    Text('Count: ${counter.value}'),
    child!,  // Doesn't rebuild
  ]),
  child: ExpensiveWidget(),
);

SignalsMixin (Auto-disposal)

class _MyState extends State<MyWidget> with SignalsMixin {
  late final count = createSignal(0);
  late final isEven = createComputed(() => count.value.isEven);

  @override
  void initState() {
    super.initState();
    createEffect(() => print('Count: ${count.value}'));  // Effects in initState!
  }
}

Async Signals

FutureSignal

final data = futureSignal(() async => fetchData());

// With dependencies (re-executes on change)
final data = futureSignal(
  () async => fetchDataFor(id.value),
  dependencies: [id],
);

data.reset();    // Return to initial
data.refresh();  // Keep state, set loading
data.reload();   // Reset to loading with preserved value

AsyncState Pattern Matching

return switch (data.value) {
  AsyncLoading() => CircularProgressIndicator(),
  AsyncData(:final value) => Text('$value'),
  AsyncError(:final error) => Text('Error: $error'),
};

// Or map()
data.value.map(
  loading: () => CircularProgressIndicator(),
  data: (value) => DataWidget(value),
  error: (err, _) => ErrorWidget(err),
);

Collections

final list = listSignal([1, 2, 3]);
final map = mapSignal({'a': 1});
final set = setSignal({1, 2, 3});

list.add(4);      // Triggers updates
map['b'] = 2;     // Triggers updates
set.remove(1);    // Triggers updates

Best Practices

1. Use Watch widget instead of .watch(context)
2. Never mutate signals inside effects without untracked()
3. Use batch() for multiple updates
4. Put effects in initState(), not as late fields
5. Access signal values BEFORE await in computedAsync()
6. Prefer computedFrom() over computedAsync() to avoid await timing issues
7. Always add debugLabel to every signal — use the variable name (no class prefix):

   final counter = Signal<int>(0, debugLabel: 'counter');
   final data = FutureSignal(() => fetch(), debugLabel: 'data');
   final items = ListSignal([], debugLabel: 'items');
   late final total = Computed(() => items.value.length, debugLabel: 'total');
   

Reference

For detailed API documentation including Flutter hooks, SignalProvider, ValueNotifier compatibility, async signals detail, collections API, advanced patterns (SignalsContainer, persisted signals, bi-directional flow, SignalsObserver), DI patterns, testing, and migration guides, see references/api_reference.md.