Signals Dart

Signals Core Advanced Primitives

This skill details advanced state management primitives, reactive collections, async features, and value options semantics in signals_core.

Related Documentation & Items

Core Primitives

Asynchronous Operations

Reactive Collections

Utility Primitives

Core Primitives Quick Start Guide

The signals library exposes five core functions which are the building blocks to model any reactive business logic.

1. signal(initialValue)

Creates a new mutable signal container. You read a signal's value or subscribe to updates by accessing .value.

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final counter = signal(0);
print(counter.value); // 0
counter.value = 1; // Mutates value and schedules dependent updates

.peek()

Reads a signal's current value without subscribing to its mutations.

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final counter = signal(0);
final logCount = signal(0);

effect(() {
  print(counter.value);
  // Read using peek to avoid subscribing/triggering loop
  logCount.value = logCount.peek() + 1;
});

2. untracked(fn)

Executes a callback that reads signals without subscribing to any of them.

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final counter = signal(0);
final count = signal(0);

effect(() {
  print(counter.value);
  count.value = untracked(() => count.value + 1);
});

3. computed(fn)

Combines the values of multiple signals into a lazy, memoized derived signal.

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final first = signal('John');
final last = signal('Doe');
final fullName = computed(() => '${first.value} ${last.value}');
print(fullName.value); // John Doe

4. effect(fn)

Orchestrates immediate synchronous side effects by running a callback and subscribing to any signals read within it.

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final name = signal('Jane');
final dispose = effect(() => print('Hello $name'));
dispose(); // Clean up subscription

5. batch(fn)

Groups multiple signal writes into a single transaction, executing all dependent computed evaluations and effects exactly once at the end.

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final a = signal(0);
final b = signal(0);

batch(() {
  a.value = 1;
  b.value = 2;
});

1. Writable Computed Signal (linkedSignal)

A writable computed signal (LinkedSignal) provides a derived default value that can be overridden manually, resetting back to the derived default whenever its upstream dependencies mutate.

Usage

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// Basic reset shorthand:
final size = signal('M');
final selection = linkedSignal(() => size.value);

// Advanced structural comparison:
final user = signal((id: 1, name: 'Alice'));
final name = linkedSignal<String, ({int id, String name})>(
  () => user.value,
  options: LinkedSignalOptions(
    computation: (u, prev) {
      if (prev != null && prev.source.id == u.id) {
        return prev.value; // Retain manual override
      }
      return u.name; // Reset on ID changes
    },
  ),
);

2. Collections and Async Primitives

A. Collections

Always use specialized collections (listSignal, setSignal, mapSignal, etc.) to wrap collections to ensure optimized mutations (only triggering updates when elements actually change).

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final list = listSignal([1, 2], options: ListSignalOptions(autoDispose: true));
list.add(3); // Reacts to changes

B. Optimized FutureSignal

FutureSignal inherits directly from AsyncSignal to bypass Stream subscription overhead and tracks its dependencies manually via computed execution, executing future computations exactly once (avoiding double-evaluation loops).

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final search = signal('');
final results = futureSignal(() => fetchResults(search.value));

3. Options Classes Value Semantics

All signal option objects support full value semantics (copyWith, custom equality, and hashCode overrides) for flexible copying and accurate comparisons:

• SignalOptions
• ComputedOptions
• AsyncSignalOptions
• LinkedSignalOptions
• ListSignalOptions / SetSignalOptions / MapSignalOptions