preact_signals

preact_signals

Complete Dart port of Preact.js Signals.

Quick Start

The signals library exposes four functions which are the building blocks to model any business logic you can think of.

signal(initialValue)

The signal function creates a new signal. A signal is a container for a value that can change over time. You can read a signal's value or subscribe to value updates by accessing its .value property.

import 'package:preact_signals/preact_signals.dart';

final counter = signal(0);

// Read value from signal, logs: 0
print(counter.value);

// Write to a signal
counter.value = 1;

Writing to a signal is done by setting its .value property. Changing a signal's value synchronously updates every computed and effect that depends on that signal, ensuring your app state is always consistent.

signal.peek()

In the rare instance that you have an effect that should write to another signal based on the previous value, but you don't want the effect to be subscribed to that signal, you can read a signals's previous value via signal.peek().

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

effect(() {
    print(counter.value);

    // Whenever this effect is triggered, increase `effectCount`.
    // But we don't want this signal to react to `effectCount`
    effectCount.value = effectCount.peek() + 1;
});

Note that you should only use signal.peek() if you really need it. Reading a signal's value via signal.value is the preferred way in most scenarios.

untracked(fn)

In case when you're receiving a callback that can read some signals, but you don't want to subscribe to them, you can use untracked to prevent any subscriptions from happening.

final counter = signal(0);
final effectCount = signal(0);
final fn = () => effectCount.value + 1;

effect(() {
    print(counter.value);

    // Whenever this effect is triggered, run `fn` that gives new value
    effectCount.value = untracked(fn);
});

computed(fn)

Data is often derived from other pieces of existing data. The computed function lets you combine the values of multiple signals into a new signal that can be reacted to, or even used by additional computeds. When the signals accessed from within a computed callback change, the computed callback is re-executed and its new return value becomes the computed signal's value.

import 'package:preact_signals/preact_signals.dart';

final name = signal("Jane");
final surname = signal("Doe");

final fullName = computed(() => name.value + " " + surname.value);

// Logs: "Jane Doe"
print(fullName.value);

// Updates flow through computed, but only if someone
// subscribes to it. More on that later.
name.value = "John";
// Logs: "John Doe"
print(fullName.value);

Any signal that is accessed inside the computed's callback function will be automatically subscribed to and tracked as a dependency of the computed signal.

effect(fn)

The effect function is the last piece that makes everything reactive. When you access a signal inside an effect's callback function, that signal and every dependency of said signal will be activated and subscribed to. In that regard it is very similar to computed(fn). By default all updates are lazy, so nothing will update until you access a signal inside effect.

import 'package:preact_signals/preact_signals.dart';

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

// Logs: "Jane Doe"
effect(() => print(fullName.value));

// Updating one of its dependencies will automatically trigger
// the effect above, and will print "John Doe" to the console.
name.value = "John";

You can destroy an effect and unsubscribe from all signals it was subscribed to, by calling the returned function.

import 'package:preact_signals/preact_signals.dart';

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

// Logs: "Jane Doe"
final dispose = effect(() => print(fullName.value));

// Destroy effect and subscriptions
dispose();

// Update does nothing, because no one is subscribed anymore.
// Even the computed `fullName` signal won't change, because it knows
// that no one listens to it.
surname.value = "Doe 2";

Warning Cycles

Mutating a signal inside an effect will cause an infinite loop, because the effect will be triggered again. To prevent this, you can use untracked(fn) to read a signal without subscribing to it.

import 'dart:async';

import 'package:preact_signals/preact_signals.dart';

Future<void> main() async {
  final completer = Completer<void>();
  final age = signal(0);

  effect(() {
    print('You are ${age.value} years old');
    age.value++; // <-- This will throw a cycle error
  });

  await completer.future;
}

batch(fn)

The batch function allows you to combine multiple signal writes into one single update that is triggered at the end when the callback completes.

import 'package:preact_signals/preact_signals.dart';

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

// Logs: "Jane Doe"
effect(() => print(fullName.value));

// Combines both signal writes into one update. Once the callback
// returns the `effect` will trigger and we'll log "Foo Bar"
batch(() {
    name.value = "Foo";
    surname.value = "Bar";
});

When you access a signal that you wrote to earlier inside the callback, or access a computed signal that was invalidated by another signal, we'll only update the necessary dependencies to get the current value for the signal you read from. All other invalidated signals will update at the end of the callback function.

import 'package:preact_signals/preact_signals.dart';

final counter = signal(0);
final _double = computed(() => counter.value * 2);
final _triple = computed(() => counter.value * 3);

effect(() => print(_double.value, _triple.value));

batch(() {
    counter.value = 1;
    // Logs: 2, despite being inside batch, but `triple`
    // will only update once the callback is complete
    print(_double.value);
});
// Now we reached the end of the batch and call the effect

Batches can be nested and updates will be flushed when the outermost batch call completes.

import 'package:preact_signals/preact_signals.dart';

final counter = signal(0);
effect(() => print(counter.value));

batch(() {
    batch(() {
        // Signal is invalidated, but update is not flushed because
        // we're still inside another batch
        counter.value = 1;
    });

    // Still not updated...
});
// Now the callback completed and we'll trigger the effect.