alien-signals

This project explores a push-pull based signal algorithm. Its current implementation is similar to or related to certain other frontend projects:

Propagation algorithm of Vue 3
Preact’s double-linked-list approach (https://preactjs.com/blog/signal-boosting/)
Inner effects scheduling of Svelte
Graph-coloring approach of Reactively (https://milomg.dev/2022-12-01/reactivity)

We impose some constraints (such as not using Array/Set/Map and disallowing function recursion in the algorithmic core) to ensure performance. We found that under these conditions, maintaining algorithmic simplicity offers more significant improvements than complex scheduling strategies.

Even though Vue 3.4 is already optimized, alien-signals is still noticeably faster. (I wrote code for both, and since they share similar algorithms, they’re quite comparable.)

Benchmark repo: https://github.com/transitive-bullshit/js-reactivity-benchmark

Background

I spent considerable time optimizing Vue 3.4’s reactivity system, gaining experience along the way. Since Vue 3.5 switched to a pull-based algorithm similar to Preact, I decided to continue researching a push-pull based implementation in a separate project. Our end goal is to implement fully incremental AST parsing and virtual code generation in Vue language tools, based on alien-signals.

Other Language Implementations

Derived Projects

Rajaniraiyn/react-alien-signals: React bindings for the alien-signals API
CCherry07/alien-deepsignals: Use alien-signals with the interface of a plain JavaScript object
hunghg255/reactjs-signal: Share Store State with Signal Pattern
gn8-ai/universe-alien-signals: Enables simple use of the Alien Signals state management system in modern frontend frameworks
WebReflection/alien-signals: Preact signals like API and a class based approach for easy brand check
@lift-html/alien: Integrating alien-signals into lift-html

Adoption

vuejs/core: The core algorithm has been ported to v3.6 (PR: vuejs/core#12349)
statelyai/xstate: The core algorithm has been ported to implement the atom architecture (PR: statelyai/xstate#5250)
flamrdevs/xignal: Infrastructure for the reactive system
vuejs/language-tools: Used in the language-core package for virtual code generation
unuse: A framework-agnostic use library inspired by VueUse

Usage

Basic APIs

import { signal, computed, effect } from 'alien-signals';

const count = signal(1);
const doubleCount = computed(() => count() * 2);

effect(() => {
  console.log(`Count is: ${count()}`);
}); // Console: Count is: 1

console.log(doubleCount()); // 2

count(2); // Console: Count is: 2

console.log(doubleCount()); // 4

Effect Scope

import { signal, effect, effectScope } from 'alien-signals';

const count = signal(1);

const stopScope = effectScope(() => {
  effect(() => {
    console.log(`Count in scope: ${count()}`);
  }); // Console: Count in scope: 1
});

count(2); // Console: Count in scope: 2

stopScope();

count(3); // No console output

Manual Triggering

The trigger() function allows you to manually trigger updates for downstream dependencies when you've directly mutated a signal's value without using the signal setter:

import { signal, computed, trigger } from 'alien-signals';

const arr = signal<number[]>([]);
const length = computed(() => arr().length);

console.log(length()); // 0

// Direct mutation doesn't automatically trigger updates
arr().push(1);
console.log(length()); // Still 0

// Manually trigger updates
trigger(arr);
console.log(length()); // 1

You can also trigger multiple signals at once:

import { signal, computed, trigger } from 'alien-signals';

const src1 = signal<number[]>([]);
const src2 = signal<number[]>([]);
const total = computed(() => src1().length + src2().length);

src1().push(1);
src2().push(2);

trigger(() => {
  src1();
  src2();
});

console.log(total()); // 2

Creating Your Own Surface API

You can reuse alien-signals’ core algorithm via createReactiveSystem() to build your own signal API. For implementation examples, see:

Starter template (implements .get() & .set() methods like the Signals proposal)
stackblitz/alien-signals/src/index.ts
proposal-signals/signal-polyfill#44

About `propagate` and `checkDirty` functions

In order to eliminate recursive calls and improve performance, we record the last link node of the previous loop in propagate and checkDirty functions, and implement the rollback logic to return to this node.

This results in code that is difficult to understand, and you don't necessarily get the same performance improvements in other languages, so we record the original implementation without eliminating recursive calls here for reference.

`propagate`

function propagate(link: Link): void {
	do {
		const sub = link.sub;

		let flags = sub.flags;

		if (!(flags & (ReactiveFlags.RecursedCheck | ReactiveFlags.Recursed | ReactiveFlags.Dirty | ReactiveFlags.Pending))) {
			sub.flags = flags | ReactiveFlags.Pending;
		} else if (!(flags & (ReactiveFlags.RecursedCheck | ReactiveFlags.Recursed))) {
			flags = ReactiveFlags.None;
		} else if (!(flags & ReactiveFlags.RecursedCheck)) {
			sub.flags = (flags & ~ReactiveFlags.Recursed) | ReactiveFlags.Pending;
		} else if (!(flags & (ReactiveFlags.Dirty | ReactiveFlags.Pending)) && isValidLink(link, sub)) {
			sub.flags = flags | ReactiveFlags.Recursed | ReactiveFlags.Pending;
			flags &= ReactiveFlags.Mutable;
		} else {
			flags = ReactiveFlags.None;
		}

		if (flags & ReactiveFlags.Watching) {
			notify(sub);
		}

		if (flags & ReactiveFlags.Mutable) {
			const subSubs = sub.subs;
			if (subSubs !== undefined) {
				propagate(subSubs);
			}
		}

		link = link.nextSub!;
	} while (link !== undefined);
}

`checkDirty`

function checkDirty(link: Link, sub: ReactiveNode): boolean {
	do {
		const dep = link.dep;
		const depFlags = dep.flags;

		if (sub.flags & ReactiveFlags.Dirty) {
			return true;
		} else if ((depFlags & (ReactiveFlags.Mutable | ReactiveFlags.Dirty)) === (ReactiveFlags.Mutable | ReactiveFlags.Dirty)) {
			if (update(dep)) {
				const subs = dep.subs!;
				if (subs.nextSub !== undefined) {
					shallowPropagate(subs);
				}
				return true;
			}
		} else if ((depFlags & (ReactiveFlags.Mutable | ReactiveFlags.Pending)) === (ReactiveFlags.Mutable | ReactiveFlags.Pending)) {
			if (checkDirty(dep.deps!, dep)) {
				if (update(dep)) {
					const subs = dep.subs!;
					if (subs.nextSub !== undefined) {
						shallowPropagate(subs);
					}
					return true;
				}
			} else {
				dep.flags = depFlags & ~ReactiveFlags.Pending;
			}
		}

		link = link.nextDep!;
	} while (link !== undefined);

	return false;
}