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) 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.)

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.

Derived Projects

• YanqingXu/alien-signals-in-lua: Lua implementation of alien-signals
• medz/alien-signals-dart: alien-signals Dart implementation of alien-signals
• 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

Adoption

• vuejs/core: The core algorithm has been ported to 3.6 or higher (PR：vuejs/core#12349)
• vuejs/language-tools: Used in the language-core package for virtual code generation

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, 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

Creating Your Own Public API

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

• https://github.com/stackblitz/alien-signals/blob/master/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, targetFlag = SubscriberFlags.Dirty): void {
	do {
		const sub = link.sub;
		const subFlags = sub.flags;

		if (
			(
				!(subFlags & (SubscriberFlags.Tracking | SubscriberFlags.Recursed | SubscriberFlags.Propagated))
				&& (sub.flags = subFlags | targetFlag | SubscriberFlags.Notified, true)
			)
			|| (
				(subFlags & SubscriberFlags.Recursed)
				&& !(subFlags & SubscriberFlags.Tracking)
				&& (sub.flags = (subFlags & ~SubscriberFlags.Recursed) | targetFlag | SubscriberFlags.Notified, true)
			)
			|| (
				!(subFlags & SubscriberFlags.Propagated)
				&& isValidLink(link, sub)
				&& (
					sub.flags = subFlags | SubscriberFlags.Recursed | targetFlag | SubscriberFlags.Notified,
					(sub as Dependency).subs !== undefined
				)
			)
		) {
			const subSubs = (sub as Dependency).subs;
			if (subSubs !== undefined) {
				propagate(
					subSubs,
					subFlags & SubscriberFlags.Effect
						? SubscriberFlags.PendingEffect
						: SubscriberFlags.PendingComputed
				);
			} else if (subFlags & SubscriberFlags.Effect) {
				if (queuedEffectsTail !== undefined) {
					queuedEffectsTail.depsTail!.nextDep = sub.deps;
				} else {
					queuedEffects = sub;
				}
				queuedEffectsTail = sub;
			}
		} else if (!(subFlags & (SubscriberFlags.Tracking | targetFlag))) {
			sub.flags = subFlags | targetFlag | SubscriberFlags.Notified;
			if ((subFlags & (SubscriberFlags.Effect | SubscriberFlags.Notified)) === SubscriberFlags.Effect) {
				if (queuedEffectsTail !== undefined) {
					queuedEffectsTail.depsTail!.nextDep = sub.deps;
				} else {
					queuedEffects = sub;
				}
				queuedEffectsTail = sub;
			}
		} else if (
			!(subFlags & targetFlag)
			&& (subFlags & SubscriberFlags.Propagated)
			&& isValidLink(link, sub)
		) {
			sub.flags = subFlags | targetFlag;
		}

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

checkDirty

function checkDirty(link: Link): boolean {
	do {
		const dep = link.dep;
		if ('flags' in dep) {
			const depFlags = dep.flags;
			if ((depFlags & (SubscriberFlags.Computed | SubscriberFlags.Dirty)) === (SubscriberFlags.Computed | SubscriberFlags.Dirty)) {
				if (updateComputed(dep)) {
					const subs = dep.subs!;
					if (subs.nextSub !== undefined) {
						shallowPropagate(subs);
					}
					return true;
				}
			} else if ((depFlags & (SubscriberFlags.Computed | SubscriberFlags.PendingComputed)) === (SubscriberFlags.Computed | SubscriberFlags.PendingComputed)) {
				if (checkDirty(dep.deps!)) {
					if (updateComputed(dep)) {
						const subs = dep.subs!;
						if (subs.nextSub !== undefined) {
							shallowPropagate(subs);
						}
						return true;
					}
				} else {
					dep.flags = depFlags & ~SubscriberFlags.PendingComputed;
				}
			}
		}
		link = link.nextDep!;
	} while (link !== undefined);

	return false;
}