[Signal] 2- Cleanup subscriptions

• 1. Basic version of signals

We have a basic version of signal:

const stack = []

export function createSignal(value) {
  const subscribers = new Set();
  const read = () => {
    // check is there any effect runnning?
    // if yes, then add to subscribers
    const observer = stack[stack.length - 1];
    if (observer) {
      subscribers.add(observer);
    }
    return value;
  };
  const write = (newValue) => {
    value = newValue;
    // notify all the subscribers there is changes
    for (const observer of subscribers) {
      // add effect to the stack again
      observer.execute();
    }
  };
  return [read, write];
}

export function createEffect(fn) {
    const effect = {
       execute() {
           stack.push(effect);
           fn();
           stack.pop();
       }
    };
    
    effect.execute();
}

One problem of current version is that, subscriptions are not cleanup. Therefore it might be triggered when not necessary.

import { createSignal, createEffect } from "./reactive";

const [count, setCount] = createSignal(0);
const [count2, setCount2] = createSignal(2);
const [show, setShow] = createSignal(true);

createEffect(() => {
  if (show()) console.log(count());
  else console.log(count2());
}); 

setShow(false);
setCount(10);

// 0
// 2
// 2

It logs 2 twice. Which is a signals cleanup is missing.

Version 3: with cleanup

In previous version, signals knows effect, but effect doesn't know signals.

We need to link bidirectional.

export function createEffect(fn) {
  const effect = {
    execute() {
      context.push(effect);
      fn();
      context.pop();
    },
    dependencies: new Set(), // add dependencies to link to signals
  };

  effect.execute();
}

When read from signals, we need to not only link effect to signals, but also link signals to effect:

function subscribe(observer, subscriptions) {
    subscriptions.add(observer)
    observer.dependencies.add(subscriptions)
}

export function createSignal(value) {
  const subscriptions = new Set();

  const read = () => {
    const observer = context[context.length - 1];
    if (observer) subscribe(observer, subscriptions); // link bidirectional
    return value;
  };
  const write = (newValue) => {
    value = newValue;
    for (const observer of [...subscriptions]) { // need to use [...subscriptions] to make a copy to avoid infinte loop
      observer.execute();
    }
  };

  return [read, write];
}

Write cleanup:

function cleanup(observer) {
  for (const dep of observer.dependencies) {
    dep.delete(observer);
  }
}

export function createEffect(fn) {
  const effect = {
    execute() {
      cleanup(effect); // call cleanup before next effect start
      context.push(effect);
      fn();
      context.pop();
    },
    dependencies: new Set(),
  };

  effect.execute();
}

---FULL CODE---

const context = [];

function cleanup(observer) {
  for (const dep of observer.dependencies) {
    dep.delete(observer);
  }
}

function subscribe(observer, subscriptions) {
  subscriptions.add(observer);
  observer.dependencies.add(subscriptions);
}

export function createSignal(value) {
  const subscriptions = new Set();

  const read = () => {
    const observer = context[context.length - 1];
    if (observer) subscribe(observer, subscriptions);
    return value;
  };
  const write = (newValue) => {
    value = newValue;
    for (const observer of [...subscriptions]) {
      observer.execute();
    }
  };

  return [read, write];
}

export function createEffect(fn) {
  const effect = {
    execute() {
      cleanup(effect);
      context.push(effect);
      fn();
      context.pop();
    },
    dependencies: new Set(),
  };

  effect.execute();
}

Test code:

import { createSignal, createEffect } from "./reactive";

const [count, setCount] = createSignal(0);
const [count2, setCount2] = createSignal(2);
const [show, setShow] = createSignal(true);

createEffect(() => {
  if (show()) console.log(count());
  else console.log(count2());
}); 

setShow(false);
setCount(10);

// 0
// 2