create-subscriber.js

import { get, tick, untrack } from '../internal/client/runtime.js';
import { effect_tracking, render_effect } from '../internal/client/reactivity/effects.js';
import { source } from '../internal/client/reactivity/sources.js';
import { increment } from './utils.js';

/**
 * Returns a `subscribe` function that, if called in an effect (including expressions in the template),
 * calls its `start` callback with an `update` function. Whenever `update` is called, the effect re-runs.
 *
 * If `start` returns a function, it will be called when the effect is destroyed.
 *
 * If `subscribe` is called in multiple effects, `start` will only be called once as long as the effects
 * are active, and the returned teardown function will only be called when all effects are destroyed.
 *
 * It's best understood with an example. Here's an implementation of [`MediaQuery`](https://svelte.dev/docs/svelte/svelte-reactivity#MediaQuery):
 *
 * ```js
 * import { createSubscriber } from 'svelte/reactivity';
 * import { on } from 'svelte/events';
 *
 * export class MediaQuery {
 * 	#query;
 * 	#subscribe;
 *
 * 	constructor(query) {
 * 		this.#query = window.matchMedia(`(${query})`);
 *
 * 		this.#subscribe = createSubscriber((update) => {
 * 			// when the `change` event occurs, re-run any effects that read `this.current`
 * 			const off = on(this.#query, 'change', update);
 *
 * 			// stop listening when all the effects are destroyed
 * 			return () => off();
 * 		});
 * 	}
 *
 * 	get current() {
 * 		this.#subscribe();
 *
 * 		// Return the current state of the query, whether or not we're in an effect
 * 		return this.#query.matches;
 * 	}
 * }
 * ```
 * @param {(update: () => void) => (() => void) | void} start
 * @since 5.7.0
 */
export function createSubscriber(start) {
	let subscribers = 0;
	let version = source(0);
	/** @type {(() => void) | void} */
	let stop;

	return () => {
		if (effect_tracking()) {
			get(version);

			render_effect(() => {
				if (subscribers === 0) {
					stop = untrack(() => start(() => increment(version)));
				}

				subscribers += 1;

				return () => {
					tick().then(() => {
						// Only count down after timeout, else we would reach 0 before our own render effect reruns,
						// but reach 1 again when the tick callback of the prior teardown runs. That would mean we
						// re-subcribe unnecessarily and create a memory leak because the old subscription is never cleaned up.
						subscribers -= 1;

						if (subscribers === 0) {
							stop?.();
							stop = undefined;
						}
					});
				};
			});
		}
	};
}