Function createSubscriber

packages/svelte/src/reactivity/create-subscriber.js:54–95

(start)

 * @since 5.7.0
 */
export function createSubscriber(start) {
	let subscribers = 0;
	let version = source(0);
	/** @type {(() => void) | void} */
	let stop;

	if (DEV) {
		tag(version, class="st">'createSubscriber version');
	}

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

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

				subscribers += 1;

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

						if (subscribers === 0) {
							stop?.();
							stop = undefined;
							class="cm">// Increment the version to ensure any dependent deriveds are marked dirty when the subscription is picked up again later.
							class="cm">// If we didn't do this then the comparison of write versions would determine that the derived has a later version than
							class="cm">// the subscriber, and it would not be re-run.
							increment(version);
						}
					});
				};
			});
		}
	};
}