Function fork

packages/svelte/src/internal/client/reactivity/batch.js:1369–1441

(fn)

 * @since 5.42
 */
export function fork(fn) {
	if (!async_mode_flag) {
		e.experimental_async_required('fork');
	}

	if (current_batch !== null) {
		e.fork_timing();
	}

	var batch = Batch.ensure();
	batch.is_fork = true;
	batch_values = new Map();

	var committed = false;
	var settled = batch.settled();

	flushSync(fn);

	return {
		commit: async () => {
			if (committed) {
				await settled;
				return;
			}

			if (!batch.linked) {
				e.fork_discarded();
			}

			committed = true;

			batch.is_fork = false;

			// apply changes and update write versions so deriveds see the change
			for (var [source, [value]] of batch.current) {
				source.v = value;
				source.wv = increment_write_version();
			}

			// trigger any `$state.eager(...)` expressions with the new state.
			// eager effects don't get scheduled like other effects, so we
			// can't just encounter them during traversal, we need to
			// proactively flush them
			// TODO maybe there's a better implementation?
			flushSync(() => {
				/** @type {Set<Effect>} */
				var eager_effects = new Set();

				for (var source of batch.current.keys()) {
					mark_eager_effects(source, eager_effects);
				}

				set_eager_effects(eager_effects);
				flush_eager_effects();
			});

			batch.flush();
			await settled;