add a way to run code when the current reaction finishes updating

packages/svelte/src/internal/client/runtime.js

@@ -1,6 +1,12 @@
 /** @import { Derived, Effect, Reaction, Signal, Source, Value } from '#client' */
 import { DEV } from 'esm-env';
-import { define_property, get_descriptors, get_prototype_of, index_of } from '../shared/utils.js';
+import {
+	define_property,
+	get_descriptors,
+	get_prototype_of,
+	index_of,
+	run_all
+} from '../shared/utils.js';
 import {
 	destroy_block_effect_children,
 	destroy_effect_children,
@@ -56,6 +62,17 @@ export function set_is_destroying_effect(value) {
 	is_destroying_effect = value;
 }
 
+/** @type {Array<() => void> | null} */
+let updated_callbacks = null;
+
+/**
+ * Run `fn` when the current reaction finishes updating
+ * @param {() => void} fn
+ */
+export function onupdated(fn) {
+	(updated_callbacks ??= []).push(fn);
+}
+
 // Handle effect queues
 
 /** @type {Effect[]} */
@@ -265,6 +282,7 @@ export function update_reaction(reaction) {
 	var previous_reaction_sources = source_ownership;
 	var previous_component_context = component_context;
 	var previous_untracking = untracking;
+	var previous_updated_callbacks = updated_callbacks;
 
 	var flags = reaction.f;
 
@@ -358,6 +376,10 @@ export function update_reaction(reaction) {
 	} catch (error) {
 		handle_error(error);
 	} finally {
+		if (updated_callbacks !== null) {
+			run_all(updated_callbacks);
+		}
+
 		new_deps = previous_deps;
 		skipped_deps = previous_skipped_deps;
 		untracked_writes = previous_untracked_writes;
@@ -366,6 +388,7 @@ export function update_reaction(reaction) {
 		source_ownership = previous_reaction_sources;
 		set_component_context(previous_component_context);
 		untracking = previous_untracking;
+		updated_callbacks = previous_updated_callbacks;
 
 		reaction.f ^= EFFECT_IS_UPDATING;
 	}

packages/svelte/src/reactivity/map.js

@@ -2,7 +2,7 @@
 import { DEV } from 'esm-env';
 import { set, source, state } from '../internal/client/reactivity/sources.js';
 import { label, tag } from '../internal/client/dev/tracing.js';
-import { active_reaction, get } from '../internal/client/runtime.js';
+import { active_reaction, get, onupdated } from '../internal/client/runtime.js';
 import { increment } from './utils.js';
 import { teardown } from '../internal/client/reactivity/effects.js';
 
@@ -68,9 +68,10 @@ export class SvelteMap extends Map {
 
 		if (active_reaction !== null) {
 			this.#initial_reaction = active_reaction;
+
 			// since we only need `initial_reaction` as long as we are in a derived/effect we can
 			// safely create a teardown function that will reset it to null and allow for GC
-			teardown(() => {
+			onupdated(() => {
 				this.#initial_reaction = null;
 			});
 		}

packages/svelte/src/reactivity/set.js

@@ -2,7 +2,7 @@
 import { DEV } from 'esm-env';
 import { source, set, state } from '../internal/client/reactivity/sources.js';
 import { label, tag } from '../internal/client/dev/tracing.js';
-import { active_reaction, get } from '../internal/client/runtime.js';
+import { active_reaction, get, onupdated } from '../internal/client/runtime.js';
 import { increment } from './utils.js';
 import { teardown } from '../internal/client/reactivity/effects.js';
 
@@ -63,9 +63,10 @@ export class SvelteSet extends Set {
 
 		if (active_reaction !== null) {
 			this.#initial_reaction = active_reaction;
+
 			// since we only need `initial_reaction` as long as we are in a derived/effect we can
 			// safely create a teardown function that will reset it to null and allow for GC
-			teardown(() => {
+			onupdated(() => {
 				this.#initial_reaction = null;
 			});
 		}