chore: more efficient effect scheduling

.changeset/rotten-taxes-float.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: more efficient effect scheduling

packages/svelte/src/internal/client/reactivity/batch.js

@@ -69,6 +69,14 @@ let last_scheduled_effect = null;
 let is_flushing = false;
 export let is_flushing_sync = false;
 
+/**
+ * During traversal, this is an array. Newly created effects are (if not immediately
+ * executed) pushed to this array, rather than going through the scheduling
+ * rigamarole that would cause another turn of the flush loop.
+ * @type {Effect[] | null}
+ */
+export let collected_effects = null;
+
 export class Batch {
 	/**
 	 * The current values of any sources that are updated in this batch
@@ -185,7 +193,7 @@ export class Batch {
 		this.apply();
 
 		/** @type {Effect[]} */
-		var effects = [];
+		var effects = (collected_effects = []);
 
 		/** @type {Effect[]} */
 		var render_effects = [];
@@ -199,6 +207,8 @@ export class Batch {
 			// log_inconsistent_branches(root);
 		}
 
+		collected_effects = null;
+
 		if (this.#is_deferred()) {
 			this.#defer_effects(render_effects);
 			this.#defer_effects(effects);
@@ -631,6 +641,7 @@ function flush_effects() {
 
 		is_flushing = false;
 		last_scheduled_effect = null;
+		collected_effects = null;
 
 		if (DEV) {
 			for (const source of /** @type {Set<Source>} */ (source_stacks)) {
@@ -836,13 +847,7 @@ export function schedule_effect(signal) {
 		// if the effect is being scheduled because a parent (each/await/etc) block
 		// updated an internal source, or because a branch is being unskipped,
 		// bail out or we'll cause a second flush
-		if (
-			is_flushing &&
-			effect === active_effect &&
-			(flags & BLOCK_EFFECT) !== 0 &&
-			(flags & HEAD_EFFECT) === 0 &&
-			(flags & REACTION_RAN) !== 0
-		) {
+		if (collected_effects !== null && effect === active_effect) {
 			return;
 		}
 

packages/svelte/src/internal/client/reactivity/effects.js

@@ -40,7 +40,7 @@ import { DEV } from 'esm-env';
 import { define_property } from '../../shared/utils.js';
 import { get_next_sibling } from '../dom/operations.js';
 import { component_context, dev_current_component_function, dev_stack } from '../context.js';
-import { Batch, schedule_effect } from './batch.js';
+import { Batch, collected_effects, schedule_effect } from './batch.js';
 import { flatten, increment_pending } from './async.js';
 import { without_reactive_context } from '../dom/elements/bindings/shared.js';
 import { set_signal_status } from './status.js';
@@ -119,15 +119,19 @@ function create_effect(type, fn, sync) {
 		effect.component_function = dev_current_component_function;
 	}
 
-	if (sync) {
-		try {
-			update_effect(effect);
-		} catch (e) {
-			destroy_effect(effect);
-			throw e;
+	if (fn !== null) {
+		if (sync) {
+			try {
+				update_effect(effect);
+			} catch (e) {
+				destroy_effect(effect);
+				throw e;
+			}
+		} else if (collected_effects !== null) {
+			collected_effects.push(effect);
+		} else {
+			schedule_effect(effect);
 		}
-	} else if (fn !== null) {
-		schedule_effect(effect);
 	}
 
 	/** @type {Effect | null} */