fix: exclude derived writes from effect abort and rescheduling

Author: paoloricciuti

.changeset/thick-mice-kick.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: exclude derived writes from effect abort and rescheduling

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

@@ -31,6 +31,7 @@ import { invoke_error_boundary } from '../error-handling.js';
 import { old_values } from './sources.js';
 import { unlink_effect } from './effects.js';
 import { unset_context } from './async.js';
+import { get_derived_writes, reset_derived_writes } from './deriveds.js';
 
 /** @type {Set<Batch>} */
 const batches = new Set();
@@ -546,6 +547,12 @@ function flush_queued_effects(effects) {
 			if (is_dirty(effect)) {
 				var wv = write_version;
 
+				// updating a derived for also increase the write version but that doesn't mean
+				// state was written to in the user effect...so we reset the derived writes
+				// before running the effect so that we can subtract the amount of derived writes
+				// from the write version when we detect if state was written to in the user effect
+				reset_derived_writes();
+
 				update_effect(effect);
 
 				// Effects with no dependencies or teardown do not get added to the effect tree.
@@ -567,7 +574,7 @@ function flush_queued_effects(effects) {
 
 				// if state is written in a user effect, abort and re-schedule, lest we run
 				// effects that should be removed as a result of the state change
-				if (write_version > wv && (effect.f & USER_EFFECT) !== 0) {
+				if (write_version - get_derived_writes() > wv && (effect.f & USER_EFFECT) !== 0) {
 					break;
 				}
 			}

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

@@ -323,6 +323,20 @@ export function execute_derived(derived) {
 	return value;
 }
 
+// in process_effects if state is written to in a user effect we reschedule the rest of the
+// tree. However if a derived is updated in an effect we also increase the write version
+// so we need to keep track of how many deriveds were written to in the effect so
+// that we can subtract that from the write version before rescheduling unnnecessarily
+let derived_writes = 0;
+
+export function get_derived_writes() {
+	return derived_writes;
+}
+
+export function reset_derived_writes() {
+	derived_writes = 0;
+}
+
 /**
  * @param {Derived} derived
  * @returns {void}
@@ -333,6 +347,7 @@ export function update_derived(derived) {
 	if (!derived.equals(value)) {
 		derived.v = value;
 		derived.wv = increment_write_version();
+		derived_writes++;
 	}
 
 	// don't mark derived clean if we're reading it inside a

packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/Component.svelte

@@ -0,0 +1,7 @@
+<script>
+	const der = $derived(false);
+
+	$effect(() => {
+		der
+	});
+</script>

packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	async test() {}
+});

packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	import Component from './Component.svelte'	
+	const arr = Array.from({length: 10001});
+</script>
+
+{#each arr}
+	<Component />
+{/each}