fix: improve behaviour of unowned derived signals (#11521)

.changeset/fluffy-ravens-juggle.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve behaviour of unowned derived signals

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

@@ -85,7 +85,7 @@ function destroy_derived_children(signal) {
 /**
  * @param {import('#client').Derived} derived
  * @param {boolean} force_schedule
- * @returns {void}
+ * @returns {boolean}
  */
 export function update_derived(derived, force_schedule) {
 	var previous_updating_derived = updating_derived;
@@ -101,7 +101,9 @@ export function update_derived(derived, force_schedule) {
 
 	set_signal_status(derived, status);
 
-	if (!derived.equals(value)) {
+	var is_equal = derived.equals(value);
+
+	if (!is_equal) {
 		derived.v = value;
 		mark_reactions(derived, DIRTY, force_schedule);
 
@@ -109,6 +111,8 @@ export function update_derived(derived, force_schedule) {
 			for (var fn of /** @type {import('#client').DerivedDebug} */ (derived).inspect) fn();
 		}
 	}
+
+	return is_equal;
 }
 
 /**

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

@@ -191,12 +191,13 @@ export function check_dirtiness(reaction) {
 
 		if (dependencies !== null) {
 			var length = dependencies.length;
+			var is_equal;
 
 			for (var i = 0; i < length; i++) {
 				var dependency = dependencies[i];
 
 				if (!is_dirty && check_dirtiness(/** @type {import('#client').Derived} */ (dependency))) {
-					update_derived(/** @type {import('#client').Derived} **/ (dependency), true);
+					is_equal = update_derived(/** @type {import('#client').Derived} **/ (dependency), true);
 				}
 
 				if (is_unowned) {
@@ -208,7 +209,7 @@ export function check_dirtiness(reaction) {
 
 					if (version > /** @type {import('#client').Derived} */ (reaction).version) {
 						/** @type {import('#client').Derived} */ (reaction).version = version;
-						return true;
+						return !is_equal;
 					}
 
 					if (!current_skip_reaction && !dependency?.reactions?.includes(reaction)) {

packages/svelte/tests/runtime-runes/samples/derived-unowned-7/_config.js

@@ -0,0 +1,14 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		let [btn1] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			btn1.click();
+		});
+
+		assert.deepEqual(logs, ['computing C', 'computing B', 'a', 'foo', 'computing B', 'aaa', 'foo']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/derived-unowned-7/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	function run(){
+		let a = $state('a');
+		let b = $derived.by(() => {
+			console.log('computing B', a);
+			return 'foo';
+		});
+		let c = $derived.by(() => {
+			console.log('computing C');
+			return b;
+		});
+
+		console.log(c);
+		a = "aaa";
+		console.log(c);
+	}
+</script>
+
+<button onclick={run}>RUN THE THING</button>