fix: improve bind:this support for each blocks (#10510)

.changeset/cool-rabbits-tickle.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve bind:this support for each blocks

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -972,7 +972,11 @@ function serialize_inline_component(node, component_name, context) {
 		const assignment = b.assignment('=', bind_this, b.id('$$value'));
 		const bind_this_id = /** @type {import('estree').Expression} */ (
 			// if expression is not an identifier, we know it can't be a signal
-			bind_this.type === 'Identifier' ? bind_this : undefined
+			bind_this.type === 'Identifier'
+				? bind_this
+				: bind_this.type === 'MemberExpression' && bind_this.object.type === 'Identifier'
+					? bind_this.object
+					: undefined
 		);
 		fn = (node_id) =>
 			b.call(
@@ -2742,7 +2746,11 @@ export const template_visitors = {
 						setter,
 						/** @type {import('estree').Expression} */ (
 							// if expression is not an identifier, we know it can't be a signal
-							expression.type === 'Identifier' ? expression : undefined
+							expression.type === 'Identifier'
+								? expression
+								: expression.type === 'MemberExpression' && expression.object.type === 'Identifier'
+									? expression.object
+									: undefined
 						)
 					);
 					break;

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

@@ -69,7 +69,7 @@ import {
 } from './utils.js';
 import { is_promise } from '../common.js';
 import { bind_transition, trigger_transitions } from './transitions.js';
-import { proxy } from './proxy.js';
+import { STATE_SYMBOL, proxy } from './proxy.js';
 
 /** @type {Set<string>} */
 const all_registerd_events = new Set();
@@ -1295,6 +1295,13 @@ export function bind_prop(props, prop, value) {
 	}
 }
 
+/**
+ * @param {unknown} value
+ */
+function is_state_object(value) {
+	return value != null && typeof value === 'object' && STATE_SYMBOL in value;
+}
+
 /**
  * @param {Element} element_or_component
  * @param {(value: unknown) => void} update
@@ -1302,9 +1309,15 @@ export function bind_prop(props, prop, value) {
  * @returns {void}
  */
 export function bind_this(element_or_component, update, binding) {
-	untrack(() => {
-		update(element_or_component);
-		render_effect(() => () => {
+	render_effect(() => {
+		// If we are reading from a proxied state binding, then we don't need to untrack
+		// the update function as it will be fine-grain.
+		if (is_state_object(binding) || (is_signal(binding) && is_state_object(binding.v))) {
+			update(element_or_component);
+		} else {
+			untrack(() => update(element_or_component));
+		}
+		return () => {
 			// Defer to the next tick so that all updates can be reconciled first.
 			// This solves the case where one variable is shared across multiple this-bindings.
 			render_effect(() => {
@@ -1314,7 +1327,7 @@ export function bind_this(element_or_component, update, binding) {
 					}
 				});
 			});
-		});
+		};
 	});
 }
 

packages/svelte/tests/runtime-runes/samples/state-each-bind-this/Paragraph.svelte

@@ -0,0 +1,13 @@
+<script>
+	const { text } = $props();
+
+	let boundParagraph = $state();
+
+	export function changeBackgroundToRed() {
+		boundParagraph.style.backgroundColor = 'red';
+	}
+</script>
+
+<p bind:this={boundParagraph}>
+	{text}
+</p>

packages/svelte/tests/runtime-runes/samples/state-each-bind-this/_config.js

@@ -0,0 +1,35 @@
+import { flushSync } from '../../../../src/main/main-client';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [btn, btn2, btn3] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<div><p style="background-color: red;">b1</p><button>change</button><button>delete</button></div><div><p>b2</p><button>change</button><button>delete</button></div>`
+		);
+
+		flushSync(() => {
+			btn2?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<div><p>b2</p><button>change</button><button>delete</button></div>`
+		);
+
+		flushSync(() => {
+			btn3?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<div><p style="background-color: red;">b2</p><button>change</button><button>delete</button></div>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/state-each-bind-this/main.svelte

@@ -0,0 +1,21 @@
+<script>
+	import Paragraph from './Paragraph.svelte';
+	let boundParagraphs = $state([]);
+
+	let store = $state([
+		{ id: 1, text: 'b1' },
+		{ id: 2, text: 'b2' }
+	]);
+</script>
+
+{#each store as text, i (text.id)}
+	<div>
+		<Paragraph bind:this={boundParagraphs[i]} text={text.text}></Paragraph>
+		<button onclick={() => boundParagraphs[i].changeBackgroundToRed()}>
+			change
+		</button>
+		<button onclick={() => store.splice(store.indexOf(text), 1)}>
+			delete
+		</button>
+	</div>
+{/each}