add invalidate fn

packages/svelte/src/ambient.d.ts

@@ -144,8 +144,8 @@ declare namespace $state {
 	 *
 	 * @param initial The initial value
 	 */
-	export function opaque<T>(initial: T): [T, () => void];
+	export function opaque<T>(initial: T): [T, (mutate?: (value: T) => void) => void];
-	export function opaque<T>(): [T | undefined, () => void];
+	export function opaque<T>(): [T | undefined, (mutate?: (value: T) => void) => void];
 
 	/**
 	 * To take a static snapshot of a deeply reactive `$state` proxy, use `$state.snapshot`:

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

@@ -164,7 +164,13 @@ export function VariableDeclaration(node, context) {
 					b.declarator(state_id, b.call('$.opaque_state', value)),
 					b.declarator(
 						invalidation_id,
-						b.thunk(b.call('$.set', state_id, b.member(state_id, b.id('v'))))
+						b.arrow(
+							[b.id('$$fn')],
+							b.sequence([
+								b.call(b.id('$$fn'), b.member(state_id, b.id('v'))),
+								b.call('$.set', state_id, b.member(state_id, b.id('v')))
+							])
+						)
 					)
 				);
 				continue;

packages/svelte/src/compiler/phases/3-transform/server/visitors/VariableDeclaration.js

@@ -98,7 +98,7 @@ export function VariableDeclaration(node, context) {
 				const invalidation_id = /** @type {Identifier} */ (pattern.elements[1]);
 				declarations.push(
 					b.declarator(state_id, value),
-					b.declarator(invalidation_id, b.thunk(b.block([])))
+					b.declarator(invalidation_id, b.arrow([b.id('$$fn')], b.call(b.id('$$fn'), state_id)))
 				);
 				continue;
 			}

packages/svelte/tests/runtime-runes/samples/opaque-state-fn/Child.svelte

@@ -0,0 +1,13 @@
+<script>
+	let { count } = $props();
+
+	let [obj, invalidate] = $state.opaque({ count: 0 });
+
+	$effect(() => {
+		invalidate((obj) => {
+			obj.count = count;
+		});
+	});
+</script>
+
+<p>{obj.count}</p>

packages/svelte/tests/runtime-runes/samples/opaque-state-fn/_config.js

@@ -0,0 +1,18 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<p>0</p><button>+1</button>`,
+
+	test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		button?.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, `<p>1</p><button>+1</button>`);
+
+		button?.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, `<p>2</p><button>+1</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/opaque-state-fn/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	import Child from './Child.svelte';
+
+	let count = $state(0);
+</script>
+
+<Child {count} />
+<button onclick={() => count += 1}>+1</button>

packages/svelte/types/index.d.ts

@@ -2691,8 +2691,8 @@ declare namespace $state {
 	 *
 	 * @param initial The initial value
 	 */
-	export function opaque<T>(initial: T): [T, () => void];
+	export function opaque<T>(initial: T): [T, (mutate?: (value: T) => void) => void];
-	export function opaque<T>(): [T | undefined, () => void];
+	export function opaque<T>(): [T | undefined, (mutate?: (value: T) => void) => void];
 
 	/**
 	 * To take a static snapshot of a deeply reactive `$state` proxy, use `$state.snapshot`: