From 591aeb0cbc18ffb383faee8766a9c274a0b84ee1 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 26 Jun 2025 12:14:55 -0400
Subject: [PATCH] groundwork for async attribute_effect

---
 .../client/visitors/shared/element.js         |  3 ++-
 .../client/dom/elements/attributes.js         | 27 ++++++++++++++++---
 .../src/internal/client/reactivity/effects.js |  6 ++---
 3 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js
index 869dd16956..b1b1a8fae4 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js
@@ -96,8 +96,9 @@ export function build_attribute_effect(
 					expressions.map(({ id }) => id),
 					b.object(values)
 				),
-				// TODO need to handle async expressions too
 				expressions.length > 0 && b.array(expressions.map(({ expression }) => b.thunk(expression))),
+				async_expressions.length > 0 &&
+					b.array(async_expressions.map(({ expression }) => b.thunk(expression))),
 				element.metadata.scoped &&
 					context.state.analysis.css.hash !== '' &&
 					b.literal(context.state.analysis.css.hash),
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index 2d3d6a921d..83035cc6a1 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -1,4 +1,4 @@
-/** @import { Effect } from '#client' */
+/** @import { Effect, Value } from '#client' */
 import { DEV } from 'esm-env';
 import { hydrating, set_hydrating } from '../hydration.js';
 import { get_descriptors, get_prototype_of } from '../../../shared/utils.js';
@@ -462,20 +462,39 @@ export function set_attributes(element, prev, next, css_hash, skip_warning = fal
 /**
  * @param {Element & ElementCSSInlineStyle} element
  * @param {(...expressions: any) => Record<string | symbol, any>} fn
- * @param {Array<() => any>} thunks
+ * @param {Array<() => any>} sync
+ * @param {Array<() => Promise<any>>} async
  * @param {string} [css_hash]
  * @param {boolean} [skip_warning]
  */
 export function attribute_effect(
 	element,
 	fn,
-	thunks = [],
+	sync = [],
+	async = [],
 	css_hash,
 	skip_warning = false,
 	d = derived
 ) {
-	const deriveds = thunks.map(d);
+	const deriveds = sync.map(d);
 
+	create_attribute_effect(element, fn, deriveds, css_hash, skip_warning);
+}
+
+/**
+ * @param {Element & ElementCSSInlineStyle} element
+ * @param {(...expressions: any) => Record<string | symbol, any>} fn
+ * @param {Value[]} deriveds
+ * @param {string} [css_hash]
+ * @param {boolean} [skip_warning]
+ */
+export function create_attribute_effect(
+	element,
+	fn,
+	deriveds = [],
+	css_hash,
+	skip_warning = false
+) {
 	/** @type {Record<string | symbol, any> | undefined} */
 	var prev = undefined;
 
diff --git a/packages/svelte/src/internal/client/reactivity/effects.js b/packages/svelte/src/internal/client/reactivity/effects.js
index 15f16a0691..1bef217476 100644
--- a/packages/svelte/src/internal/client/reactivity/effects.js
+++ b/packages/svelte/src/internal/client/reactivity/effects.js
@@ -341,10 +341,10 @@ export function render_effect(fn, flags = 0) {
  * @param {<T>(fn: () => T) => Derived<T>} d
  */
 export function template_effect(fn, sync = [], async = [], d = derived) {
-	var batch = current_batch;
-	var parent = /** @type {Effect} */ (active_effect);
-
 	if (async.length > 0) {
+		var batch = current_batch;
+		var parent = /** @type {Effect} */ (active_effect);
+
 		var restore = capture();
 
 		Promise.all(async.map((expression) => async_derived(expression))).then((result) => {