From 31882d1d2d948611d6826b5e9bf55a1d7fad6aa0 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 28 Feb 2025 08:28:27 -0500
Subject: [PATCH] add `$effect.pending()`

---
 .../3-transform/client/visitors/CallExpression.js   |  3 +++
 .../3-transform/server/visitors/CallExpression.js   |  4 ++++
 packages/svelte/src/internal/client/index.js        |  1 +
 .../svelte/src/internal/client/reactivity/forks.js  | 13 +++++++++++++
 packages/svelte/src/utils.js                        |  1 +
 5 files changed, 22 insertions(+)

diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/CallExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/CallExpression.js
index 7a3057451a..e7e20dc150 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/CallExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/CallExpression.js
@@ -30,6 +30,9 @@ export function CallExpression(node, context) {
 				.../** @type {Expression[]} */ (node.arguments.map((arg) => context.visit(arg)))
 			);
 
+		case '$effect.pending':
+			return b.call('$.get', b.id('$.pending'));
+
 		case '$inspect':
 		case '$inspect().with':
 			return transform_inspect_rune(node, context);
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js
index 386c6b6ff3..727947be89 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js
@@ -25,6 +25,10 @@ export function CallExpression(node, context) {
 		return b.arrow([], b.block([]));
 	}
 
+	if (rune === '$effect.pending') {
+		return b.false;
+	}
+
 	if (rune === '$state.snapshot') {
 		return b.call(
 			'$.snapshot',
diff --git a/packages/svelte/src/internal/client/index.js b/packages/svelte/src/internal/client/index.js
index a20e1f67dc..fea7ac1ada 100644
--- a/packages/svelte/src/internal/client/index.js
+++ b/packages/svelte/src/internal/client/index.js
@@ -115,6 +115,7 @@ export {
 	user_effect,
 	user_pre_effect
 } from './reactivity/effects.js';
+export { pending } from './reactivity/forks.js';
 export { mutable_state, mutate, set, state, update, update_pre } from './reactivity/sources.js';
 export {
 	prop,
diff --git a/packages/svelte/src/internal/client/reactivity/forks.js b/packages/svelte/src/internal/client/reactivity/forks.js
index 19894db94f..1abefbfe34 100644
--- a/packages/svelte/src/internal/client/reactivity/forks.js
+++ b/packages/svelte/src/internal/client/reactivity/forks.js
@@ -1,6 +1,7 @@
 /** @import { Effect, Source } from '#client' */
 import { noop } from '../../shared/utils.js';
 import { flushSync } from '../runtime.js';
+import { internal_set, source } from './sources.js';
 
 /** @type {Set<Fork>} */
 const forks = new Set();
@@ -12,6 +13,12 @@ export function remove_active_fork() {
 	active_fork = null;
 }
 
+export let pending = source(false);
+
+function update_pending() {
+	internal_set(pending, forks.size > 0);
+}
+
 let uid = 1;
 
 export class Fork {
@@ -97,6 +104,8 @@ export class Fork {
 				}
 			}
 		}
+
+		update_pending();
 	}
 
 	/**
@@ -134,6 +143,10 @@ export class Fork {
 
 	static ensure() {
 		if (active_fork === null) {
+			if (forks.size === 0) {
+				requestAnimationFrame(update_pending);
+			}
+
 			active_fork = new Fork();
 			forks.add(active_fork); // TODO figure out where we remove this
 		}
diff --git a/packages/svelte/src/utils.js b/packages/svelte/src/utils.js
index d4d106d56d..bce4e091e2 100644
--- a/packages/svelte/src/utils.js
+++ b/packages/svelte/src/utils.js
@@ -441,6 +441,7 @@ const RUNES = /** @type {const} */ ([
 	'$effect.pre',
 	'$effect.tracking',
 	'$effect.root',
+	'$effect.pending',
 	'$inspect',
 	'$inspect().with',
 	'$inspect.trace',