From c34e44f7812b15f94990213da13d770f9214c832 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 22 Jan 2025 15:23:49 -0500
Subject: [PATCH] async props

---
 .../client/visitors/shared/component.js       | 81 +++++++++++++++----
 .../src/internal/client/dom/blocks/async.js   | 17 ++++
 packages/svelte/src/internal/client/index.js  |  1 +
 .../samples/async-prop/Child.svelte           |  4 +-
 .../samples/async-prop/_config.js             |  4 +-
 5 files changed, 86 insertions(+), 21 deletions(-)
 create mode 100644 packages/svelte/src/internal/client/dom/blocks/async.js

diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js
index 15e4f68e9e49..55f632e53054 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js
@@ -1,13 +1,19 @@
 /** @import { BlockStatement, Expression, ExpressionStatement, Identifier, MemberExpression, Pattern, Property, SequenceExpression, Statement } from 'estree' */
 /** @import { AST } from '#compiler' */
-/** @import { ComponentContext } from '../../types.js' */
+/** @import { ComponentContext, MemoizedExpression } from '../../types.js' */
 import { dev, is_ignored } from '../../../../../state.js';
 import { get_attribute_chunks, object } from '../../../../../utils/ast.js';
 import * as b from '../../../../../utils/builders.js';
-import { build_bind_this, memoize_expression, validate_binding } from '../shared/utils.js';
+import {
+	build_bind_this,
+	get_expression_id,
+	memoize_expression,
+	validate_binding
+} from '../shared/utils.js';
 import { build_attribute_value } from '../shared/element.js';
 import { build_event_handler } from './events.js';
 import { determine_slot } from '../../../../../utils/slot.js';
+import { create_derived } from '../../utils.js';
 
 /**
  * @param {AST.Component | AST.SvelteComponent | AST.SvelteSelf} node
@@ -40,6 +46,12 @@ export function build_component(node, component_name, context, anchor = context.
 	/** @type {Record<string, Expression[]>} */
 	const events = {};
 
+	/** @type {MemoizedExpression[]} */
+	const expressions = [];
+
+	/** @type {MemoizedExpression[]} */
+	const async_expressions = [];
+
 	/** @type {Property[]} */
 	const custom_css_props = [];
 
@@ -115,16 +127,21 @@ export function build_component(node, component_name, context, anchor = context.
 			(events[attribute.name] ||= []).push(handler);
 		} else if (attribute.type === 'SpreadAttribute') {
 			const expression = /** @type {Expression} */ (context.visit(attribute));
-			if (attribute.metadata.expression.has_state) {
-				let value = expression;
 
-				if (attribute.metadata.expression.has_call) {
-					const id = b.id(context.state.scope.generate('spread_element'));
-					context.state.init.push(b.var(id, b.call('$.derived', b.thunk(value))));
-					value = b.call('$.get', id);
-				}
-
-				props_and_spreads.push(b.thunk(value));
+			if (attribute.metadata.expression.has_state) {
+				props_and_spreads.push(
+					b.thunk(
+						attribute.metadata.expression.is_async || attribute.metadata.expression.has_call
+							? b.call(
+									'$.get',
+									get_expression_id(
+										attribute.metadata.expression.is_async ? async_expressions : expressions,
+										expression
+									)
+								)
+							: expression
+					)
+				);
 			} else {
 				props_and_spreads.push(expression);
 			}
@@ -133,10 +150,15 @@ export function build_component(node, component_name, context, anchor = context.
 				custom_css_props.push(
 					b.init(
 						attribute.name,
-						build_attribute_value(attribute.value, context, (value, metadata) =>
+						build_attribute_value(attribute.value, context, (value, metadata) => {
 							// TODO put the derived in the local block
-							metadata.has_call ? memoize_expression(context.state, value) : value
-						).value
+							return metadata.has_call || metadata.is_async
+								? b.call(
+										'$.get',
+										get_expression_id(metadata.is_async ? async_expressions : expressions, value)
+									)
+								: value;
+						}).value
 					)
 				);
 				continue;
@@ -154,7 +176,7 @@ export function build_component(node, component_name, context, anchor = context.
 				attribute.value,
 				context,
 				(value, metadata) => {
-					if (!metadata.has_state) return value;
+					if (!metadata.has_state && !metadata.is_async) return value;
 
 					// When we have a non-simple computation, anything other than an Identifier or Member expression,
 					// then there's a good chance it needs to be memoized to avoid over-firing when read within the
@@ -167,7 +189,12 @@ export function build_component(node, component_name, context, anchor = context.
 						);
 					});
 
-					return should_wrap_in_derived ? memoize_expression(context.state, value) : value;
+					return should_wrap_in_derived
+						? b.call(
+								'$.get',
+								get_expression_id(metadata.is_async ? async_expressions : expressions, value)
+							)
+						: value;
 				}
 			);
 
@@ -420,7 +447,12 @@ export function build_component(node, component_name, context, anchor = context.
 		};
 	}
 
-	const statements = [...snippet_declarations];
+	const statements = [
+		...snippet_declarations,
+		...expressions.map((memo) =>
+			b.let(memo.id, create_derived(context.state, b.thunk(memo.expression)))
+		)
+	];
 
 	if (node.type === 'SvelteComponent') {
 		const prev = fn;
@@ -457,5 +489,20 @@ export function build_component(node, component_name, context, anchor = context.
 		statements.push(b.stmt(fn(anchor)));
 	}
 
+	[...async_expressions, ...expressions].forEach((memo, i) => {
+		memo.id.name = `$${i}`;
+	});
+
+	if (async_expressions.length > 0) {
+		return b.stmt(
+			b.call(
+				'$.async',
+				anchor,
+				b.array(async_expressions.map(({ expression }) => b.thunk(expression, true))),
+				b.arrow([b.id('$$anchor'), ...async_expressions.map(({ id }) => id)], b.block(statements))
+			)
+		);
+	}
+
 	return statements.length > 1 ? b.block(statements) : statements[0];
 }
diff --git a/packages/svelte/src/internal/client/dom/blocks/async.js b/packages/svelte/src/internal/client/dom/blocks/async.js
new file mode 100644
index 000000000000..0ffeb0591b1c
--- /dev/null
+++ b/packages/svelte/src/internal/client/dom/blocks/async.js
@@ -0,0 +1,17 @@
+/** @import { TemplateNode, Value } from '#client' */
+
+import { async_derived } from '../../reactivity/deriveds.js';
+import { suspend } from './boundary.js';
+
+/**
+ * @param {TemplateNode} node
+ * @param {Array<() => Promise<any>>} expressions
+ * @param {(anchor: TemplateNode, ...deriveds: Value[]) => void} fn
+ */
+export function async(node, expressions, fn) {
+	// TODO handle hydration
+
+	suspend(Promise.all(expressions.map(async_derived))).then((result) => {
+		fn(node, ...result.exit());
+	});
+}
diff --git a/packages/svelte/src/internal/client/index.js b/packages/svelte/src/internal/client/index.js
index c9b259c4dfbb..842343a11932 100644
--- a/packages/svelte/src/internal/client/index.js
+++ b/packages/svelte/src/internal/client/index.js
@@ -14,6 +14,7 @@ export {
 export { check_target, legacy_api } from './dev/legacy.js';
 export { trace } from './dev/tracing.js';
 export { inspect } from './dev/inspect.js';
+export { async } from './dom/blocks/async.js';
 export { await_block as await } from './dom/blocks/await.js';
 export { if_block as if } from './dom/blocks/if.js';
 export { key_block as key } from './dom/blocks/key.js';
diff --git a/packages/svelte/tests/runtime-runes/samples/async-prop/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-prop/Child.svelte
index 00f8df7c0a89..85d212b1a835 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-prop/Child.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/async-prop/Child.svelte
@@ -1,5 +1,5 @@
 <script>
-	let { num } = $props();
+	let { value } = $props();
 </script>
 
-<p>{num}</p>
+<h1>{value}</h1>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-prop/_config.js b/packages/svelte/tests/runtime-runes/samples/async-prop/_config.js
index 91daba25a933..24882c56cd16 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-prop/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/async-prop/_config.js
@@ -22,7 +22,7 @@ export default test({
 		await Promise.resolve();
 		await Promise.resolve();
 		await tick();
-		assert.htmlEqual(target.innerHTML, '<p>hello</p>');
+		assert.htmlEqual(target.innerHTML, '<h1>hello</h1>');
 
 		d = deferred();
 		component.promise = d.promise;
@@ -32,6 +32,6 @@ export default test({
 		d.resolve('hello again');
 		await Promise.resolve();
 		await tick();
-		assert.htmlEqual(target.innerHTML, '<p>hello again</p>');
+		assert.htmlEqual(target.innerHTML, '<h1>hello again</h1>');
 	}
 });