fix: wait a microtask for await blocks to reduce UI churn

.changeset/gentle-eagles-walk.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: wait a microtask for await blocks to reduce UI churn

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

@@ -1,5 +1,6 @@
 import * as b from '../../../utils/builders.js';
 import {
+	extract_identifiers,
 	extract_paths,
 	is_expression_async,
 	is_simple_expression,
@@ -684,3 +685,44 @@ export function with_loc(target, source) {
 	}
 	return target;
 }
+
+/**
+ * @param {import("estree").Pattern} node
+ * @param {import("zimmerframe").Context<import("#compiler").SvelteNode, import("./types").ComponentClientTransformState>} context
+ * @returns {{ id: import("estree").Pattern, declarations: null | import("estree").Statement[] }}
+ */
+export function create_derived_block_argument(node, context) {
+	if (node.type === 'Identifier') {
+		return { id: node, declarations: null };
+	}
+
+	const pattern = /** @type {import('estree').Pattern} */ (context.visit(node));
+	const identifiers = extract_identifiers(node);
+
+	const id = b.id('$$source');
+	const value = b.id('$$value');
+
+	const block = b.block([
+		b.var(pattern, b.call('$.get', id)),
+		b.return(b.object(identifiers.map((identifier) => b.prop('init', identifier, identifier))))
+	]);
+
+	const declarations = [b.var(value, create_derived(context.state, b.thunk(block)))];
+
+	for (const id of identifiers) {
+		declarations.push(
+			b.var(id, create_derived(context.state, b.thunk(b.member(b.call('$.get', value), id))))
+		);
+	}
+
+	return { id, declarations };
+}
+
+/**
+ * Svelte legacy mode should use safe equals in most places, runes mode shouldn't
+ * @param {import('./types.js').ComponentClientTransformState} state
+ * @param {import('estree').Expression} arg
+ */
+export function create_derived(state, arg) {
+	return b.call(state.analysis.runes ? '$.derived' : '$.derived_safe_equal', arg);
+}

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

@@ -21,7 +21,9 @@ import {
 	function_visitor,
 	get_assignment_value,
 	serialize_get_binding,
-	serialize_set_binding
+	serialize_set_binding,
+	create_derived,
+	create_derived_block_argument
 } from '../utils.js';
 import {
 	AttributeAliases,
@@ -646,15 +648,6 @@ function collect_parent_each_blocks(context) {
 	);
 }
 
-/**
- * Svelte legacy mode should use safe equals in most places, runes mode shouldn't
- * @param {import('../types.js').ComponentClientTransformState} state
- * @param {import('estree').Expression} arg
- */
-function create_derived(state, arg) {
-	return b.call(state.analysis.runes ? '$.derived' : '$.derived_safe_equal', arg);
-}
-
 /**
  * @param {import('#compiler').Component | import('#compiler').SvelteComponent | import('#compiler').SvelteSelf} node
  * @param {string} component_name
@@ -2594,6 +2587,45 @@ export const template_visitors = {
 	AwaitBlock(node, context) {
 		context.state.template.push('<!>');
 
+		let then_block;
+		let catch_block;
+
+		if (node.then) {
+			/** @type {import('estree').Pattern[]} */
+			const args = [b.id('$$anchor')];
+			const block = /** @type {import('estree').BlockStatement} */ (context.visit(node.then));
+
+			if (node.value) {
+				const argument = create_derived_block_argument(node.value, context);
+
+				args.push(argument.id);
+
+				if (argument.declarations !== null) {
+					block.body.unshift(...argument.declarations);
+				}
+			}
+
+			then_block = b.arrow(args, block);
+		}
+
+		if (node.catch) {
+			/** @type {import('estree').Pattern[]} */
+			const args = [b.id('$$anchor')];
+			const block = /** @type {import('estree').BlockStatement} */ (context.visit(node.catch));
+
+			if (node.error) {
+				const argument = create_derived_block_argument(node.error, context);
+
+				args.push(argument.id);
+
+				if (argument.declarations !== null) {
+					block.body.unshift(...argument.declarations);
+				}
+			}
+
+			catch_block = b.arrow(args, block);
+		}
+
 		context.state.init.push(
 			b.stmt(
 				b.call(
@@ -2606,28 +2638,8 @@ export const template_visitors = {
 								/** @type {import('estree').BlockStatement} */ (context.visit(node.pending))
 							)
 						: b.literal(null),
-					node.then
-						? b.arrow(
-								node.value
-									? [
-											b.id('$$anchor'),
-											/** @type {import('estree').Pattern} */ (context.visit(node.value))
-										]
-									: [b.id('$$anchor')],
-								/** @type {import('estree').BlockStatement} */ (context.visit(node.then))
-							)
-						: b.literal(null),
-					node.catch
-						? b.arrow(
-								node.error
-									? [
-											b.id('$$anchor'),
-											/** @type {import('estree').Pattern} */ (context.visit(node.error))
-										]
-									: [b.id('$$anchor')],
-								/** @type {import('estree').BlockStatement} */ (context.visit(node.catch))
-							)
-						: b.literal(null)
+					then_block,
+					catch_block
 				)
 			)
 		);

packages/svelte/src/compiler/phases/scope.js

@@ -613,7 +613,7 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 					scopes.set(node.value, value_scope);
 					context.visit(node.value, { scope: value_scope });
 					for (const id of extract_identifiers(node.value)) {
-						then_scope.declare(id, 'normal', 'const');
+						then_scope.declare(id, 'derived', 'const');
 						value_scope.declare(id, 'normal', 'const');
 					}
 				}
@@ -627,7 +627,7 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 					scopes.set(node.error, error_scope);
 					context.visit(node.error, { scope: error_scope });
 					for (const id of extract_identifiers(node.error)) {
-						catch_scope.declare(id, 'normal', 'const');
+						catch_scope.declare(id, 'derived', 'const');
 						error_scope.declare(id, 'normal', 'const');
 					}
 				}

packages/svelte/src/internal/client/dom/blocks/await.js

@@ -7,111 +7,135 @@ import {
 	set_current_reaction,
 	set_dev_current_component_function
 } from '../../runtime.js';
-import { block, branch, destroy_effect, pause_effect } from '../../reactivity/effects.js';
-import { INERT } from '../../constants.js';
+import { block, branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
 import { DEV } from 'esm-env';
+import { queue_micro_task } from '../task.js';
+import { hydrating } from '../hydration.js';
+import { set, source } from '../../reactivity/sources.js';
+
+const PENDING = 0;
+const THEN = 1;
+const CATCH = 2;
 
 /**
  * @template V
  * @param {Comment} anchor
  * @param {(() => Promise<V>)} get_input
  * @param {null | ((anchor: Node) => void)} pending_fn
- * @param {null | ((anchor: Node, value: V) => void)} then_fn
+ * @param {null | ((anchor: Node, value: import('#client').Source<V>) => void)} then_fn
  * @param {null | ((anchor: Node, error: unknown) => void)} catch_fn
  * @returns {void}
  */
 export function await_block(anchor, get_input, pending_fn, then_fn, catch_fn) {
-	const component_context = current_component_context;
-	/** @type {any} */
-	let component_function;
-	if (DEV) {
-		component_function = component_context?.function ?? null;
-	}
+	var component_context = current_component_context;
 
 	/** @type {any} */
-	let input;
+	var component_function = DEV ? component_context?.function : null;
+
+	/** @type {V | Promise<V>} */
+	var input;
 
 	/** @type {import('#client').Effect | null} */
-	let pending_effect;
+	var pending_effect;
 
 	/** @type {import('#client').Effect | null} */
-	let then_effect;
+	var then_effect;
 
 	/** @type {import('#client').Effect | null} */
-	let catch_effect;
+	var catch_effect;
+
+	var input_source = source(/** @type {V} */ (undefined));
+	var error_source = source(undefined);
+	var resolved = false;
 
 	/**
-	 * @param {(anchor: Comment, value: any) => void} fn
-	 * @param {any} value
+	 * @param {PENDING | THEN | CATCH} state
+	 * @param {boolean} restore
 	 */
-	function create_effect(fn, value) {
-		set_current_effect(effect);
-		set_current_reaction(effect); // TODO do we need both?
-		set_current_component_context(component_context);
-		if (DEV) {
-			set_dev_current_component_function(component_function);
+	function update(state, restore) {
+		resolved = true;
+
+		if (restore) {
+			set_current_effect(effect);
+			set_current_reaction(effect); // TODO do we need both?
+			set_current_component_context(component_context);
+			if (DEV) set_dev_current_component_function(component_function);
+		}
+
+		if (state === PENDING && pending_fn) {
+			if (pending_effect) resume_effect(pending_effect);
+			else pending_effect = branch(() => pending_fn(anchor));
+		}
+
+		if (state === THEN && then_fn) {
+			if (then_effect) resume_effect(then_effect);
+			else then_effect = branch(() => then_fn(anchor, input_source));
+		}
+
+		if (state === CATCH && catch_fn) {
+			if (catch_effect) resume_effect(catch_effect);
+			else catch_effect = branch(() => catch_fn(anchor, error_source));
+		}
+
+		if (state !== PENDING && pending_effect) {
+			pause_effect(pending_effect, () => (pending_effect = null));
 		}
-		var e = branch(() => fn(anchor, value));
-		if (DEV) {
-			set_dev_current_component_function(null);
+
+		if (state !== THEN && then_effect) {
+			pause_effect(then_effect, () => (then_effect = null));
+		}
+
+		if (state !== CATCH && catch_effect) {
+			pause_effect(catch_effect, () => (catch_effect = null));
 		}
-		set_current_component_context(null);
-		set_current_reaction(null);
-		set_current_effect(null);
 
-		// without this, the DOM does not update until two ticks after the promise,
-		// resolves which is unexpected behaviour (and somewhat irksome to test)
-		flush_sync();
+		if (restore) {
+			if (DEV) set_dev_current_component_function(null);
+			set_current_component_context(null);
+			set_current_reaction(null);
+			set_current_effect(null);
 
-		return e;
+			// without this, the DOM does not update until two ticks after the promise
+			// resolves, which is unexpected behaviour (and somewhat irksome to test)
+			flush_sync();
+		}
 	}
 
-	const effect = block(() => {
+	var effect = block(() => {
 		if (input === (input = get_input())) return;
 
 		if (is_promise(input)) {
-			const promise = /** @type {Promise<any>} */ (input);
+			var promise = input;
 
-			if (pending_fn) {
-				if (pending_effect && (pending_effect.f & INERT) === 0) {
-					destroy_effect(pending_effect);
-				}
-
-				pending_effect = branch(() => pending_fn(anchor));
-			}
-
-			if (then_effect) pause_effect(then_effect);
-			if (catch_effect) pause_effect(catch_effect);
+			resolved = false;
 
 			promise.then(
 				(value) => {
 					if (promise !== input) return;
-					if (pending_effect) pause_effect(pending_effect);
-
-					if (then_fn) {
-						then_effect = create_effect(then_fn, value);
-					}
+					set(input_source, value);
+					update(THEN, true);
 				},
 				(error) => {
 					if (promise !== input) return;
-					if (pending_effect) pause_effect(pending_effect);
-
-					if (catch_fn) {
-						catch_effect = create_effect(catch_fn, error);
-					}
+					set(error_source, error);
+					update(CATCH, true);
 				}
 			);
-		} else {
-			if (pending_effect) pause_effect(pending_effect);
-			if (catch_effect) pause_effect(catch_effect);
 
-			if (then_fn) {
-				if (then_effect) {
-					destroy_effect(then_effect);
+			if (hydrating) {
+				if (pending_fn) {
+					pending_effect = branch(() => pending_fn(anchor));
 				}
-
-				then_effect = branch(() => then_fn(anchor, input));
+			} else {
+				// Wait a microtask before checking if we should show the pending state as
+				// the promise might have resolved by the next microtask.
+				queue_micro_task(() => {
+					if (!resolved) update(PENDING, true);
+				});
 			}
+		} else {
+			set(input_source, input);
+			update(THEN, false);
 		}
 
 		// Inert effects are proactively detached from the effect tree. Returning a noop

packages/svelte/tests/runtime-legacy/samples/await-then-destruct-computed-props/_config.js

@@ -22,6 +22,7 @@ export default test({
 			prop3: { prop7: 'seven' },
 			prop4: { prop10: 'ten' }
 		}));
+		await Promise.resolve();
 		assert.htmlEqual(
 			target.innerHTML,
 			`

packages/svelte/tests/runtime-legacy/samples/await-then-no-expression/main.svelte

@@ -20,4 +20,4 @@
 	<p>the promise is pending</p>
 {:then}
 	<p>the promise is resolved</p>
-{/await}
+{/await}