feat: simplify HMR implementation (#11132)

* chore: simplify HMR implementation

* changeset

* unused

* prettier

Author: Rich-Harris

.changeset/fair-bags-smoke.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: simplify HMR implementation

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

@@ -422,32 +422,31 @@ export function client_component(source, analysis, options) {
 		)
 	];
 
-	// In order for hmr to work correctly, we need associate each component with a unique key.
-	// This is because bundlers might put many components into a the same module (usuaully as a chunk).
-	// `import.meta.hot` will then be the same object for all components in that modules.
-	if (options.hmr && options.filename) {
+	if (options.hmr) {
 		body.push(
-			b.export_default(
-				b.conditional(
-					b.import_meta_hot(),
-					b.call(
-						'$.hmr',
-						b.member(b.import_meta_hot(), b.id('data')),
-						b.id(analysis.name),
-						b.literal(options.filename)
-					),
-					b.id(analysis.name)
-				)
-			),
 			b.if(
-				b.import_meta_hot(),
-				b.stmt(b.call('import.meta.hot.acceptExports', b.literal('default')))
+				b.id('import.meta.hot'),
+				b.block([
+					b.const(b.id('s'), b.call('$.source', b.id(analysis.name))),
+					b.stmt(b.assignment('=', b.id(analysis.name), b.call('$.hmr', b.id('s')))),
+					b.stmt(
+						b.call(
+							'import.meta.hot.accept',
+							b.arrow(
+								[b.id('module')],
+								b.block([
+									b.stmt(b.call('$.set', b.id('s'), b.member(b.id('module'), b.id('default'))))
+								])
+							)
+						)
+					)
+				])
 			)
 		);
-	} else {
-		body.push(b.export_default(b.id(analysis.name)));
 	}
 
+	body.push(b.export_default(b.id(analysis.name)));
+
 	if (options.dev) {
 		if (options.filename) {
 			let filename = options.filename;

packages/svelte/src/compiler/utils/builders.js

@@ -600,20 +600,6 @@ export function throw_error(str) {
 	};
 }
 
-/**
- * @return {import('estree').MemberExpression}
- */
-export function import_meta_hot() {
-	return member(
-		{
-			type: 'MetaProperty',
-			meta: id('import'),
-			property: id('meta')
-		},
-		id('hot')
-	);
-}
-
 export {
 	await_builder as await,
 	let_builder as let,

packages/svelte/src/internal/client/dev/hmr.js

@@ -1,55 +1,38 @@
 import { block, branch, destroy_effect } from '../reactivity/effects.js';
-import { set, source } from '../reactivity/sources.js';
 import { set_should_intro } from '../render.js';
 import { get } from '../runtime.js';
 
 /**
  * @template {(anchor: Comment, props: any) => any} Component
- * @param {{ components: Map<string, { source: import("#client").Source<Component>; wrapper: null | Component; }> }} hot_data
- * @param {string} key
- * @param {Component} component
+ * @param {import("#client").Source<Component>} source
  */
-export function hmr(hot_data, component, key) {
-	var components = (hot_data.components ??= new Map());
-	var data = components.get(key);
-
-	if (data === undefined) {
-		components.set(
-			key,
-			(data = {
-				source: source(component),
-				wrapper: null
-			})
-		);
-	} else {
-		set(data.source, component);
-	}
-	const component_source = data.source;
-
-	return (data.wrapper ??= /** @type {Component} */ (
-		(anchor, props) => {
-			let instance = {};
-
-			/** @type {import("#client").Effect} */
-			let effect;
-
-			block(() => {
-				const component = get(component_source);
-
-				if (effect) {
-					// @ts-ignore
-					for (var k in instance) delete instance[k];
-					destroy_effect(effect);
-				}
-
-				effect = branch(() => {
-					set_should_intro(false);
-					Object.assign(instance, component(anchor, props));
-					set_should_intro(true);
-				});
+export function hmr(source) {
+	/**
+	 * @param {Comment} anchor
+	 * @param {any} props
+	 */
+	return (anchor, props) => {
+		let instance = {};
+
+		/** @type {import("#client").Effect} */
+		let effect;
+
+		block(() => {
+			const component = get(source);
+
+			if (effect) {
+				// @ts-ignore
+				for (var k in instance) delete instance[k];
+				destroy_effect(effect);
+			}
+
+			effect = branch(() => {
+				set_should_intro(false);
+				Object.assign(instance, component(anchor, props));
+				set_should_intro(true);
 			});
+		});
 
-			return instance;
-		}
-	));
+		return instance;
+	};
 }

packages/svelte/tests/snapshot/samples/hmr/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		hmr: true
+	}
+});

packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js

@@ -0,0 +1,28 @@
+// index.svelte (Svelte VERSION)
+// Note: compiler output will change before 5.0 is released!
+import "svelte/internal/disclose-version";
+import * as $ from "svelte/internal/client";
+
+var root = $.template(`<h1>hello world</h1>`);
+
+function Hmr($$anchor, $$props) {
+	$.push($$props, false);
+	$.init();
+
+	var h1 = root();
+
+	$.append($$anchor, h1);
+	$.pop();
+}
+
+if (import.meta.hot) {
+	const s = $.source(Hmr);
+
+	Hmr = $.hmr(s);
+
+	import.meta.hot.accept((module) => {
+		$.set(s, module.default);
+	});
+}
+
+export default Hmr;

packages/svelte/tests/snapshot/samples/hmr/_expected/server/index.svelte.js

@@ -0,0 +1,9 @@
+// index.svelte (Svelte VERSION)
+// Note: compiler output will change before 5.0 is released!
+import * as $ from "svelte/internal/server";
+
+export default function Hmr($$payload, $$props) {
+	$.push(false);
+	$$payload.out += `<h1>hello world</h1>`;
+	$.pop();
+}

packages/svelte/tests/snapshot/samples/hmr/index.svelte

@@ -0,0 +1 @@
+<h1>hello world</h1>