Raw snippet alternative (#12425)

* feat: add createRawSnippet API

* handle missing hydrate function, improve types

* fix

* tweak types

* beef up test

* build

* types

* oops this was temporary

* typo

* regenerate types

* make mount/render optional, error if missing

* move code to new module

* test hydration

* simpler createRawSnippet API

* regenerate types

* change signature

* docs

* h1 -> node

* allow `setup` to return a teardown function

---------

Co-authored-by: Dominic Gannaway <[email protected]>

Author: Rich-Harris

.changeset/fresh-zoos-burn.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: add createRawSnippet API

packages/svelte/src/index-client.js

@@ -190,3 +190,5 @@ export {
 	tick,
 	untrack
 } from './internal/client/runtime.js';
+
+export { createRawSnippet } from './internal/client/dom/blocks/snippet.js';

packages/svelte/src/index-server.js

@@ -35,3 +35,5 @@ export function unmount() {
 export async function tick() {}
 
 export { getAllContexts, getContext, hasContext, setContext } from './internal/server/context.js';
+
+export { createRawSnippet } from './internal/server/blocks/snippet.js';

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

@@ -1,12 +1,16 @@
+/** @import { Snippet } from 'svelte' */
 /** @import { Effect, TemplateNode } from '#client' */
+/** @import { Getters } from '#shared' */
 import { add_snippet_symbol } from '../../../shared/validate.js';
 import { EFFECT_TRANSPARENT } from '../../constants.js';
-import { branch, block, destroy_effect } from '../../reactivity/effects.js';
+import { branch, block, destroy_effect, teardown } from '../../reactivity/effects.js';
 import {
 	dev_current_component_function,
 	set_dev_current_component_function
 } from '../../runtime.js';
-import { hydrate_node, hydrating } from '../hydration.js';
+import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
+import { create_fragment_from_html } from '../reconciler.js';
+import { assign_nodes } from '../template.js';
 
 /**
  * @template {(node: TemplateNode, ...args: any[]) => void} SnippetFn
@@ -60,3 +64,40 @@ export function wrap_snippet(component, fn) {
 		}
 	});
 }
+
+/**
+ * Create a snippet programmatically
+ * @template {unknown[]} Params
+ * @param {(...params: Getters<Params>) => {
+ *   render: () => string
+ *   setup?: (element: Element) => void
+ * }} fn
+ * @returns {Snippet<Params>}
+ */
+export function createRawSnippet(fn) {
+	return add_snippet_symbol(
+		(/** @type {TemplateNode} */ anchor, /** @type {Getters<Params>} */ ...params) => {
+			var snippet = fn(...params);
+
+			/** @type {Element} */
+			var element;
+
+			if (hydrating) {
+				element = /** @type {Element} */ (hydrate_node);
+				hydrate_next();
+			} else {
+				var html = snippet.render().trim();
+				var fragment = create_fragment_from_html(html);
+				element = /** @type {Element} */ (fragment.firstChild);
+				anchor.before(element);
+			}
+
+			const result = snippet.setup?.(element);
+			assign_nodes(element, element);
+
+			if (typeof result === 'function') {
+				teardown(result);
+			}
+		}
+	);
+}

packages/svelte/src/internal/server/blocks/snippet.js

@@ -0,0 +1,22 @@
+/** @import { Snippet } from 'svelte' */
+/** @import { Payload } from '#server' */
+/** @import { Getters } from '#shared' */
+import { add_snippet_symbol } from '../../shared/validate.js';
+
+/**
+ * Create a snippet programmatically
+ * @template {unknown[]} Params
+ * @param {(...params: Getters<Params>) => {
+ *   render: () => string
+ *   setup?: (element: Element) => void
+ * }} fn
+ * @returns {Snippet<Params>}
+ */
+export function createRawSnippet(fn) {
+	return add_snippet_symbol((/** @type {Payload} */ payload, /** @type {Params} */ ...args) => {
+		var getters = /** @type {Getters<Params>} */ (args.map((value) => () => value));
+		payload.out += fn(...getters)
+			.render()
+			.trim();
+	});
+}

packages/svelte/src/internal/shared/types.d.ts

@@ -7,4 +7,8 @@ export type SourceLocation =
 	| [line: number, column: number]
 	| [line: number, column: number, SourceLocation[]];
 
+export type Getters<T> = {
+	[K in keyof T]: () => T[K];
+};
+
 export type Snapshot<T> = ReturnType<typeof $state.snapshot<T>>;

packages/svelte/src/internal/shared/validate.js

@@ -1,3 +1,5 @@
+/** @import { TemplateNode } from '#client' */
+/** @import { Getters } from '#shared' */
 import { is_void } from '../../constants.js';
 import * as w from './warnings.js';
 import * as e from './errors.js';
@@ -6,6 +8,7 @@ const snippet_symbol = Symbol.for('svelte.snippet');
 
 /**
  * @param {any} fn
+ * @returns {import('svelte').Snippet}
  */
 export function add_snippet_symbol(fn) {
 	fn[snippet_symbol] = true;

packages/svelte/tests/hydration/samples/snippet-raw-hydrate/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	snapshot(target) {
+		return {
+			p: target.querySelector('p')
+		};
+	}
+});

packages/svelte/tests/hydration/samples/snippet-raw-hydrate/_expected.html

@@ -0,0 +1 @@
+<!--[--><p>hydrated</p><!--]-->

packages/svelte/tests/hydration/samples/snippet-raw-hydrate/main.svelte

@@ -0,0 +1,14 @@
+<script>
+	import { createRawSnippet } from 'svelte';
+
+	const snippet = createRawSnippet(() => ({
+		render: () => `
+			<p>rendered</p>
+		`,
+		setup(p) {
+			p.textContent = 'hydrated';
+		}
+	}));
+</script>
+
+{@render snippet()}

packages/svelte/tests/runtime-runes/samples/snippet-raw-teardown/_config.js

@@ -0,0 +1,11 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ target, assert, logs }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+		assert.deepEqual(logs, ['tearing down']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/snippet-raw-teardown/main.svelte

@@ -0,0 +1,18 @@
+<script>
+	import { createRawSnippet } from 'svelte';
+
+	let show = $state(true);
+
+	const snippet = createRawSnippet(() => ({
+		render: () => `<hr>`,
+		setup(p) {
+			return () => console.log('tearing down')
+		}
+	}));
+</script>
+
+<button onclick={() => show = !show}>click</button>
+
+{#if show}
+	{@render snippet()}
+{/if}

packages/svelte/tests/runtime-runes/samples/snippet-raw/_config.js

@@ -0,0 +1,17 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true // Render in dev mode to check that the validation error is not thrown
+	},
+
+	html: `<button>click</button><p>clicks: 0</p>`,
+
+	test({ target, assert }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+		assert.htmlEqual(target.innerHTML, `<button>click</button><p>clicks: 1</p>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/snippet-raw/main.svelte

@@ -0,0 +1,20 @@
+<script>
+	import { createRawSnippet } from 'svelte';
+
+	let count = $state(0);
+
+	const hello = createRawSnippet((count) => ({
+		render: () => `
+			<p>clicks: ${count()}</p>
+		`,
+		setup(p) {
+			$effect(() => {
+				p.textContent = `clicks: ${count()}`
+			});
+		}
+	}));
+</script>
+
+<button onclick={() => count += 1}>click</button>
+
+{@render hello(count)}

packages/svelte/types/index.d.ts

@@ -365,6 +365,13 @@ declare module 'svelte' {
 	export function flushSync(fn?: (() => void) | undefined): void;
 	/** Anything except a function */
 	type NotFunction<T> = T extends Function ? never : T;
+	/**
+	 * Create a snippet programmatically
+	 * */
+	export function createRawSnippet<Params extends unknown[]>(fn: (...params: Getters<Params>) => {
+		render: () => string;
+		setup?: (element: Element) => void;
+	}): Snippet<Params>;
 	/**
 	 * Mounts a component to the given target and returns the exports and potentially the props (if compiled with `accessors: true`) of the component.
 	 * Transitions will play during the initial render unless the `intro` option is set to `false`.
@@ -450,6 +457,9 @@ declare module 'svelte' {
 	 * https://svelte.dev/docs/svelte#getallcontexts
 	 * */
 	export function getAllContexts<T extends Map<any, any> = Map<any, any>>(): T;
+	type Getters<T> = {
+		[K in keyof T]: () => T[K];
+	};
 
 	export {};
 }

sites/svelte-5-preview/src/routes/docs/content/01-api/03-snippets.md

@@ -256,6 +256,10 @@ We can tighten things up further by declaring a generic, so that `data` and `row
 </script>
 ```
 
+## Creating snippets programmatically
+
+In advanced scenarios, you may need to create a snippet programmatically. For this, you can use [`createRawSnippet`](/docs/imports#svelte-createrawsnippet)
+
 ## Snippets and slots
 
 In Svelte 4, content can be passed to components using [slots](https://svelte.dev/docs/special-elements#slot). Snippets are more powerful and flexible, and as such slots are deprecated in Svelte 5.

sites/svelte-5-preview/src/routes/docs/content/01-api/05-imports.md

@@ -93,6 +93,37 @@ To prevent something from being treated as an `$effect`/`$derived` dependency, u
 </script>
 ```
 
+### `createRawSnippet`
+
+An advanced API designed for people building frameworks that integrate with Svelte, `createRawSnippet` allows you to create [snippets](/docs/snippets) programmatically for use with `{@render ...}` tags:
+
+```js
+import { createRawSnippet } from 'svelte';
+
+const greet = createRawSnippet((name) => {
+	return {
+		render: () => `
+			<h1>Hello ${name()}!</h1>
+		`,
+		setup: (node) => {
+			$effect(() => {
+				node.textContent = `Hello ${name()}!`;
+			});
+		}
+	};
+});
+```
+
+The `render` function is called during server-side rendering, or during `mount` (but not during `hydrate`, because it already ran on the server), and must return HTML representing a single element.
+
+The `setup` function is called during `mount` or `hydrate` with that same element as its sole argument. It is responsible for ensuring that the DOM is updated when the arguments change their value — in this example, when `name` changes:
+
+```svelte
+{@render greet(name)}
+```
+
+If `setup` returns a function, it will be called when the snippet is unmounted. If the snippet is fully static, you can omit the `setup` function altogether.
+
 ## `svelte/reactivity`
 
 Svelte provides reactive `SvelteMap`, `SvelteSet`, `SvelteDate` and `SvelteURL` classes. These can be imported from `svelte/reactivity` and used just like their native counterparts. [Demo:](https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE32QwUrEMBBAf2XMpQrb9t7tFrx7UjxZYWM6NYFkEpJJ16X03yWK9OQeZ3iPecwqZmMxie5tFSQdik48hiAOgq-hDGlByygOIvkcVdn0SUUTeBhpZOOCjwwrvPxgr89PsMEcvYPqV2wjSsVmMXytjiMVR3lKDDlaOAHhZVfvK80cUte2-CVdsNgo79ogWVcPx5H6dj9M_V1dg9KSPjEBe2CNCZumgboeRuoNhczwYWjqFmkzntYcbROiZ6-83f5HtE9c3nADKUF_yEi9jnvQxVgLOUySEc464nwGSRMsRiEsGJO8mVeEbRAH4fxkZoOT6Dhm3N63b9_bGfOlAQAA)