feat: add remote function query.stream

.changeset/smart-nails-allow.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: add remote function `query.stream`

documentation/docs/20-core-concepts/60-remote-functions.md

@@ -225,6 +225,90 @@ export const getWeather = query.batch(v.string(), async (cities) => {
 {/if}
 ```
 
+## query.stream
+
+`query.stream` allows you to stream continuous data from the server to the client.
+
+```js
+/// file: src/routes/time.remote.js
+// @filename: ambient.d.ts
+declare module '$lib/server/database' {
+	export function sql(strings: TemplateStringsArray, ...values: any[]): Promise<any[]>;
+}
+// @filename: index.js
+// ---cut---
+import { query } from '$app/server';
+
+export const time = query.stream(async function* () {
+	while (true) {
+		yield new Date();
+		await new Promise(r => setTimeout(r, 1000))
+	}
+});
+```
+
+You can consume the stream like a promise or via the `current` property. In both cases, if it's used in a reactive context, it will automatically update to the latest version upon retrieving new data.
+
+```svelte
+<!--- file: src/routes/+page.svelte --->
+<script>
+	import { time } from './time.remote.js';
+</script>
+
+<p>{await time()}</p>
+<p>{time().current}</p>
+```
+
+Apart from that you can iterate over it like any other async iterable, including using `for await (...)`.
+
+```svelte
+<!--- file: src/routes/+page.svelte --->
+<script>
+	import { time } from './time.remote.js';
+
+	let times = $state([]);
+
+	async function stream() {
+		times = []
+		let count = 0;
+
+		for await (const entry of time()) {
+			times.push(time);
+			count++;
+			if (count >= 5) {
+				break;
+			}
+		}
+	})
+</script>
+
+<button onclick={stream}>stream for five seconds</button>
+
+{#each times as time}
+	<span>{time}</time>
+{/each}
+```
+
+Unlike other `query` methods, stream requests to the same resource with the same payload are _not_ deduplicated. That means you can start the same stream multiple times in parallel and it will start from the beginning each time.
+
+```svelte
+<!--- file: src/routes/+page.svelte --->
+<script>
+	import { oneToTen } from './count.remote.js';
+
+	const stream = oneToTen();
+</script>
+
+<!-- these are one single ReadableStream request since they share the same stream instance -->
+{#await stream()}
+{#await stream()}
+
+<!-- this is a separate instance and will create a new ReadableStream request to the backend -->
+{await oneToTen()}
+```
+
+> [!NOTE] Be careful when using `query.stream` in combination with service workers. Specifically, make sure to never pass the promise of a `ReadableStream` (which `query.stream` uses) to `event.respondWith(...)`, as the promise never settles.
+
 ## form
 
 The `form` function makes it easy to write data to the server. It takes a callback that receives the current [`FormData`](https://developer.mozilla.org/en-US/docs/Web/API/FormData)...

packages/kit/src/exports/internal/remote-functions.js

@@ -17,6 +17,7 @@ export function validate_remote_functions(module, file) {
 			type !== 'command' &&
 			type !== 'query' &&
 			type !== 'query_batch' &&
+			type !== 'query_stream' &&
 			type !== 'prerender'
 		) {
 			throw new Error(

packages/kit/src/exports/public.d.ts

@@ -1822,6 +1822,16 @@ export interface RemoteQueryOverride {
 	release(): void;
 }
 
+/**
+ * The return value of a remote `query.stream` function. See [Remote functions](https://svelte.dev/docs/kit/remote-functions#query-stream) for full documentation.
+ */
+export type RemoteQueryStream<T> = RemoteResource<T> & AsyncIterable<Awaited<T>>;
+
+/**
+ * The return value of a remote `query.stream` function. See [Remote functions](https://svelte.dev/docs/kit/remote-functions#query-stream) for full documentation.
+ */
+export type RemoteQueryStreamFunction<Input, Output> = (arg: Input) => RemoteQueryStream<Output>;
+
 /**
  * The return value of a remote `prerender` function. See [Remote functions](https://svelte.dev/docs/kit/remote-functions#prerender) for full documentation.
  */

packages/kit/src/runtime/app/server/remote/query.js

@@ -1,4 +1,4 @@
-/** @import { RemoteQuery, RemoteQueryFunction } from '@sveltejs/kit' */
+/** @import { RemoteQuery, RemoteQueryFunction, RemoteQueryStream, RemoteQueryStreamFunction } from '@sveltejs/kit' */
 /** @import { RemoteInfo, MaybePromise } from 'types' */
 /** @import { StandardSchemaV1 } from '@standard-schema/spec' */
 import { get_request_store } from '@sveltejs/kit/internal/server';
@@ -248,5 +248,120 @@ function batch(validate_or_fn, maybe_fn) {
 	return wrapper;
 }
 
-// Add batch as a property to the query function
+/**
+ * Creates a streaming remote query. When called from the browser, the generator function will be invoked on the server and values will be streamed via Server-Sent Events.
+ *
+ * See [Remote functions](https://svelte.dev/docs/kit/remote-functions#query.stream) for full documentation.
+ *
+ * @template Output
+ * @overload
+ * @param {() => Generator<Output, void, unknown> | AsyncGenerator<Output, void, unknown>} fn
+ * @returns {RemoteQueryStreamFunction<void, Output>}
+ * @since 2.36
+ */
+/**
+ * Creates a streaming remote query. When called from the browser, the generator function will be invoked on the server and values will be streamed via Server-Sent Events.
+ *
+ * See [Remote functions](https://svelte.dev/docs/kit/remote-functions#query.stream) for full documentation.
+ *
+ * @template Input
+ * @template Output
+ * @overload
+ * @param {'unchecked'} validate
+ * @param {(arg: Input) => Generator<Output, void, unknown> | AsyncGenerator<Output, void, unknown>} fn
+ * @returns {RemoteQueryStreamFunction<Input, Output>}
+ * @since 2.36
+ */
+/**
+ * Creates a streaming remote query. When called from the browser, the generator function will be invoked on the server and values will be streamed via Server-Sent Events.
+ *
+ * See [Remote functions](https://svelte.dev/docs/kit/remote-functions#query.stream) for full documentation.
+ *
+ * @template {StandardSchemaV1} Schema
+ * @template Output
+ * @overload
+ * @param {Schema} schema
+ * @param {(arg: StandardSchemaV1.InferOutput<Schema>) => Generator<Output, void, unknown> | AsyncGenerator<Output, void, unknown>} fn
+ * @returns {RemoteQueryStreamFunction<StandardSchemaV1.InferInput<Schema>, Output>}
+ * @since 2.36
+ */
+/**
+ * @template Input
+ * @template Output
+ * @param {any} validate_or_fn
+ * @param {(arg?: Input) => Generator<Output, void, unknown> | AsyncGenerator<Output, void, unknown>} [maybe_fn]
+ * @returns {RemoteQueryStreamFunction<Input, Output>}
+ * @since 2.36
+ */
+/*@__NO_SIDE_EFFECTS__*/
+function stream(validate_or_fn, maybe_fn) {
+	/** @type {(arg?: Input) => Generator<Output, void, unknown> | AsyncGenerator<Output, void, unknown>} */
+	const fn = maybe_fn ?? validate_or_fn;
+
+	/** @type {(arg?: any) => MaybePromise<Input>} */
+	const validate = create_validator(validate_or_fn, maybe_fn);
+
+	/** @type {RemoteInfo} */
+	const __ = { type: 'query_stream', id: '', name: '' };
+
+	/** @type {RemoteQueryStreamFunction<Input, Output> & { __: RemoteInfo }} */
+	const wrapper = (/** @type {Input} */ arg) => {
+		if (prerendering) {
+			throw new Error(
+				`Cannot call query.stream '${__.name}' while prerendering, as prerendered pages need static data. Use 'prerender' from $app/server instead`
+			);
+		}
+
+		const { event, state } = get_request_store();
+
+		/** @type {IteratorResult<Output> | undefined} */
+		let first_value;
+
+		const promise = (async () => {
+			// We only care about the generator when doing a remote request
+			if (event.isRemoteRequest) return;
+
+			const generator = await run_remote_function(event, state, false, arg, validate, fn);
+			first_value = await generator.next();
+			await generator.return();
+			return first_value.done ? undefined : first_value.value;
+		})();
+
+		// Catch promise to avoid unhandled rejection
+		promise.catch(() => {});
+
+		// eslint-disable-next-line @typescript-eslint/no-floating-promises
+		Object.assign(promise, {
+			async *[Symbol.asyncIterator]() {
+				if (event.isRemoteRequest) {
+					const generator = await run_remote_function(event, state, false, arg, validate, fn);
+					yield* generator;
+				} else {
+					// TODO how would we subscribe to the stream on the server while deduplicating calls and knowing when to stop?
+					throw new Error(
+						'Cannot iterate over a stream on the server. This restriction may be lifted in a future version.'
+					);
+				}
+			},
+			get error() {
+				return undefined;
+			},
+			get ready() {
+				return !!first_value;
+			},
+			get current() {
+				return first_value?.value;
+			}
+		});
+
+		return /** @type {RemoteQueryStream<Output>} */ (promise);
+	};
+
+	Object.defineProperty(wrapper, '__', { value: __ });
+
+	return wrapper;
+}
+
+// Add batch and stream as properties to the query function
 Object.defineProperty(query, 'batch', { value: batch, enumerable: true });
+Object.defineProperty(query, 'stream', { value: stream, enumerable: true });

packages/kit/src/runtime/client/remote-functions/index.js

@@ -1,4 +1,4 @@
 export { command } from './command.svelte.js';
 export { form } from './form.svelte.js';
 export { prerender } from './prerender.svelte.js';
-export { query, query_batch } from './query.svelte.js';
+export { query, query_batch, query_stream } from './query.svelte.js';