feat: Export createReactiveFunction

packages/svelte/src/reactivity/create-reactive-function.js

@@ -0,0 +1,48 @@
+import { ReactiveValue } from "./reactive-value.js";
+
+/**
+ * @typedef {() => void} VoidFn
+ */
+
+/**
+ * Used to synchronize external state with Svelte's reactivity system.
+ * 
+ * In order to synchronize external state, you need to know two things: 
+ * - The value of the external state at a given time
+ * - When you should update that value
+ * 
+ * These correspond to the two arguments to this function:
+ * 
+ * - {@link getSnapshot} is a function that returns the current value of the external state
+ * - {@link onSubscribe} is a callback that sets up reactivity:
+ *   - It is called the first time the function returned by {@link createReactiveFunction} becomes tracked by a dependent
+ *   - It receives a function, `update`, as its first argument. Calling `update` tells Svelte that the value in the external
+ *     store has changed, and it needs to push that change to all of its listeners
+ *   - If it returns a cleanup function, that cleanup function will be called when the number of listeners to the reactive function
+ *     returns to zero
+ * 
+ * Combined with `$derived.by`, this enables seamless integration with external data sources, including destructuring support:
+ * 
+ * @example
+ * ```js
+ * import { observer } from 'external-datafetching-library';
+ * 
+ * const { data, loading, refetch } = $derived.by(
+ *   createReactiveFunction(
+ *     observer.getCurrentResult,
+ *     observer.subscribe
+ *   )
+ * );
+ * ```
+ * 
+ * (For our React friends, this is a similar model to `useSyncExternalStore`.)
+ * 
+ * @template T
+ * @param {() => T} getSnapshot
+ * @param {(update: VoidFn) => void | VoidFn} onSubscribe
+ * @returns {() => T}
+ */
+export function createReactiveFunction(getSnapshot, onSubscribe) {
+  const value = new ReactiveValue(getSnapshot, onSubscribe);
+  return () => value.current
+}

packages/svelte/src/reactivity/index-client.js

@@ -5,3 +5,4 @@ export { SvelteURL } from './url.js';
 export { SvelteURLSearchParams } from './url-search-params.js';
 export { MediaQuery } from './media-query.js';
 export { createSubscriber } from './create-subscriber.js';
+export { createReactiveFunction } from './create-reactive-function.js';

packages/svelte/src/reactivity/index-server.js

@@ -21,3 +21,13 @@ export class MediaQuery {
 export function createSubscriber(_) {
 	return () => {};
 }
+
+/**
+ * @template T
+ * @param {() => T} getSnapshot 
+ * @param {any} __ 
+ * @returns {() => T}
+ */
+export function createReactiveFunction(getSnapshot, __) {
+	return getSnapshot
+}