feat: provide MediaQuery / prefersReducedMotion

closes #5346

packages/svelte/src/motion/index.js

@@ -1,2 +1,10 @@
+import { MediaQuery } from 'svelte/reactivity';
+
 export * from './spring.js';
 export * from './tweened.js';
+
+/**
+ * A media query that matches if the user has requested reduced motion.
+ * @type {MediaQuery}
+ */
+export const prefersReducedMotion = new MediaQuery('(prefers-reduced-motion: reduce)');

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

@@ -3,3 +3,4 @@ export { SvelteSet } from './set.js';
 export { SvelteMap } from './map.js';
 export { SvelteURL } from './url.js';
 export { SvelteURLSearchParams } from './url-search-params.js';
+export { MediaQuery } from './media-query.js';

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

@@ -3,3 +3,7 @@ export const SvelteSet = globalThis.Set;
 export const SvelteMap = globalThis.Map;
 export const SvelteURL = globalThis.URL;
 export const SvelteURLSearchParams = globalThis.URLSearchParams;
+
+export class MediaQuery {
+	matches = false;
+}

packages/svelte/src/reactivity/media-query.js

@@ -0,0 +1,49 @@
+import { get, tick } from '../internal/client/runtime.js';
+import { set, source } from '../internal/client/reactivity/sources.js';
+import { effect_tracking, render_effect } from '../internal/client/reactivity/effects.js';
+
+/**
+ * Creates a media query and provides a `matches` property that reflects its current state.
+ */
+export class MediaQuery {
+	#matches = source(false);
+	#subscribers = 0;
+	#query;
+	/** @type {any} */
+	#listener;
+
+	get matches() {
+		if (effect_tracking()) {
+			render_effect(() => {
+				if (this.#subscribers === 0) {
+					this.#listener = () => set(this.#matches, this.#query.matches);
+					this.#query.addEventListener('change', this.#listener);
+				}
+
+				this.#subscribers += 1;
+
+				return () => {
+					tick().then(() => {
+						// Only count down after timeout, else we would reach 0 before our own render effect reruns,
+						// but reach 1 again when the tick callback of the prior teardown runs. That would mean we
+						// re-subcribe unnecessarily and create a memory leak because the old subscription is never cleaned up.
+						this.#subscribers -= 1;
+
+						if (this.#subscribers === 0) {
+							this.#query.removeEventListener('change', this.#listener);
+						}
+					});
+				};
+			});
+		}
+
+		return get(this.#matches);
+	}
+
+	/** @param {string} query */
+	constructor(query) {
+		this.#query = window.matchMedia(query);
+		console.log('MediaQuery.constructor', query, this.#query);
+		this.#matches.v = this.#query.matches;
+	}
+}

packages/svelte/types/index.d.ts

@@ -1630,6 +1630,7 @@ declare module 'svelte/legacy' {
 }
 
 declare module 'svelte/motion' {
+	import type { MediaQuery } from 'svelte/reactivity';
 	export interface Spring<T> extends Readable<T> {
 		set: (new_value: T, opts?: SpringUpdateOpts) => Promise<void>;
 		update: (fn: Updater<T>, opts?: SpringUpdateOpts) => Promise<void>;
@@ -1676,6 +1677,10 @@ declare module 'svelte/motion' {
 		easing?: (t: number) => number;
 		interpolate?: (a: T, b: T) => (t: number) => T;
 	}
+	/**
+	 * A media query that matches if the user has requested reduced motion.
+	 * */
+	export const prefersReducedMotion: MediaQuery;
 	/**
 	 * The spring function in Svelte creates a store whose value is animated, with a motion that simulates the behavior of a spring. This means when the value changes, instead of transitioning at a steady rate, it "bounces" like a spring would, depending on the physics parameters provided. This adds a level of realism to the transitions and can enhance the user experience.
 	 *
@@ -1720,6 +1725,15 @@ declare module 'svelte/reactivity' {
 		[REPLACE](params: URLSearchParams): void;
 		#private;
 	}
+	/**
+	 * Creates a media query and provides a `matches` property that reflects its current state.
+	 */
+	export class MediaQuery {
+
+		constructor(query: string);
+		get matches(): boolean;
+		#private;
+	}
 
 	export {};
 }