feat: add Spring and Tween classes (#11519)

* feat: add Spring class

* add some docs, Spring.of static method

* add Tween class

* lint

* preserveMomentum in milliseconds

* deprecate tweened

* changeset

* wrestle with types

* more consolidation

* flesh out the distinction a bit more, deprecate `subscribe`

---------

Co-authored-by: Simon Holthausen <[email protected]>

Author: Rich-Harris

.changeset/tame-bottles-switch.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: add `Spring` and `Tween` classes to `svelte/motion`

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

@@ -44,6 +44,27 @@ export function run_all(arr) {
 	}
 }
 
+/**
+ * TODO replace with Promise.withResolvers once supported widely enough
+ * @template T
+ */
+export function deferred() {
+	/** @type {(value: T) => void} */
+	var resolve;
+
+	/** @type {(reason: any) => void} */
+	var reject;
+
+	/** @type {Promise<T>} */
+	var promise = new Promise((res, rej) => {
+		resolve = res;
+		reject = rej;
+	});
+
+	// @ts-expect-error
+	return { promise, resolve, reject };
+}
+
 /**
  * @template V
  * @param {V} value

packages/svelte/src/motion/private.d.ts

@@ -1,9 +1,11 @@
-import { Spring } from './public.js';
-
-export interface TickContext<T> {
+export interface TickContext {
 	inv_mass: number;
 	dt: number;
-	opts: Spring<T>;
+	opts: {
+		stiffness: number;
+		damping: number;
+		precision: number;
+	};
 	settled: boolean;
 }
 
@@ -14,8 +16,22 @@ export interface SpringOpts {
 }
 
 export interface SpringUpdateOpts {
+	/**
+	 * @deprecated Only use this for the spring store; does nothing when set on the Spring class
+	 */
 	hard?: any;
+	/**
+	 * @deprecated Only use this for the spring store; does nothing when set on the Spring class
+	 */
 	soft?: string | number | boolean;
+	/**
+	 * Only use this for the Spring class; does nothing when set on the spring store
+	 */
+	instant?: boolean;
+	/**
+	 * Only use this for the Spring class; does nothing when set on the spring store
+	 */
+	preserveMomentum?: number;
 }
 
 export type Updater<T> = (target_value: T, value: T) => T;

packages/svelte/src/motion/public.d.ts

@@ -1,17 +1,87 @@
-import { Readable } from '../store/public.js';
-import { SpringUpdateOpts, TweenedOptions, Updater } from './private.js';
+import { Readable, type Unsubscriber } from '../store/public.js';
+import { SpringUpdateOpts, TweenedOptions, Updater, SpringOpts } from './private.js';
+
+// TODO we do declaration merging here in order to not have a breaking change (renaming the Spring interface)
+// this means both the Spring class and the Spring interface are merged into one with some things only
+// existing on one side. In Svelte 6, remove the type definition and move the jsdoc onto the class in spring.js
 
 export interface Spring<T> extends Readable<T> {
-	set: (new_value: T, opts?: SpringUpdateOpts) => Promise<void>;
+	set(new_value: T, opts?: SpringUpdateOpts): Promise<void>;
+	/**
+	 * @deprecated Only exists on the legacy `spring` store, not the `Spring` class
+	 */
 	update: (fn: Updater<T>, opts?: SpringUpdateOpts) => Promise<void>;
+	/**
+	 * @deprecated Only exists on the legacy `spring` store, not the `Spring` class
+	 */
+	subscribe(fn: (value: T) => void): Unsubscriber;
 	precision: number;
 	damping: number;
 	stiffness: number;
 }
 
+/**
+ * A wrapper for a value that behaves in a spring-like fashion. Changes to `spring.target` will cause `spring.current` to
+ * move towards it over time, taking account of the `spring.stiffness` and `spring.damping` parameters.
+ *
+ * ```svelte
+ * <script>
+ * 	import { Spring } from 'svelte/motion';
+ *
+ * 	const spring = new Spring(0);
+ * </script>
+ *
+ * <input type="range" bind:value={spring.target} />
+ * <input type="range" bind:value={spring.current} disabled />
+ * ```
+ */
+export class Spring<T> {
+	constructor(value: T, options?: SpringOpts);
+
+	/**
+	 * Create a spring whose value is bound to the return value of `fn`. This must be called
+	 * inside an effect root (for example, during component initialisation).
+	 *
+	 * ```svelte
+	 * <script>
+	 * 	import { Spring } from 'svelte/motion';
+	 *
+	 * 	let { number } = $props();
+	 *
+	 * 	const spring = Spring.of(() => number);
+	 * </script>
+	 * ```
+	 */
+	static of<U>(fn: () => U, options?: SpringOpts): Spring<U>;
+
+	/**
+	 * Sets `spring.target` to `value` and returns a `Promise` that resolves if and when `spring.current` catches up to it.
+	 *
+	 * If `options.instant` is `true`, `spring.current` immediately matches `spring.target`.
+	 *
+	 * If `options.preserveMomentum` is provided, the spring will continue on its current trajectory for
+	 * the specified number of milliseconds. This is useful for things like 'fling' gestures.
+	 */
+	set(value: T, options?: SpringUpdateOpts): Promise<void>;
+
+	damping: number;
+	precision: number;
+	stiffness: number;
+	/**
+	 * The end value of the spring.
+	 * This property only exists on the `Spring` class, not the legacy `spring` store.
+	 */
+	target: T;
+	/**
+	 * The current value of the spring.
+	 * This property only exists on the `Spring` class, not the legacy `spring` store.
+	 */
+	get current(): T;
+}
+
 export interface Tweened<T> extends Readable<T> {
 	set(value: T, opts?: TweenedOptions<T>): Promise<void>;
 	update(updater: Updater<T>, opts?: TweenedOptions<T>): Promise<void>;
 }
 
-export * from './index.js';
+export { spring, tweened, Tween } from './index.js';