svelte/src/runtime/store/index.ts

import { run_all, subscribe, noop, safe_not_equal, is_function, get_store_value } from 'svelte/internal';

/** Callback to inform of a value updates. */
export type Subscriber<T> = (value: T) => void;

/** Unsubscribes from value updates. */
export type Unsubscriber = () => void;

/** Callback to update a value. */
export type Updater<T> = (value: T) => T;

/** Cleanup logic callback. */
type Invalidator<T> = (value?: T) => void;

/** Start and stop notification callbacks. */
export type StartStopNotifier<T> = (set: Subscriber<T>, invalidate: Invalidator<T>) => Unsubscriber | void;

/** Readable interface for subscribing. */
export interface Readable<T> {
	/**
	 * Subscribe on value changes.
	 * @param run subscription callback
	 * @param invalidate cleanup callback
	 */
	subscribe(this: void, run: Subscriber<T>, invalidate?: Invalidator<T>, revalidate?: Revalidator): Unsubscriber;
}

/** Writable interface for both updating and subscribing. */
export interface Writable<T> extends Readable<T> {
	/**
	 * Set value and inform subscribers.
	 * @param value to set
	 */
	set(this: void, value: T): void;

	/**
	 * Update value using callback and inform subscribers.
	 * @param updater callback
	 */
	update(this: void, updater: Updater<T>): void;
}

type Revalidator = () => void;
/** Tuple of subscriber, invalidator, and revalidator. */
type Subscription<T> = [Subscriber<T>, Invalidator<T>, Revalidator];

const subscriber_queue = [];

/**
 * Creates a `Readable` store that allows reading by subscription.
 * @param value initial value
 * @param {StartStopNotifier}start start and stop notifications for subscriptions
 */
export function readable<T>(value?: T, start?: StartStopNotifier<T>): Readable<T> {
	return {
		subscribe: writable(value, start).subscribe
	};
}

/**
 * Create a `Writable` store that allows both updating and reading by subscription.
 * @param {*=}value initial value
 * @param {StartStopNotifier=}start start and stop notifications for subscriptions
 */
export function writable<T>(value?: T, start: StartStopNotifier<T> = noop): Writable<T> {
	let stop: Unsubscriber;
	const subscriptions: Array<Subscription<T>> = [];

	function invalidate() {
		for (let i = 0; i < subscriptions.length; i += 1) {
			subscriptions[i][1]();
		}
	}
	function revalidate() {
		for (let i = 0; i < subscriptions.length; i += 1) {
			subscriptions[i][2]();
		}
	}

	function set(new_value: T): void {
		if (safe_not_equal(value, new_value)) {
			value = new_value;
			if (stop) { // store is ready
				const run_queue = !subscriber_queue.length;
				invalidate();
				for (let i = 0; i < subscriptions.length; i += 1) {
					subscriber_queue.push(subscriptions[i], value);
				}
				if (run_queue) {
					for (let i = 0; i < subscriber_queue.length; i += 2) {
						subscriber_queue[i][0](subscriber_queue[i + 1]);
					}
					subscriber_queue.length = 0;
				}
			}
		} else {
			revalidate();
		}
	}

	function update(fn: Updater<T>): void {
		set(fn(value));
	}

	function subscribe(run: Subscriber<T>, invalidate: Invalidator<T> = noop, revalidate: Revalidator = noop): Unsubscriber {
		const subscription: Subscription<T> = [run, invalidate, revalidate];
		subscriptions.push(subscription);
		if (subscriptions.length === 1) {
			stop = start(set, invalidate) || noop;
		}
		run(value);

		return () => {
			const index = subscriptions.indexOf(subscription);
			if (index !== -1) {
				subscriptions.splice(index, 1);
			}
			if (subscriptions.length === 0) {
				stop();
				stop = null;
			}
		};
	}

	return { set, update, subscribe };
}

/** One or more `Readable`s. */
type Stores = Readable<any> | [Readable<any>, ...Array<Readable<any>>];

/** One or more values from `Readable` stores. */
type StoresValues<T> = T extends Readable<infer U> ? U :
	{ [K in keyof T]: T[K] extends Readable<infer U> ? U : never };

/**
 * Derived value store by synchronizing one or more readable stores and
 * applying an aggregation function over its input values.
 *
 * @param stores - input stores
 * @param fn - function callback that aggregates the values
 * @param initial_value - when used asynchronously
 */
export function derived<S extends Stores, T>(
	stores: S,
	fn: (values: StoresValues<S>, set: (value: T) => void) => Unsubscriber | void,
	initial_value?: T
): Readable<T>;

/**
 * Derived value store by synchronizing one or more readable stores and
 * applying an aggregation function over its input values.
 *
 * @param stores - input stores
 * @param fn - function callback that aggregates the values
 * @param initial_value - initial value
 */
export function derived<S extends Stores, T>(
	stores: S,
	fn: (values: StoresValues<S>) => T,
	initial_value?: T
): Readable<T>;

/**
 * Derived value store by synchronizing one or more readable stores and
 * applying an aggregation function over its input values.
 *
 * @param stores - input stores
 * @param fn - function callback that aggregates the values
 */
export function derived<S extends Stores, T>(
	stores: S,
	fn: (values: StoresValues<S>) => T
): Readable<T>;

export function derived<T>(stores: Stores, fn: Function, initial_value?: T): Readable<T> {
	const single = !Array.isArray(stores);
	const stores_array: Array<Readable<any>> = single
		? [stores as Readable<any>]
		: stores as Array<Readable<any>>;

	const auto = fn.length < 2;

	return readable(initial_value, (set, invalidate) => {
		let inited = false;
		const values = [];

		let pending = 0;
		let cleanup = noop;

		const sync = () => {
			if (pending) {
				return;
			}
			cleanup();
			const result = fn(single ? values[0] : values, set);
			if (auto) {
				set(result as T);
			} else {
				cleanup = is_function(result) ? result as Unsubscriber : noop;
			}
		};

		const unsubscribers = stores_array.map((store, i) => subscribe(
			store,
			(value) => {
				values[i] = value;
				pending &= ~(1 << i);
				if (inited) {
					sync();
				}
			},
			() => {
				const invalidated = pending & (1 << i);
				pending |= (1 << i);
				if (!invalidated) {
					invalidate();
				}
			},
			() => pending &= ~(1 << i)
		));

		inited = true;
		sync();

		return function stop() {
			run_all(unsubscribers);
			cleanup();
		};
	});
}

/**
 * Get the current value from a store by subscribing and immediately unsubscribing.
 * @param store readable
 */
export { get_store_value as get };