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feat: pass changed boolean array to derived callbacks #6786

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feat: pass changed boolean array to derived callbacks #6786

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48 changes: 39 additions & 9 deletions site/content/docs/03-run-time.md
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Original file line number Diff line number Diff line change
Expand Up @@ -268,7 +268,7 @@ This makes it possible to wrap almost any other reactive state handling library
store = writable(value?: any)
```
```js
store = writable(value?: any, start?: (set: (value: any) => void) => () => void)
store = writable(value?: any, start?: (set: (value: any) => void, update: (fn: any => any) => void) => () => void)
```

---
Expand All @@ -295,7 +295,7 @@ count.update(n => n + 1); // logs '2'

---

If a function is passed as the second argument, it will be called when the number of subscribers goes from zero to one (but not from one to two, etc). That function will be passed a `set` function which changes the value of the store. It must return a `stop` function that is called when the subscriber count goes from one to zero.
If a function is passed as the second argument, it will be called when the number of subscribers goes from zero to one (but not from one to two, etc). That function will be passed a `set` function which changes the value of the store, and an `update` function which works like the `update` method on the store, taking a callback to calculate the store's new value from its old value. It must return a `stop` function that is called when the subscriber count goes from one to zero.

```js
import { writable } from 'svelte/store';
Expand All @@ -319,7 +319,7 @@ Note that the value of a `writable` is lost when it is destroyed, for example wh
#### `readable`

```js
store = readable(value?: any, start?: (set: (value: any) => void) => () => void)
store = readable(value?: any, start?: (set: (value: any) => void, update: (fn: any => any) => void) => () => void)
```

---
Expand All @@ -338,6 +338,16 @@ const time = readable(null, set => {

return () => clearInterval(interval);
});

const ticktock = readable(null, (set, update) => {
set('tick');

const interval = setInterval(() => {
update(sound => sound === 'tick' ? 'tock' : 'tick');
}, 1000);

return () => clearInterval(interval);
});
```

#### `derived`
Expand All @@ -346,13 +356,13 @@ const time = readable(null, set => {
store = derived(a, callback: (a: any) => any)
```
```js
store = derived(a, callback: (a: any, set: (value: any) => void) => void | () => void, initial_value: any)
store = derived(a, callback: (a: any, set: (value: any) => void, update: (fn: any => any) => void) => void | () => void, initial_value: any)
```
```js
store = derived([a, ...b], callback: ([a: any, ...b: any[]]) => any)
```
```js
store = derived([a, ...b], callback: ([a: any, ...b: any[]], set: (value: any) => void) => void | () => void, initial_value: any)
store = derived([a, ...b], callback: ([a: any, ...b: any[]], set: (value: any) => void, update: (fn: any => any) => void, changed: boolean[]) => void | () => void, initial_value: any)
```

---
Expand All @@ -369,16 +379,23 @@ const doubled = derived(a, $a => $a * 2);

---

The callback can set a value asynchronously by accepting a second argument, `set`, and calling it when appropriate.
The callback can set a value asynchronously by accepting a second argument, `set`, and an optional third argument, `update`, calling either or both of them when appropriate.

In this case, you can also pass a third argument to `derived` — the initial value of the derived store before `set` is first called.
In this case, you can also pass a third argument to `derived` — the initial value of the derived store before `set` or `update` is first called. If no initial value is specified, the store's initial value will be `undefined`.

```js
import { derived } from 'svelte/store';

const delayed = derived(a, ($a, set) => {
setTimeout(() => set($a), 1000);
}, 'one moment...');

const delayedIncrement = derived(a, ($a, set, update) => {
set($a);
setTimeout(() => update(x => x + 1), 1000);
// every time $a produces a value, this produces two
// values, $a immediately and then $a + 1 a second later
});
```

---
Expand All @@ -401,7 +418,7 @@ const tick = derived(frequency, ($frequency, set) => {

---

In both cases, an array of arguments can be passed as the first argument instead of a single store.
In both cases, an array of stores can be passed as the first argument instead of a single store. In this case, the callback can optionally take a fourth argument, `changed`, which will be an array of Booleans describing which store value(s) changed since the last time the callback was called. (In the single-store case, the `changed` array would be pointless since it would always be equal to `[true]`.)

```js
import { derived } from 'svelte/store';
Expand All @@ -411,6 +428,19 @@ const summed = derived([a, b], ([$a, $b]) => $a + $b);
const delayed = derived([a, b], ([$a, $b], set) => {
setTimeout(() => set($a + $b), 1000);
});

const loggingSum = derived([a, b], ([$a, $b], set, _, changed) => {
const [aChanged, bChanged] = changed;
if (aChanged) console.log('New value of a', $a);
if (bChanged) console.log('New value of b', $b);
set($a + $b);
});

const complexLogic = derived([a, b], ([$a, $b], set, update, changed) => {
const [aChanged, bChanged] = changed;
if (aChanged) set($a + $b);
if (bChanged) update(n => n * 2 - $b);
});
```

#### `get`
Expand Down Expand Up @@ -808,7 +838,7 @@ The `crossfade` function creates a pair of [transitions](docs#transition_fn) cal
* `delay` (`number`, default 0) — milliseconds before starting
* `duration` (`number` | `function`, default 800) — milliseconds the transition lasts
* `easing` (`function`, default `cubicOut`) — an [easing function](docs#svelte_easing)
* `fallback` (`function`) — A fallback [transition](docs#transition_fn) to use for send when there is no matching element being received, and for receive when there is no element being sent.
* `fallback` (`function`) — A fallback [transition](docs#transition_fn) to use for send when there is no matching element being received, and for receive when there is no element being sent.

```sv
<script>
Expand Down
41 changes: 36 additions & 5 deletions src/runtime/store/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,7 @@ export type Updater<T> = (value: T) => T;
type Invalidator<T> = (value?: T) => void;

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

/** Readable interface for subscribing. */
export interface Readable<T> {
Expand Down Expand Up @@ -92,7 +92,7 @@ export function writable<T>(value?: T, start: StartStopNotifier<T> = noop): Writ
const subscriber: SubscribeInvalidateTuple<T> = [run, invalidate];
subscribers.add(subscriber);
if (subscribers.size === 1) {
stop = start(set) || noop;
stop = start(set, update) || noop;
}
run(value);

Expand All @@ -115,6 +115,34 @@ type Stores = Readable<any> | [Readable<any>, ...Array<Readable<any>>] | Array<R
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: Subscriber<T>, update: (fn: Updater<T>) => void, changed: boolean[]) => 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 - when used asynchronously
*/
export function derived<S extends Stores, T>(
stores: S,
fn: (values: StoresValues<S>, set: Subscriber<T>, update: (fn: Updater<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.
Expand All @@ -125,7 +153,7 @@ type StoresValues<T> = T extends Readable<infer U> ? U :
*/
export function derived<S extends Stores, T>(
stores: S,
fn: (values: StoresValues<S>, set: (value: T) => void) => Unsubscriber | void,
fn: (values: StoresValues<S>, set: Subscriber<T>) => Unsubscriber | void,
initial_value?: T
): Readable<T>;

Expand Down Expand Up @@ -163,19 +191,21 @@ export function derived<T>(stores: Stores, fn: Function, initial_value?: T): Rea

const auto = fn.length < 2;

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

let pending = 0;
const changed = [];
let cleanup = noop;

const sync = () => {
if (pending) {
return;
}
cleanup();
const result = fn(single ? values[0] : values, set);
const result = fn(single ? values[0] : values, set, update, changed);
changed.fill(false);
if (auto) {
set(result as T);
} else {
Expand All @@ -188,6 +218,7 @@ export function derived<T>(stores: Stores, fn: Function, initial_value?: T): Rea
(value) => {
values[i] = value;
pending &= ~(1 << i);
changed[i] = true;
if (inited) {
sync();
}
Expand Down
39 changes: 39 additions & 0 deletions test/store/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -281,6 +281,45 @@ describe('store', () => {
unsubscribe();
});

it('provides a boolean array to easily tell what changed', () => {
const count = writable(0);
const values = [];

const a = derived(count, $count => {
return 'a' + $count;
});

const b = derived(count, $count => {
return 'b' + $count;
});

const c = writable(0);

const combined = derived([a, b, c], ([a, b, c], set, _u, changes) => {
const [aChanged, bChanged, cChanged] = changes;
if (aChanged && bChanged) {
set(a + b);
} else if (cChanged) {
set('c' + c);
} else {
set('a or b changed without the other one changing');
}
});

const unsubscribe = combined.subscribe(v => {
values.push(v);
});

assert.deepEqual(values, ['a0b0']);

c.set(2);
count.set(1);

assert.deepEqual(values, ['a0b0', 'c2', 'a1b1']);

unsubscribe();
});

it('derived dependency does not update and shared ancestor updates', () => {
const root = writable({ a: 0, b:0 });
const values = [];
Expand Down