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useSyncExternalStore React Native version (#22367)
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‎packages/use-sync-external-store/index.native.js‎

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/**
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*
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* @flow
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*/
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'use strict';
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export * from './src/useSyncExternalStoreClient';

‎packages/use-sync-external-store/npm/index.native.js‎

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'use strict';
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if (process.env.NODE_ENV === 'production') {
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module.exports = require('./cjs/use-sync-external-store.native.production.min.js');
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} else {
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module.exports = require('./cjs/use-sync-external-store.native.development.js');
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}

‎packages/use-sync-external-store/package.json‎

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"build-info.json",
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"index.js",
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"extra.js",
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"index.native.js",
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"cjs/"
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],
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"license": "MIT",

‎packages/use-sync-external-store/src/__tests__/useSyncExternalStoreNative-test.js‎

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/**
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*
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* @emails react-core
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*
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* @jest-environment node
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*/
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'use strict';
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let React;
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let ReactNoop;
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let Scheduler;
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let useSyncExternalStore;
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let useSyncExternalStoreExtra;
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let act;
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// This tests the userspace shim of `useSyncExternalStore` in a server-rendering
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// (Node) environment
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describe('useSyncExternalStore (userspace shim, server rendering)', () => {
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beforeEach(() => {
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jest.resetModules();
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// Remove useSyncExternalStore from the React imports so that we use the
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// shim instead. Also removing startTransition, since we use that to detect
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// outdated 18 alphas that don't yet include useSyncExternalStore.
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//
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// Longer term, we'll probably test this branch using an actual build of
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// React 17.
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jest.mock('react', () => {
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const {
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// eslint-disable-next-line no-unused-vars
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startTransition: _,
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// eslint-disable-next-line no-unused-vars
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useSyncExternalStore: __,
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// eslint-disable-next-line no-unused-vars
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unstable_useSyncExternalStore: ___,
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...otherExports
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} = jest.requireActual('react');
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return otherExports;
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});
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jest.mock('use-sync-external-store', () =>
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jest.requireActual('use-sync-external-store/index.native'),
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);
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React = require('react');
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ReactNoop = require('react-noop-renderer');
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Scheduler = require('scheduler');
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act = require('jest-react').act;
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useSyncExternalStore = require('use-sync-external-store')
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.useSyncExternalStore;
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useSyncExternalStoreExtra = require('use-sync-external-store/extra')
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.useSyncExternalStoreExtra;
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});
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function Text({text}) {
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Scheduler.unstable_yieldValue(text);
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return text;
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}
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function createExternalStore(initialState) {
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const listeners = new Set();
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let currentState = initialState;
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return {
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set(text) {
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currentState = text;
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ReactNoop.batchedUpdates(() => {
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listeners.forEach(listener => listener());
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});
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},
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subscribe(listener) {
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listeners.add(listener);
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return () => listeners.delete(listener);
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},
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getState() {
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return currentState;
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},
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getSubscriberCount() {
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return listeners.size;
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},
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};
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}
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test('native version', async () => {
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const store = createExternalStore('client');
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function App() {
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const text = useSyncExternalStore(
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store.subscribe,
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store.getState,
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() => 'server',
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);
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return <Text text={text} />;
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}
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const root = ReactNoop.createRoot();
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await act(() => {
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root.render(<App />);
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});
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expect(Scheduler).toHaveYielded(['client']);
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expect(root).toMatchRenderedOutput('client');
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});
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test('native version', async () => {
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const store = createExternalStore('client');
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function App() {
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const text = useSyncExternalStore(
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store.subscribe,
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store.getState,
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() => 'server',
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);
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return <Text text={text} />;
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}
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const root = ReactNoop.createRoot();
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await act(() => {
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root.render(<App />);
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});
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expect(Scheduler).toHaveYielded(['client']);
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expect(root).toMatchRenderedOutput('client');
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});
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// @gate !(enableUseRefAccessWarning && __DEV__)
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test('Using isEqual to bailout', async () => {
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const store = createExternalStore({a: 0, b: 0});
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function A() {
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const {a} = useSyncExternalStoreExtra(
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store.subscribe,
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store.getState,
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null,
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state => ({a: state.a}),
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(state1, state2) => state1.a === state2.a,
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);
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return <Text text={'A' + a} />;
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}
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function B() {
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const {b} = useSyncExternalStoreExtra(
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store.subscribe,
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store.getState,
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null,
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state => {
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return {b: state.b};
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},
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(state1, state2) => state1.b === state2.b,
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);
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return <Text text={'B' + b} />;
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}
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function App() {
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return (
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<>
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<A />
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<B />
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</>
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);
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}
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const root = ReactNoop.createRoot();
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act(() => root.render(<App />));
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expect(Scheduler).toHaveYielded(['A0', 'B0']);
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expect(root).toMatchRenderedOutput('A0B0');
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// Update b but not a
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await act(() => {
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store.set({a: 0, b: 1});
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});
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// Only b re-renders
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expect(Scheduler).toHaveYielded(['B1']);
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expect(root).toMatchRenderedOutput('A0B1');
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// Update a but not b
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await act(() => {
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store.set({a: 1, b: 1});
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});
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// Only a re-renders
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expect(Scheduler).toHaveYielded(['A1']);
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expect(root).toMatchRenderedOutput('A1B1');
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});
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});

‎packages/use-sync-external-store/src/useSyncExternalStore.js‎

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* @flow
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*/
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import * as React from 'react';
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import is from 'shared/objectIs';
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import invariant from 'shared/invariant';
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import {canUseDOM} from 'shared/ExecutionEnvironment';
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import {useSyncExternalStore as client} from './useSyncExternalStoreClient';
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import {useSyncExternalStore as server} from './useSyncExternalStoreServer';
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// Intentionally not using named imports because Rollup uses dynamic
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// dispatch for CommonJS interop named imports.
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const {
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useState,
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useEffect,
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useLayoutEffect,
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useDebugValue,
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// The built-in API is still prefixed.
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unstable_useSyncExternalStore: builtInAPI,
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} = React;
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// TODO: This heuristic doesn't work in React Native. We'll need to provide a
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// special build, using the `.native` extension.
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const isServerEnvironment = !canUseDOM;
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// Prefer the built-in API, if it exists. If it doesn't exist, then we assume
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// we're in version 16 or 17, so rendering is always synchronous. The shim
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// does not support concurrent rendering, only the built-in API.
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export const useSyncExternalStore =
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builtInAPI !== undefined
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? ((builtInAPI: any): typeof useSyncExternalStore_client)
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: isServerEnvironment
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? useSyncExternalStore_server
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: useSyncExternalStore_client;
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let didWarnOld18Alpha = false;
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let didWarnUncachedGetSnapshot = false;
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function useSyncExternalStore_server<T>(
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subscribe: (() => void) => () => void,
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getSnapshot: () => T,
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getServerSnapshot?: () => T,
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): T {
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if (getServerSnapshot === undefined) {
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invariant(
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false,
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'Missing getServerSnapshot, which is required for server-' +
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'rendered content.',
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);
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}
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return getServerSnapshot();
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}
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// Disclaimer: This shim breaks many of the rules of React, and only works
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// because of a very particular set of implementation details and assumptions
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// -- change any one of them and it will break. The most important assumption
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// is that updates are always synchronous, because concurrent rendering is
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// only available in versions of React that also have a built-in
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// useSyncExternalStore API. And we only use this shim when the built-in API
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// does not exist.
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//
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// Do not assume that the clever hacks used by this hook also work in general.
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// The point of this shim is to replace the need for hacks by other libraries.
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function useSyncExternalStore_client<T>(
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subscribe: (() => void) => () => void,
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getSnapshot: () => T,
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// Note: The client shim does not use getServerSnapshot, because pre-18
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// versions of React do not expose a way to check if we're hydrating. So
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// users of the shim will need to track that themselves and return the
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// correct value from `getSnapshot`.
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getServerSnapshot?: () => T,
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): T {
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if (__DEV__) {
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if (!didWarnOld18Alpha) {
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if (React.startTransition !== undefined) {
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didWarnOld18Alpha = true;
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console.error(
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'You are using an outdated, pre-release alpha of React 18 that ' +
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'does not support useSyncExternalStore. The ' +
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'use-sync-external-store shim will not work correctly. Upgrade ' +
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'to a newer pre-release.',
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);
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}
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}
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}
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// Read the current snapshot from the store on every render. Again, this
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// breaks the rules of React, and only works here because of specific
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// implementation details, most importantly that updates are
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// always synchronous.
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const value = getSnapshot();
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if (__DEV__) {
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if (!didWarnUncachedGetSnapshot) {
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if (value !== getSnapshot()) {
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console.error(
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'The result of getSnapshot should be cached to avoid an infinite loop',
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);
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didWarnUncachedGetSnapshot = true;
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}
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}
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}
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// Because updates are synchronous, we don't queue them. Instead we force a
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// re-render whenever the subscribed state changes by updating an some
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// arbitrary useState hook. Then, during render, we call getSnapshot to read
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// the current value.
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//
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// Because we don't actually use the state returned by the useState hook, we
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// can save a bit of memory by storing other stuff in that slot.
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//
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// To implement the early bailout, we need to track some things on a mutable
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// object. Usually, we would put that in a useRef hook, but we can stash it in
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// our useState hook instead.
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//
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// To force a re-render, we call forceUpdate({inst}). That works because the
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// new object always fails an equality check.
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const [{inst}, forceUpdate] = useState({inst: {value, getSnapshot}});
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// Track the latest getSnapshot function with a ref. This needs to be updated
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// in the layout phase so we can access it during the tearing check that
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// happens on subscribe.
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useLayoutEffect(() => {
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inst.value = value;
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inst.getSnapshot = getSnapshot;
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// Whenever getSnapshot or subscribe changes, we need to check in the
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// commit phase if there was an interleaved mutation. In concurrent mode
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// this can happen all the time, but even in synchronous mode, an earlier
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// effect may have mutated the store.
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if (checkIfSnapshotChanged(inst)) {
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// Force a re-render.
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forceUpdate({inst});
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}
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}, [subscribe, value, getSnapshot]);
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useEffect(() => {
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// Check for changes right before subscribing. Subsequent changes will be
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// detected in the subscription handler.
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if (checkIfSnapshotChanged(inst)) {
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// Force a re-render.
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forceUpdate({inst});
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}
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const handleStoreChange = () => {
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// TODO: Because there is no cross-renderer API for batching updates, it's
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// up to the consumer of this library to wrap their subscription event
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// with unstable_batchedUpdates. Should we try to detect when this isn't
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// the case and print a warning in development?
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// The store changed. Check if the snapshot changed since the last time we
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// read from the store.
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if (checkIfSnapshotChanged(inst)) {
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// Force a re-render.
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forceUpdate({inst});
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}
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};
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// Subscribe to the store and return a clean-up function.
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return subscribe(handleStoreChange);
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}, [subscribe]);
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useDebugValue(value);
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return value;
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}
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function checkIfSnapshotChanged(inst) {
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const latestGetSnapshot = inst.getSnapshot;
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const prevValue = inst.value;
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try {
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const nextValue = latestGetSnapshot();
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return !is(prevValue, nextValue);
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} catch (error) {
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return true;
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}
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}
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export const useSyncExternalStore = canUseDOM ? client : server;

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