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# use-context-selector [](https://github.com/dai-shi/use-context-selector/actions?query=workflow%3ACI) [](https://www.npmjs.com/package/use-context-selector) [](https://bundlephobia.com/result?p=use-context-selector) [](https://discord.gg/MrQdmzd) React useContextSelector hook in userland ## Introduction React Context and useContext is often used to avoid prop drilling, however it's known that there's a performance issue. When a context value is changed, all components that useContext will re-render. To solve this issue, [useContextSelector](https://github.com/reactjs/rfcs/pull/119) is proposed and later proposed [Speculative Mode](https://github.com/reactjs/rfcs/pull/150) with context selector support. This library provides the API in userland. Prior to v1.3, it uses `changedBits=0` feature to stop propagation, v1.3 no longer depends on this undocumented feature. ## Install This package requires some peer dependencies, which you need to install by yourself. ```bash yarn add use-context-selector react scheduler ``` Notes for library authors: Please do not forget to keep `"peerDependencies"` and note instructions to let users to install peer dependencies. ## Technical memo To make it work like original React context, it uses [useReducer cheat mode](https://overreacted.io/a-complete-guide-to-useeffect/#why-usereducer-is-the-cheat-mode-of-hooks) intentionally. It also requires `useContextUpdate` to behave better in concurrent rendering. Its usage is optional and only required if the default behavior is unexpected. ## Usage ```javascript import React, { useState } from 'react'; import ReactDOM from 'react-dom'; import { createContext, useContextSelector } from 'use-context-selector'; const context = createContext(null); const Counter1 = () => { const count1 = useContextSelector(context, v => v[0].count1); const setState = useContextSelector(context, v => v[1]); const increment = () => setState(s => ({ ...s, count1: s.count1 + 1, })); return ( <div> <span>Count1: {count1}</span> <button type="button" onClick={increment}>+1</button> {Math.random()} </div> ); }; const Counter2 = () => { const count2 = useContextSelector(context, v => v[0].count2); const setState = useContextSelector(context, v => v[1]); const increment = () => setState(s => ({ ...s, count2: s.count2 + 1, })); return ( <div> <span>Count2: {count2}</span> <button type="button" onClick={increment}>+1</button> {Math.random()} </div> ); }; const StateProvider = ({ children }) => ( <context.Provider value={useState({ count1: 0, count2: 0 })}> {children} </context.Provider> ); const App = () => ( <StateProvider> <Counter1 /> <Counter2 /> </StateProvider> ); ReactDOM.render(<App />, document.getElementById('app')); ``` ## API <!-- Generated by documentation.js. Update this documentation by updating the source code. --> ### createContext This creates a special context for `useContextSelector`. #### Parameters * `defaultValue` **Value** #### Examples ```javascript import { createContext } from 'use-context-selector'; const PersonContext = createContext({ firstName: '', familyName: '' }); ``` ### useContextSelector This hook returns context selected value by selector. It will only accept context created by `createContext`. It will trigger re-render if only the selected value is referentially changed. The selector should return referentially equal result for same input for better performance. #### Parameters * `context` **Context\<Value>** * `selector` **function (value: Value): Selected** #### Examples ```javascript import { useContextSelector } from 'use-context-selector'; const firstName = useContextSelector(PersonContext, state => state.firstName); ``` ### useContext This hook returns the entire context value. Use this instead of React.useContext for consistent behavior. #### Parameters * `context` **Context\<Value>** #### Examples ```javascript import { useContext } from 'use-context-selector'; const person = useContext(PersonContext); ``` ### useContextUpdate This hook returns an update function that accepts a thunk function Use this for a function that will change a value in concurrent rendering in React 18. Otherwise, there's no need to use this hook. #### Parameters * `context` **Context\<Value>** #### Examples ```javascript import { useContextUpdate } from 'use-context-selector'; const update = useContextUpdate(); // Wrap set state function update(() => setState(...)); // Experimental suspense mode update(() => setState(...), { suspense: true }); ``` ### BridgeProvider This is a Provider component for bridging multiple react roots #### Parameters * `$0` **{context: Context\<any>, value: any, children: ReactNode}** * `$0.context` * `$0.value` * `$0.children` #### Examples ```javascript const valueToBridge = useBridgeValue(PersonContext); return ( <Renderer> <BridgeProvider context={PersonContext} value={valueToBridge}> {children} </BridgeProvider> </Renderer> ); ``` ### useBridgeValue This hook return a value for BridgeProvider #### Parameters * `context` **Context\<any>** ## Limitations * In order to stop propagation, `children` of a context provider has to be either created outside of the provider or memoized with `React.memo`. * Provider trigger re-renders only if the context value is referentially changed. * Neither context consumers or class components are supported. * The [stale props](https://react-redux.js.org/api/hooks#stale-props-and-zombie-children) issue can't be solved in userland. * Tearing is only avoided if all consumers get data using `useContextSelector`. If you use both props and `use-context-selector` to pass the same data, they may provide inconsistence data for a brief moment. (`02_tearing_spec` fails) ## Examples The [examples](examples) folder contains working examples. You can run one of them with ```bash PORT=8080 yarn run examples:01_minimal ``` and open <http://localhost:8080> in your web browser. You can also try them in codesandbox.io: [01](https://codesandbox.io/s/github/dai-shi/use-context-selector/tree/main/examples/01\_minimal) [02](https://codesandbox.io/s/github/dai-shi/use-context-selector/tree/main/examples/02\_typescript) [03](https://codesandbox.io/s/github/dai-shi/use-context-selector/tree/main/examples/03\_suspense) ## Projects that use use-context-selector * [react-tracked](https://github.com/dai-shi/react-tracked) * [use-atom](https://github.com/dai-shi/use-atom) * [react-hooks-fetch](https://github.com/dai-shi/react-hooks-fetch)