use-context-selector


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
is proposed and later proposed
Speculative Mode
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
npm install 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 intentionally.

It also requires useContextUpdate to behave better in concurrent rendering.
Its usage is optional and only required if the default behavior is unexpected.

If you need a simpler solution, you can use useSyncExternalStore without any libraries. See an example.

Usage

`javascript
import { useState } from 'react';
import { createRoot } from 'react-dom/client';

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 (


createRoot(document.getElementById('app')).render();
`

API

$3

This creates a special context for useContextSelector.

#### Parameters

* defaultValue Value

#### Examples

`javascript
import { createContext } from 'use-context-selector';

const PersonContext = createContext({ firstName: '', familyName: '' });
`

$3

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\
* selector function (value: Value): Selected

#### Examples

`javascript
import { useContextSelector } from 'use-context-selector';

const firstName = useContextSelector(PersonContext, (state) => state.firstName);
`

$3

This hook returns the entire context value.
Use this instead of React.useContext for consistent behavior.

#### Parameters

* context Context\

#### Examples

`javascript
import { useContext } from 'use-context-selector';

const person = useContext(PersonContext);
`

$3

This hook returns an update function to wrap an updating 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\

#### Examples

`javascript
import { useContextUpdate } from 'use-context-selector';

const update = useContextUpdate();

// Wrap set state function
update(() => setState(...));

// Experimental suspense mode
update(() => setState(...), { suspense: true });
`

$3

This is a Provider component for bridging multiple react roots

#### Parameters

* $0 {context: Context\, value: any, children: ReactNode}

* $0.context
* $0.value
* $0.children

#### Examples

`javascript
const valueToBridge = useBridgeValue(PersonContext);
return (


{children}


);
`

$3

This hook return a value for BridgeProvider

#### Parameters

* context Context\

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 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 folder contains working examples.
You can run one of them with

`bash
PORT=8080 yarn run examples:01_counter
``

and open in your web browser.

You can also try them in codesandbox.io:
01
02
03

Projects that use use-context-selector

* react-tracked
* use-atom
* react-hooks-fetch