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

npm install use-context-selector

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 Mode. (You don't need to use it in Legacy Mode.)

Usage

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 }) => {
  const [state, setState] = useState({ count1: 0, count2: 0 });
  return (
    <context.Provider value={[state, setState]}>
      {children}
    </context.Provider>
  );
};

const App = () => (
  <StateProvider>
    <Counter1 />
    <Counter2 />
  </StateProvider>
);

ReactDOM.render(<App />, document.getElementById('app'));

API

createContext

This creates a special context for useContextSelector.

Parameters

• defaultValue Value

Examples

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

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

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.

Parameters

• context Context<Value>

Examples

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

const update = useContextUpdate();
update(() => setState(...));

BridgeProvider

This is a Provider component for bridging multiple react roots

Type: FC<{context: Context<any>, value: any}>

Parameters

• $0 Object
  • $0.context
  • $0.value
  • $0.children

Examples

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 issue can't be solved in userland.
• Tearing is only avoided within the Provider tree. A value outside the Provider will tear. (02_tearing_spec fails)

Examples

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

PORT=8080 npm run examples:01_minimal

and open http://localhost:8080 in your web browser.

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

Related projects

• react-tracked
• reactive-react-redux