use-context-selector

What is use-context-selector?

The `use-context-selector` package is a React hook that allows for more granular context updates by enabling the selection of specific parts of the context. This can help optimize performance by reducing unnecessary re-renders in components that consume context.

What are use-context-selector's main functionalities?

Context Selection

This feature allows you to select a specific part of the context value, which can help in optimizing performance by preventing unnecessary re-renders of components that consume the context.

```jsx
import React, { createContext } from 'react';
import { useContextSelector } from 'use-context-selector';

const MyContext = createContext();

const MyComponent = () => {
  const value = useContextSelector(MyContext, (v) => v.someValue);
  return <div>{value}</div>;
};

const App = () => {
  const contextValue = { someValue: 'Hello, World!' };
  return (
    <MyContext.Provider value={contextValue}>
      <MyComponent />
    </MyContext.Provider>
  );
};

export default App;
```

Avoiding Unnecessary Re-renders

This example demonstrates how `use-context-selector` can be used to avoid unnecessary re-renders. The `MyComponent` only re-renders when `someValue` changes, not when `otherValue` changes.

```jsx
import React, { createContext, useState } from 'react';
import { useContextSelector } from 'use-context-selector';

const MyContext = createContext();

const MyComponent = () => {
  const value = useContextSelector(MyContext, (v) => v.someValue);
  console.log('MyComponent re-rendered');
  return <div>{value}</div>;
};

const App = () => {
  const [contextValue, setContextValue] = useState({ someValue: 'Hello, World!', otherValue: 42 });

  return (
    <MyContext.Provider value={contextValue}>
      <MyComponent />
      <button onClick={() => setContextValue({ ...contextValue, otherValue: contextValue.otherValue + 1 })}>
        Update Other Value
      </button>
    </MyContext.Provider>
  );
};

export default App;
```

Other packages similar to use-context-selector

react-redux

React-Redux is a popular library for managing state in React applications using the Redux pattern. It provides a `connect` function and hooks like `useSelector` and `useDispatch` to connect React components to the Redux store. While `react-redux` is more comprehensive and suited for global state management, `use-context-selector` is more lightweight and focused on optimizing context usage.

recoil

Recoil is a state management library for React that provides a way to manage shared state with atoms and selectors. It offers fine-grained control over state updates and dependencies, similar to `use-context-selector`. However, Recoil is more feature-rich and provides a more structured approach to state management compared to the more lightweight `use-context-selector`.

zustand

Zustand is a small, fast, and scalable state management solution for React. It uses hooks to manage state and provides a simple API for creating and consuming state. Like `use-context-selector`, it aims to optimize performance by minimizing re-renders, but it is more focused on global state management rather than context-specific optimizations.

README

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.

useContextSelector is recently proposed. While waiting for the process, this library provides the API in userland.

Install

npm install use-context-selector

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'));

Technical memo

React context by nature triggers propagation of component re-rendering if a value is changed. To avoid this, this library uses undocumented feature of calculateChangedBits. It then uses a subscription model to force update when a component needs to re-render.

API

createContext

This creates a special context for useContextSelector.

Parameters

• defaultValue any

Examples

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

Returns React.Context

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.

Parameters

• context React.Context
• selector Function

Examples

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

Returns any

useContext

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

Parameters

• context React.Context

Examples

const person = useContext(PersonContext);

Returns any

BridgeProvider

This is a Provider component for bridging multiple react roots

Parameters

• props Object
  • props.context React.Context
  • props.value any
  • props.children React.ReactNote

Examples

const valueToBridge = useContext(PersonContext);
return (
  <Renderer>
    <BridgeProvider context={PersonContext} value={valueToBridge}>
      {children}
    </Bidge>
  </Renderer>
);

Returns React.ReactElement

Limitations

• Subscriptions are per-context basis. So, even if there are multiple context providers in a component tree, all components are subscribed to all providers. This may lead false positives (extra re-renders).
• 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.
• Context consumers are not supported.
• The stale props issue can't be solved in userland. (workaround with try-catch)

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