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 useContext with selector support 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.

v1 uses calculateChangedBits=()=>0 technique to stop propagation, while v2 uses useMutableSource.

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'));

Migrating from v1 to v2

In v1:

useContextSelector(context, state => state.count);

In v2:

useContext(context, useCallback(state => state.count, []));

In this case, you can (should) also define the selector function outside render.

API

createContext

This creates a special context for selector-enabled useContext.

It doesn't pass its value but a ref of the value. Unlike the original context provider, this context provider expects the context value to be immutable and stable.

Parameters

• defaultValue Value

Examples

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

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

useContext

This hook returns context value with optional selector.

It will only accept context created by createContext. It will trigger re-render if only the selected value is referentially changed. The selector must be stable. Either define selector outside render or wrap with useCallback.

The selector should return referentially equal result for same input for better performance.

Parameters

• context Context<Value>
• selector function (value: Value): Selected (optional, default identity as(value:Value)=>Selected)

Examples

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

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

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<any>

Examples

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

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

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.
• 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