React useContextSelector hook in userland
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.
This package requires some peer dependencies, which you need to install by yourself.
npm install use-context-selector react schedulerNotes for library authors:
Please do not forget to keep "peerDependencies" and
note instructions to let users to install peer dependencies.
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.
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 (
<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>
);
createRoot(document.getElementById('app')).render(<App />);This creates a special context for useContextSelector.
defaultValueValue
import { createContext } from 'use-context-selector';
const PersonContext = createContext({ firstName: '', familyName: '' });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.
contextContext<Value>selectorfunction (value: Value): Selected
import { useContextSelector } from 'use-context-selector';
const firstName = useContextSelector(PersonContext, (state) => state.firstName);This hook returns the entire context value. Use this instead of React.useContext for consistent behavior.
contextContext<Value>
import { useContext } from 'use-context-selector';
const person = useContext(PersonContext);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.
contextContext<Value>
import { useContextUpdate } from 'use-context-selector';
const update = useContextUpdate();
// Wrap set state function
update(() => setState(...));
// Experimental suspense mode
update(() => setState(...), { suspense: true });This is a Provider component for bridging multiple react roots
-
$0{context: Context<any>, value: any, children: ReactNode}$0.context$0.value$0.children
const valueToBridge = useBridgeValue(PersonContext);
return (
<Renderer>
<BridgeProvider context={PersonContext} value={valueToBridge}>
{children}
</BridgeProvider>
</Renderer>
);This hook return a value for BridgeProvider
contextContext<any>
- In order to stop propagation,
childrenof a context provider has to be either created outside of the provider or memoized withReact.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 anduse-context-selectorto pass the same data, they may provide inconsistence data for a brief moment. (02_tearing_specfails)
The examples folder contains working examples. You can run one of them with
PORT=8080 pnpm run examples:01_counterand open http://localhost:8080 in your web browser.
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