Implement useSelector equivalent for React Context? - reactjs

Question

There's a bunch of articles out there that show how Redux can be replaced with context and hooks (see this one from Kent Dodds, for instance). The basic idea is to make your global state available through a context instead of putting it inside a Redux store. But there's one big problem with that approach: components that subscribe to the context will be rerendered whenever any change happens to the context, regardless of whether or not your component cares about the part of the state that just changed. For functional components, React-redux solves this problem with the useSelector hook. So my question is: can a hook like useSelector be created that would grab a piece of the context instead of the Redux store, would have the same signature as useSelector, and, just like useSelector, would only cause rerenders to the component when the "selected" part of the context has changed?
(note: this discussion on the React Github page suggests that it can't be done)

Answer 1

No, it's not possible. Any time you put a new context value into a provider, all consumers will re-render, even if they only need part of that context value.
That's specifically one of the reasons why we gave up on using context to propagate state updates in React-Redux v6, and switched back to using direct store subscriptions in v7.
There's a community-written React RFC to add selectors to context, but no indication the React team will actually pursue implementing that RFC at all.

Answer 2

As markerikson answers, it is not possible, but you can work around it without using external dependencies and without falling back to doing manual subscriptions.
As a workaround, you can let the component re-render, but skip the VDOM reconciliation by memoizing the returned React element with useMemo.
function Section(props) {
const partOfState = selectPartOfState(useContext(StateContext))
// Memoize the returned node
return useMemo(() => {
return <div>{partOfState}</div>
}, [partOfState])
}
This is because internally, when React diffs 2 versions of virtual DOM nodes, if it encountered the exact same reference, it will skip reconciling that node entirely.

Answer 3

I created a toolkit for managing state using ContextAPI. It provides useSelector (with autocomplete) as well as useDispatch.
The library is available here:
https://www.npmjs.com/package/react-context-toolkit
https://github.com/bergkvist/react-context-toolkit
It uses:
use-context-selector to avoid unneccesary rerenders.
createSlice from #reduxjs/toolkit to make the state more modular and to avoid boilerplate.

Answer 4

I've created this small package, react-use-context-selector, and it just does the job.
I used the same approach as used in Redux's useSelector. It also comes with type declarations and the return type matches the selector function's return type making it suitable for using in TS project.
function MyComponent() {
// This component will re-render only when the `name` within the context object changes.
const name = useContextSelector(context, value => value.name);
return <div>{name}</div>;
}

Answer 5

Here is my take on this problem:
I used the function as child pattern with useMemo to create a generic selector component:
import React, {
useContext,
useReducer,
createContext,
Reducer,
useMemo,
FC,
Dispatch
} from "react";
export function createStore<TState>(
rootReducer: Reducer<TState, any>,
initialState: TState
) {
const store = createContext({
state: initialState,
dispatch: (() => {}) as Dispatch<any>
});
const StoreProvider: FC = ({ children }) => {
const [state, dispatch] = useReducer(rootReducer, initialState);
return (
<store.Provider value={{ state, dispatch }}>{children}</store.Provider>
);
};
const Connect: FC<{
selector: (value: TState) => any;
children: (args: { dispatch: Dispatch<any>; state: any }) => any;
}> = ({ children, selector }) => {
const { state, dispatch } = useContext(store);
const selected = selector(state);
return useMemo(() => children({ state: selected, dispatch }), [
selected,
dispatch,
children
]);
};
return { StoreProvider, Connect };
}
Counter component:
import React, { Dispatch } from "react";
interface CounterProps {
name: string;
count: number;
dispatch: Dispatch<any>;
}
export function Counter({ name, count, dispatch }: CounterProps) {
console.count("rendered Counter " + name);
return (
<div>
<h1>
Counter {name}: {count}
</h1>
<button onClick={() => dispatch("INCREMENT_" + name)}>+</button>
</div>
);
}
Usage:
import React, { Reducer } from "react";
import { Counter } from "./counter";
import { createStore } from "./create-store";
import "./styles.css";
const initial = { counterA: 0, counterB: 0 };
const counterReducer: Reducer<typeof initial, any> = (state, action) => {
switch (action) {
case "INCREMENT_A": {
return { ...state, counterA: state.counterA + 1 };
}
case "INCREMENT_B": {
return { ...state, counterB: state.counterB + 1 };
}
default: {
return state;
}
}
};
const { Connect, StoreProvider } = createStore(counterReducer, initial);
export default function App() {
return (
<StoreProvider>
<div className="App">
<Connect selector={(state) => state.counterA}>
{({ dispatch, state }) => (
<Counter name="A" dispatch={dispatch} count={state} />
)}
</Connect>
<Connect selector={(state) => state.counterB}>
{({ dispatch, state }) => (
<Counter name="B" dispatch={dispatch} count={state} />
)}
</Connect>
</div>
</StoreProvider>
);
}
Working example: CodePen

Answer 6

Solution with external store (Redux or Zustand like approach) with new hook useSyncExternalStore comes with React 18.
For React 18: Define createStore and useStore functions:
import React, { useCallback } from "react";
import { useSyncExternalStore } from "react";
const createStore = (initialState) => {
let state = initialState;
const getState = () => state;
const listeners = new Set();
const setState = (fn) => {
state = fn(state);
listeners.forEach((l) => l());
};
const subscribe = (listener) => {
listeners.add(listener);
return () => listeners.delete(listener);
};
return { getState, setState, subscribe };
};
const useStore = (store, selector) =>
useSyncExternalStore(
store.subscribe,
useCallback(() => selector(store.getState()), [store, selector])
);
Now use it :
const store = createStore({ count: 0, text: "hello" });
const Counter = () => {
const count = useStore(store, (state) => state.count);
const inc = () => {
store.setState((prev) => ({ ...prev, count: prev.count + 1 }));
};
return (
<div>
{count} <button onClick={inc}>+1</button>
</div>
);
};
For React 17 and any React version that supports hooks:
Option 1: You may use the external library (maintained by React team)
use-sync-external-store/shim :
import { useSyncExternalStore } from "use-sync-external-store/shim";
Option 2: If you don't want to add new library and don't care about concurency problems:
const createStore = (initialState) => {
let state = initialState;
const getState = () => state;
const listeners = new Set();
const setState = (fn) => {
state = fn(state);
listeners.forEach((l) => l());
}
const subscribe = (listener) => {
listeners.add(listener);
return () => listeners.delete(listener);
}
return {getState, setState, subscribe}
}
const useStore = (store, selector) => {
const [state, setState] = useState(() => selector(store.getState()));
useEffect(() => {
const callback = () => setState(selector(store.getState()));
const unsubscribe = store.subscribe(callback);
callback();
return unsubscribe;
}, [store, selector]);
return state;
}
Sources:
A conference talk from Daishi Kato from React Conf 2021
A blog post about same conference talk by Chetan Gawai

Answer 7

Simple approach to prevent additional renders with HoC and React.memo:
const withContextProps = (WrappedComponent) => {
const MemoizedComponent = React.memo(WrappedComponent);
return (props) => {
const state = useContext(myContext);
const mySelectedState = state.a.b.c;
return (
<MemoizedComponent
{...props}
mySelectedState={mySelectedState} // inject your state here
/>
);
};
};
withContextProps(MyComponent)

Answer 8

I have made a library, react-context-slices, which can solve what you are looking for. The idea is to break the store or state in slices of state, that is, smaller objects, and create a context for each one. That library which I told you does this, exposes a function createSlice which accepts a reducer, initial state, name of the slice, and a function for creating the actions. You create as slices as you want ('todos', 'counter', etc) and integrate them in a unique interface easily, exposing at the end two custom hooks, useValues and useActions, which can 'attack' all the slices (that is, in your client components you do not use useTodosValues but useValues). The key is that useValues accepts a name of the slice, so would be equivalent to the useSelector from redux. The library use immer as redux does. It's a very tiny library which the key point is how is used, which is explained in the readme file. I have also made a post about it. The library exposes only two functions, createSlice and composeProviders.