React To Many Re Renders In Context [duplicate] - reactjs

Question

Whenever I update user (object) in users (array) in context - all components which uses users re-renders.
What I've tried
I have a component which is using values from context:
const DashboardCardList=()=> {
const context = useContext(StaticsContext);
const users = context.users.filter(user=>user.visible);
return !users
? <Loading/>
: (
<Container>
{users.map(user=>
<DashboardCard key={user._id} user={user}/>
)}
</Container>
);
};
My update function (updates context state):
const onUserUpdate=(user)=>{
const index = this.state.users.findIndex(item => item._id === user._id);
const users = [...this.state.users]
users[index] = user;
this.setState({users:users});
}
Final component:
const DashboardCard=({user})=> {
console.log("I'm here!", user);
return ...;
}
Question
Why it keeps re-rendering? Is it because of context?
How to write this properly?

Answer 1

There is no render bailout for context consumers (v17).
Here is a demonstration, where the Consumer will always re-render just because he is a Context Consumer, even though he doesn't consume anything.
import React, { useState, useContext, useMemo } from "react";
import ReactDOM from "react-dom";
// People wonder why the component gets render although the used value didn't change
const Context = React.createContext();
const Provider = ({ children }) => {
const [counter, setCounter] = useState(0);
const value = useMemo(() => {
const count = () => setCounter(p => p + 1);
return [counter, count];
}, [counter]);
return <Context.Provider value={value}>{children}</Context.Provider>;
};
const Consumer = React.memo(() => {
useContext(Context);
console.log("rendered");
return <>Consumer</>;
});
const ContextChanger = () => {
const [, count] = useContext(Context);
return <button onClick={count}>Count</button>;
};
const App = () => {
return (
<Provider>
<Consumer />
<ContextChanger />
</Provider>
);
};
ReactDOM.render(
<React.StrictMode>
<App />
</React.StrictMode>,
document.getElementById("root")
);
To fix it:
Use a single context for each consumed value. Meaning that context holds a single value, (and no, there is no problem with multiple contexts in an application).
Use a state management solution like Recoil.js, Redux, MobX, etc. (although it might be overkill, think good about app design beforehand).
Minor optimization can be achieved by memoizing Provider's values with useMemo.