React component's internal state is not updating when it is inside a context-CodePudding

I have a simple React component named Child which increments its count when clicked. Also, an outside state is saved which get updated when the Child component is incremented.

const Child = ({ onChange }) => {
  const [count, setCount] = useState(0);
  return (
    <div>
      <h1
        onClick={() => {
          setCount(prev => prev   1);
          onChange(1);
        }}
      >
        Child count = {count}
      </h1>
    </div>
  );
};

const App = () => {
  const [count, setCount] = useState(0);

  return (
    <div className="App">
      <h1> Parent Count = {count} </h1>

      <Child onChange={(i) => setCount(count   i)} />
    </div>
  );
}

Above code works as expected. However, when I use the Context API to manage the outside state, the Child component's internal count remains at 0.

const App = () => {
  const [count, setCount] = useState(0);

  const AppContext = React.createContext();

  return (
    <div className="App">
      <AppContext.Provider value={{ state: count, setState: setCount }}>
        <>
          <AppContext.Consumer>
            {({ state }) => <h1> Parent Count = {state} </h1>}
          </AppContext.Consumer>

          <AppContext.Consumer>
            {({ state, setState }) => (
              <Child onChange={(i) => setState(state   i)} />
            )}
          </AppContext.Consumer>
        </>
      </AppContext.Provider>
    </div>
  );
};

Sample CodeSandbox.

Why the internal state of the element inside AppContext.Provider always reset to its initial state?

CodePudding user response：

First, the creation of a context should be outside of a Component, which means this line:

const AppContext = React.createContext()

should be outside of App. Second, a context setup can be simpler, as below (I separated each component in its own file, so it resembles to common use cases), and working forked CodeSandbox here.

App:

import { useState, createContext } from "react";
import Child from "./Child";
export const AppContext = createContext();

const App = () => {
  const [count, setCount] = useState(0);

  return (
    <div className="App">
      <h1> Parent Count = {count} </h1>
      <AppContext.Provider value={{ state: count, setState: setCount }}>
        <Child />
      </AppContext.Provider>
    </div>
  );
};

export default App;

Child:

import { useContext, useState } from "react";
import { AppContext } from "./App";

const Child = () => {
  const { state, setState } = useContext(AppContext);
  const [count, setCount] = useState(0);
  return (
    <div>
      <h1
        onClick={() => {
          setCount((prev) => prev   1);
          setState(state   1);
        }}
      >
        Child count = {count}
      </h1>
    </div>
  );
};

export default Child;