React：子组件中的陈旧道具

Question

Here is the CodeSandBox link: https://codesandbox.io/s/stale-prop-one-g92sv?file=/src/App.js

I find the child components will not show the correct counter values after two button clicks, though the counter is actually incrementing:

import "./styles.css";
import { useState } from "react";

const MyComponent = ({ value }) => {
  const [counter] = useState(value);
  return <span>{counter}</span>;
};

export default function App() {
  const [counter, setCounter] = useState(0);
  const isVisible = counter !== 1;
  console.log(counter);
  return (
    <div className="App">
      <div>
        <button onClick={() => setCounter((counter) => counter + 1)}>
          Click me
        </button>
      </div>
      {isVisible && (
        <div>
          Message 1 is: <MyComponent value={counter} />
        </div>
      )}
      <div style={isVisible ? { display: "block" } : { display: "none" }}>
        Message 2 is: <MyComponent value={counter} />
      </div>
    </div>
  );
}

I try to force child component re-render by assigning counter to its key:

export default function App() {
  const [counter, setCounter] = useState(0);
  const isVisible = counter !== 1;
  console.log(counter);
  return (
    <div className="App">
      <div>
        <button onClick={() => setCounter((counter) => counter + 1)}>
          Click me
        </button>
      </div>
      {isVisible && (
        <div>
          Message 1 is: <MyComponent  key = {counter} value={counter} />
        </div>
      )}
      <div style={isVisible ? { display: "block" } : { display: "none" }}>
        Message 2 is: <MyComponent key = {counter} value={counter} />
      </div>
    </div>
  );
}

It works, but I still have no idea why the previous one does not work, since the props.value in MyComponent has changed... Thanks in advance.

Answer 1

With this:

const MyComponent = ({ value }) => {
  const [counter] = useState(value);
  return <span>{counter}</span>;
};

You're telling React to set the initial state to the first value prop passed to the component, on mount.

When the component re-renders, the component has already been mounted, so the value passed to useState is ignored - instead, the counter in that child is taken from the state of MyComponent - which is equal to the initial state in MyComponent , the initial value prop passed.

For what you're trying to do, you only have a single value throughout the app here that you want to use everywhere, so you should only have one useState call, in the parent - and then render the counter in the child from the prop, which will change with the parent state.

const MyComponent = ({ value }) => {
  return <span>{value}</span>;
};