How to stop the infinite loop inside this React useEffect?-CodePudding

I want React to render key presses from a non-React context, more specifically the string array keys:

import * as React from "react";
import { render } from "react-dom";

let keys: string[] = [];

function handleKeypress(event: any) {
  keys.push(event.key);
  console.log(keys);

  // there will be more code here unrelated to React.
}

document.removeEventListener("keypress", handleKeypress);
document.addEventListener("keypress", handleKeypress);

function App() {
  const [keysState, setKeysState] = React.useState<string[]>([]);

  React.useEffect(() => {
    function updateKeysState() {
      setKeysState([...keys]);
    }

    // if you uncomment this, the code inside useEffect will run forever
    // updateKeysState()

    console.log("Hello world");
  }, [keysState]);

  return (
    <div>
      {keys.map((key: string, id) => (
        <li key={id}>{key}</li>
      ))}
    </div>
  );
}

const rootElement = document.getElementById("root");
render(<App />, rootElement);

I almost accomplished that ... the problem is, the code inside React.useEffect runs in an infinite loop.

I thought passing [keysState] as a second argument to React.useEffect would stop the infinite loop. But it didn't.

Why is this and how to fix it?

Live code: https://codesandbox.io/s/changing-props-on-react-root-component-forked-eu16oj?file=/src/index.tsx

CodePudding user response：

The best way would be to integrate the non-React code into the App, so that setting state as a result of a keypress is natural and trivial.

function App() {
    const [keys, setKeys] = React.useState<string[]>([]);
    useEffect(() => {
        function handleKeypress(event: KeyboardEvent) {
            setKeys([...keys, event.key]);
            // There will be more code here that's unrelated to React.
        }
        document.addEventListener("keypress", handleKeypress);
        return () => {
            document.removeEventListener("keypress", handleKeypress);
        };
    }, []);

Then you can drop your current React.useEffect (and its infinite loop) entirely.

If that's not an option, you'd have to trigger the React state setter from outside of React - any way you look at it, that'll be pretty ugly. I suppose you could assign it to an outside variable:

let setKeysOuter;

function handleKeypress(event: KeyboardEvent) {
  setKeysOuter?.(keys => [...keys, event.key]);
  // There will be more code here that's unrelated to React.
}

function App() {
  const [keys, setKeys] = React.useState<string[]>([]);
  setKeysOuter = setKeys;

CodePudding user response：

You need to pass two arguments to useEffect:

1 => A setup function with setup code that connects to that system. It should return a cleanup function with cleanup code that disconnects from that system. 2 => A list of dependencies including every value from your component used inside of those functions.