javascript - React Hooks useState+useEffect+event gives stale state

Question

I'm trying to use an event emitter with React useEffect and useState, but it always gets the initial state instead of the updated state. It works if I call the event handler directly, even with a setTimeout.

If I pass the value to the useEffect() 2nd argument, it makes it work, however this causes a resubscription to the event emitter every time the value changes (which is triggered off of keystrokes).

What am I doing wrong? I've tried useState, useRef, useReducer, and useCallback, and couldn't get any working.

Here's a reproduction:

import React, { useState, useEffect } from "react";
import { Controlled as CodeMirror } from "react-codemirror2";
import "codemirror/lib/codemirror.css";
import EventEmitter from "events";

let ee = new EventEmitter();

const initialValue = "initial value";

function App(props) {
  const [value, setValue] = useState(initialValue);

  // Should get the latest value, both after the initial server load, and whenever the Codemirror input changes.
  const handleEvent = (msg, data) => {
    console.info("Value in event handler: ", value);
    // This line is only for demoing the problem. If we wanted to modify the DOM in this event, we would instead call some setState function and rerender in a React-friendly fashion.
    document.getElementById("result").innerHTML = value;
  };

  // Get value from server on component creation (mocked)
  useEffect(() => {
    setTimeout(() => {
      setValue("value from server");
    }, 1000);
  }, []);

  // Subscribe to events on component creation
  useEffect(() => {
    ee.on("some_event", handleEvent);
    return () => {
      ee.off(handleEvent);
    };
  }, []);

  return (
    <React.Fragment>
      <CodeMirror
        value={value}
        options={{ lineNumbers: true }}
        onBeforeChange={(editor, data, newValue) => {
          setValue(newValue);
        }}
      />
      {/* Everything below is only for demoing the problem. In reality the event would come from some other source external to this component. */}
      <button
        onClick={() => {
          ee.emit("some_event");
        }}
      >
        EventEmitter (doesnt work)
      </button>
      <div id="result" />
    </React.Fragment>
  );
}

export default App;

question from:https://stackoverflow.com/questions/55154186/react-hooks-usestateuseeffectevent-gives-stale-state

Answer 1

value is stale in the event handler because it gets its value from the closure where it was defined. Unless we re-subscribe a new event handler every time value changes, it will not get the new value.

Solution 1: Make the second argument to the publish effect [value]. This makes the event handler get the correct value, but also causes the effect to run again on every keystroke.

Solution 2: Use a ref to store the latest value in a component instance variable. Then, make an effect which does nothing but update this variable every time value state changes. In the event handler, use the ref, not value.

const [value, setValue] = useState(initialValue);
const refValue = useRef(value);
useEffect(() => {
    refValue.current = value;
});
const handleEvent = (msg, data) => {
    console.info("Value in event handler: ", refValue.current);
};

https://reactjs.org/docs/hooks-faq.html#what-can-i-do-if-my-effect-dependencies-change-too-often

Looks like there are some other solutions on that page which might work too. Much thanks to @Dinesh for the assistance.