How to use useState in addEventListener?-CodePudding

If I rightClick the red DivElement, I want to change my state

As the state has been changed, so I thought I could see console.log

But It does not work

Here is my code

 import { useState, useRef, useEffect } from "react";

function Main() {
  const [testState, setTestState] = useState(0);
  const testRef = useRef(null);

  function clickHandler(event) {
    setTestState(testState   1);
  }

  useEffect(() => {
    if (!testRef) return;

    testRef.current.addEventListener('contextmenu', (event) => { clickHandler(event) });
  }, [testRef]);

  useEffect(() => {
    console.log('testState: ', testState);
  }, [testState]);

  return (
    <div
      ref={testRef}
      style={{width: '100%', display: 'block', height: '32px', backgroundColor: 'red'}}
    />
  );
}

export default Main;

CodePudding user response：

the problem comes when you are referencing your state, keep in mind that setState is executed asynchronously (think about set state as a request to rerender the component).

by changing your setTestState(testState 1); setTestState((oldVal) => oldVal 1); your example will work. take a look to this stackblitz example

regards,

CodePudding user response：

You should use a cleanup function and preventDefault on clickHandler.

import { useState, useRef, useEffect, useCallback } from "react";

function Main() {
  const [testState, setTestState] = useState(0);
  const testRef = useRef(null);

  const clickHandler = useCallback((event) => {
    event.preventDefault();
    setTestState((test) => test   1);
  }, []);

  useEffect(() => {
    const refElement = testRef.current;
    refElement.addEventListener("contextmenu", clickHandler);
    return () => refElement.removeEventListener("contextmenu", clickHandler); //cleanup function
  }, [clickHandler]);

  useEffect(() => {
    console.log("testState: ", testState);
  }, [testState]);

  return (
    <div
      ref={testRef}
      style={{
        width: "100%",
        display: "block",
        height: "32px",
        backgroundColor: "red"
      }}
    />
  );
}

export default Main;

Working Codesandbox

CodePudding user response：

Here is a working code snippet:

function Main() {
  const [testState, setTestState] = useState(0);
  const testRef = useRef(null);

  useEffect(() => {
    function clickHandler(event) {
      setTestState((old) => old   1);
    }
    if (!testRef) return;
    const ref = testRef.current;
    ref.addEventListener("contextmenu", clickHandler);
    return () => ref.removeEventListener("contextmenu", clickHandler);
  }, [testRef, testState]);

  useEffect(() => {
    console.log("testState: ", testState);
  }, [testState]);

  return (
    <div
      ref={testRef}
      style={{
        width: "100%",
        display: "block",
        height: "32px",
        backgroundColor: "red",
      }}
    />
  );
}