React, can't access updated value of state variable inside function passed to setInterval() in-CodePudding

I am building a simple clock app with React. Currently the countDown() function works, but I would like the user to be able to stop/start the clock by pressing a button. I have a state boolean called paused that is inverted when the user clicks a button. The trouble is that after the value of paused is inverted, the reference to paused inside the countDown() function passed to setInterval() seems to be accessing the default value of paused, instead of the updated value.

function Clock(){
  const [sec, setSecs] = useState(sessionLength * 60);
  const [paused, setPaused] = useState(false);
 
  const playPause = () => {
    setPaused(paused => !paused);
  };
  
  const countDown = () => {
    if(!paused){
        setSecs(sec => sec - 1)
      }
  }
  
  useEffect(() => {
    const interval = setInterval(() => {
        countDown();
    }, 1000);
    return () => {
      clearInterval(interval);
    };
  }, []);

I'm assuming it has something to do with the asynchronous nature of calls to setState() in React, and/or the nature of scoping/context when using regular expressions. However I haven't been able to determine what is going on by reading documentation related to these concepts.

I can think of some workarounds that would allow my app to function as desired. However I want to understand what is wrong with my current approach. I would appreciate any light anyone can shed on this!

CodePudding user response：

In your code, the useEffect is called only once when mounting the component.

The countdown function registered inside will have its initial value at the time when the useEffect/setInterval is called. So paused will only have the value when you initially mount the component. Because you are not calling countDown directly or updating its value inside your useEffect, it is not updated.

I think you could solve this issue like this:

  interval.current = useRef(null);
  const countDown = () => {
    if(!paused){
        setSecs(sec => sec - 1)
      }
  }
  
  useEffect(() => {
    clearInterval(interval.current);
    interval.current = setInterval(countDown, 1000);
    return () => {
      clearInterval(interval.current);
    };
  }, [paused]);

your useEffect is dependent on the value of paused as it needs to create a new interval (with a different countdown function). This will trigger the useEffect not only on mount but every time paused changes. So one solution would be to clear the interval and start a new one with a different callback function.

Edit: You could actually improve it as you only want the interval to be running if the countDown function actually does something so this should work too:

 useEffect(() => {
    clearInterval(interval.current);
    if(!paused) {
      interval.current = setInterval(countDown, 1000);
    }
    return () => {
      clearInterval(interval.current);
    };
  }, [paused]);