Function is executing even after leaving the page React js

Question

When I navigate from home i.e, "/" to "/realtime" useEffect hook start the video from webcam, then I added a function handleVideoPlay for video onPlay event as shown below.

<video
  className="video"
  ref={videoRef}
  autoPlay
  muted
  onPlay={handleVideoPlay}
/>

For every interval of 100ms, the code inside setInterval( which is inside the handleVideoPlay function) will run, which detects facial emotions using faceapi and draw canvas.

Here is my handleVideoPlay function

const [ isOnPage, setIsOnPage] = useState(true);

  const handleVideoPlay = () => {

    setInterval(async () => {
    if(isOnPage){
      canvasRef.current.innerHTML = faceapi.createCanvasFromMedia(videoRef.current);
      const displaySize = {
        width: videoWidth,
        height: videoHeight,
      };
      faceapi.matchDimensions(canvasRef.current, displaySize);

      const detections = await faceapi.detectAllFaces(videoRef.current, new 
      faceapi.TinyFaceDetectorOptions()).withFaceLandmarks().withFaceExpressions();

      const resizeDetections = faceapi.resizeResults(detections, displaySize);

      canvasRef.current.getContext("2d").clearRect(0, 0, videoWidth, videoHeight);

      faceapi.draw.drawDetections(canvasRef.current, resizeDetections);

      faceapi.draw.drawFaceLandmarks(canvasRef.current, resizeDetections);

      faceapi.draw.drawFaceExpressions(canvasRef.current, resizeDetections);
     }else{
      return;
     }
    }, 100);
  };

The problem is when I go back the handleVideoFunction is still running, so for canvasRef it is getting null value, and it's throwing this error as shown below

Unhandled Rejection (TypeError): Cannot read property 'getContext' of null

I want to stop the setInterval block on leaving the page. I tried by putting a state isOnPage to true and I set it to false in useEffect cleanup so that if isOnPage is true the code in setInterval runs else it returns. but that doesn't worked. The other code in useEffect cleanup function is running but the state is not changing. Please help me with this, and I'm sorry if haven't asked the question correctly and I'll give you if you need more information about this to resolve. Thank you

useEffect should do the job in this case. Can you add your useEffect block too? or if possible please create and share a fiddle/sandbox for this issue. — Manish Sundriyal, Mar 05 '21 at 18:22
sorry, I'm unable to create sandbox. If you have no problem I can you the code in any other way — Pranay, Mar 06 '21 at 06:35

score 0 · Accepted Answer · answered Mar 05 '21 at 18:23

You need to clear your setInterval from running when the component is unmounted.

You can do this using the useEffect hook:

useEffect(() => {
  const interval = setInterval(() => {
    console.log('I will log every second until I am cleared');
  }, 1000);
  return () => clearInterval(interval);
}, []);

Passing an empty array to useEffect ensures the effect will only trigger once when it is mounted.

The return of the effect is called when the component is unmounted.

If you clear the interval here, you will no longer have the interval running once the component is unmounted. Not only that, but you are ensuring that you are not leaking memory (i.e. by indefinitely increasing the number of setIntervals that are running in the background).

I tried using this but i have no reference to that interval,so I declared variable interval like this -> let interval; then I set this interval = that interval in handleVIdeoPlay function still its continuing the execution — Pranay, Mar 06 '21 at 06:29

Function is executing even after leaving the page React js

1 Answers1