CODEWITHSUNDEEP
javascriptreactjsaddeventlistenerreact-hooks
zilijonas
zilijonas

Reputation: 3755

React setState hook from scroll event listener

First of all, with a class component, this works fine and does not cause any issues.

However, in functional component with hooks, whenever I try to set state from my scroll event listener's function handleScroll, my state fails to get updated or app's performance gets affected drastically even though I am using debounce.

import React, { useState, useEffect } from "react";
import debounce from "debounce";

let prevScrollY = 0;

const App = () => {
  const [goingUp, setGoingUp] = useState(false);

  const handleScroll = () => {
    const currentScrollY = window.scrollY;
    if (prevScrollY < currentScrollY && goingUp) {
      debounce(() => {
        setGoingUp(false);
      }, 1000);
    }
    if (prevScrollY > currentScrollY && !goingUp) {
      debounce(() => {
        setGoingUp(true);
      }, 1000);
    }

    prevScrollY = currentScrollY;
    console.log(goingUp, currentScrollY);
  };

  useEffect(() => {
    window.addEventListener("scroll", handleScroll);
    return () => window.removeEventListener("scroll", handleScroll);
  }, []);

  return (
    <div>
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
    </div>
  );
};

export default App;

Tried to use useCallback hook in handleScroll function but it did not help much.

What am I doing wrong? How can I set state from handleScroll without a huge impact on performance?

I've created a sandbox with this issue.

Edit React setState from event listener

Upvotes: 24

Views: 49533

Answers (5)

Jagannath
Jagannath

Reputation: 1

const objDiv = document.getElementById('chat_body');
objDiv.scrollTop = objDiv.scrollHeight;

Upvotes: -1

Maged Mohamed
Maged Mohamed

Reputation: 895

Here is my solution for this case

const [pageUp, setPageUp] = useState(false)
const lastScroll = useRef(0)

const checkScrollTop = () => {
  const currentScroll = window.pageYOffset
  setPageUp(lastScroll.current > currentScroll)
  lastScroll.current = currentScroll
}

// lodash throttle function
const throttledFunc = throttle(checkScrollTop, 400, { leading: true })

useEffect(() => {
  window.addEventListener("scroll", throttledFunc, false)
  return () => {
    window.removeEventListener("scroll", throttledFunc)
  }
}, [])

Adding event listeners should be once when componentDidMount only. not in every changes causes by pageUp. So, we don't need to add pageUp to useEffect dependencies.

Edit toggle on scroll

Hint: you can add throttledFunc to useEffect if it changes and you need to re-render the component

Upvotes: 1

NCM
NCM

Reputation: 368

In short, you need to add goingUp as the dependency of useEffect.

If you use [], you will only create/remove a listener with a function(handleScroll, which is created in the initial render). In other words, when re-render, the scroll event listener is still using the old handleScroll from the initial render. 

  useEffect(() => {
    window.addEventListener("scroll", handleScroll);
    return () => window.removeEventListener("scroll", handleScroll);
  }, [goingUp]);

Using custom hooks

I recommend move the whole logic into a custom hooks, which can make your code more clear and easy to reuse. I use useRef to store the previous value. 

export function useScrollDirection() {
  const prevScrollY = useRef(0)
  const [goingUp, setGoingUp] = useState(false);

  const handleScroll = () => {
    const currentScrollY = window.scrollY;
    if (prevScrollY.current < currentScrollY && goingUp) {
      setGoingUp(false);
    }
    if (prevScrollY.current > currentScrollY && !goingUp) {
      setGoingUp(true);
    }
    prevScrollY.current = currentScrollY;
  };

  useEffect(() => {
    window.addEventListener("scroll", handleScroll);
    return () => window.removeEventListener("scroll", handleScroll);
  }, [goingUp]); 
  return goingUp ? 'up' : 'down';
}

Upvotes: 9

Yozi
Yozi

Reputation: 12727

In your code I see several issues:

1) [] in useEffect means it will not see any changes of state, like changes of goingUp. It will always see initial value of goingUp

2) debounce does not work so. It returns a new debounced function.

3) usually global variables is an anti-pattern, thought it works just in your case.

4) your scroll listener is not passive, as mentioned by @skyboyer.

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

const App = () => {
  const prevScrollY = useRef(0);

  const [goingUp, setGoingUp] = useState(false);

  useEffect(() => {
    const handleScroll = () => {
      const currentScrollY = window.scrollY;
      if (prevScrollY.current < currentScrollY && goingUp) {
        setGoingUp(false);
      }
      if (prevScrollY.current > currentScrollY && !goingUp) {
        setGoingUp(true);
      }

      prevScrollY.current = currentScrollY;
      console.log(goingUp, currentScrollY);
    };

    window.addEventListener("scroll", handleScroll, { passive: true });

    return () => window.removeEventListener("scroll", handleScroll);
  }, [goingUp]);

  return (
    <div>
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
    </div>
  );
};

export default App;

https://codesandbox.io/s/react-setstate-from-event-listener-q7to8

Upvotes: 25

skyboyer
skyboyer

Reputation: 23705

Your are re-creating handleScroll function on each render so it's refers to actual data(goingup) so you don't need useCallback here.

But besides you recreate handleScroll you are set as event listener only first instance of it - since you give useEffect(..., []) it runs only once on mount.

The one solution is to re-subscribe to scroll with up-to-date handleScroll. To do that you have to return cleanup function within useEffect(by now it returns one more subscribing) and remove [] to run it on each render:

useEffect(() => {
  window.addEventListener("scroll", handleScroll);
  return () => window.removeEventListener("scroll", handleScroll);
});

PS and you better use passive event listener for scroll.

Upvotes: 3

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