Listen for scroll up / down?

This is general advice for hooking into any listeners:

Attach stuff in componentDidMount, unattach in componentWillUnmount

class Whatever extends Component {
  componentDidMount() {
    window.addEventListener('scroll', this.handleScroll, { passive: true })
  }

  componentWillUnmount() {
    window.removeEventListener('scroll', this.handleScroll)
  }

  handleScroll(event) {
    // do something like call `this.setState`
    // access window.scrollY etc
  }
}

Actually there are many ways to do it, but using React hooks was the easier one for me, all you need to do is:

• define a state for scroll position
• a function to update your scroll position state
• start a scroll listen when component mounts
• stop the listen when component unmount

Something like:

import React, { useState, useEffect } from 'react';

...
// inside component:

const [scrollPosition, setSrollPosition] = useState(0);
const handleScroll = () => {
    const position = window.pageYOffset;
    setSrollPosition(position);
};

useEffect(() => {
    window.addEventListener('scroll', handleScroll, { passive: true });

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

Now you have scrollPosition in pixels to work on

In this particular case useEffect will be equivalent to componentDidMount because it will be fired once the component mounts, but also after all updates, but addEventListener is smart enough to not start duplicated listeners.

The return inside of useEffect is equivalent to componentWillUnmount, it'll "called" when the component unmounts.