metamask-extension/ui/hooks/useScrollRequired.js

import { useEffect, useRef, useState } from 'react';
import { debounce } from 'lodash';

/**
 * Utility hook for requiring users to scroll through content.
 * Returns an object containing state and helpers to accomplish this.
 *
 * The hook expects both the `ref` and the `onScroll` handler to be passed to the scrolling element.
 *
 * @param dependencies - Any optional hook dependencies for updating the scroll state.
 * @returns Flags for isScrollable and isScrollToBottom, a ref to use for the scrolling content, a scrollToBottom function and a onScroll handler.
 */
export const useScrollRequired = (dependencies = []) => {
  const ref = useRef();
  const [isScrollableState, setIsScrollable] = useState(false);
  const [isScrolledToBottomState, setIsScrolledToBottom] = useState(false);

  const update = () => {
    const isScrollable =
      ref.current && ref.current.scrollHeight > ref.current.clientHeight;
    const isScrolledToBottom = isScrollable
      ? Math.round(ref.current.scrollTop) + ref.current.offsetHeight >=
        ref.current.scrollHeight
      : true;
    setIsScrollable(isScrollable);
    setIsScrolledToBottom(isScrolledToBottom);
  };

  useEffect(update, [ref, ...dependencies]);

  const scrollToBottom = () => {
    if (ref.current) {
      ref.current.scrollTo(0, ref.current.scrollHeight);
    }
  };

  return {
    isScrollable: isScrollableState,
    isScrolledToBottom: isScrolledToBottomState,
    scrollToBottom,
    ref,
    onScroll: debounce(update, 25),
  };
};