These React hooks for the scrollmonitor provide an object with the current scroll state of an element. Use the withScrollContainer HOC to create a container element for in-page scrolling,

see a demo

1. Create a ref
2. Pass it to const scrollState = useScrollState(ref)
3. Use the scrollState object to know the current scroll state.
4. Pass the ref to the item you want to watch.

import { useScrollState } from 'scrollmonitor-hooks';

const WatchedElement = ({children}) => {
  // 1. Create a ref
  const ref = useRef(null);
  // 2. Pass it to `useScrollState`
  const scrollState = useScrollState(ref);
  
  // 3. Get the current scroll state.
  let className;
  if (!scrollState.isInViewport) {
    className = 'in';
  } else {
    className = 'out';
  }

  return <span
    className={className}
    ref={ref} // <----- 4. be sure to pass the ref!
  >
    {children}
  </span>;
}

useScrollState(ref, offsets);

• ref - this should be the return value of React's useRef hook. It must be passed as a ref to the element you want to watch.
• offsets - same as scrollmonitor

This has the same data properties as the scrollmonitor's watcher object, except it is immutable.

• scrollState.isInViewport - true if any part of the element is visible, false if not.
• scrollState.isFullyInViewport - true if the entire element is visible [1].
• scrollState.isAboveViewport - true if any part of the element is above the viewport.
• scrollState.isBelowViewport - true if any part of the element is below the viewport.
• scrollState.top - distance from the top of the document to the top of this component.
• scrollState.bottom - distance from the top of the document to the bottom of this component.
• scrollState.height - top - bottom.

1. If the element is larger than the viewport isFullyInViewport is true when the element spans the entire viewport.

note: all values will be false the first render because React has not yet created the DOM.

If you have a scrolling container on your page (for example, a div with overflow: auto), you must wrap it in the withScrollContainer HOC. This container will be passed to all child components with React's Context API.

Be sure to call withScrollContainer at the top level of your module and not in a render function.

const BoxesInContainer = withScrollContainer(<MyContainer />);

If you need side effects or additional speed, useScrollMonitor will provide direct access to scrollmonitor's callbacks.

import { useScrollMonitor } from 'scrollmonitor-hooks';

const callbacks = {
  enterViewport: (watcher) => {
    track('item entered viewport', watcher.watchItem.getAttribute('data-tracking-id'));
  }
}

const Component = ({children}) => {
  const ref = useRef(null);
  useScrollMonitor(ref, callbacks);

  return <span
    className={`box ${className}`}
    ref={ref}
  >
    {children}
  </span>;
}