function MeasureExample() {
  const [rect, ref] = useClientRect();
  return (
    <React.Fragment>
      <h1 ref={ref}>Hello, world</h1>
      {rect !== null &&
        <h2>The above header is {Math.round(rect.width)}px wide</h2>
      }
    </ React.Fragment >
  );}function useClientRect() {
  const [rect, setRect] = React.useState(null);
  const ref = React.useCallback(node => {
    if (node !== null) {
      setRect(node.getBoundingClientRect());
    }
  }, []);
  return [rect, ref];}