function MeasureExample() {
  const [width, setWidth] = React.useState(0);
  const measuredRef = React.useCallback(node => {
    if (node !== null) {
      setWidth(node.getBoundingClientRect().width);
    }
  }, []);
  return (
    <React.Fragment>
      <h1 ref={measuredRef}>Hello, world</h1>
      <h2>The above header is {Math.round(width)}px wide.</h2>
    </React.Fragment >
  );}