通过 ref 来手动获取上一轮props或者state的值：

function Counter() {
  const [count, setCount] = useState(0);

  const prevCountRef = useRef();
  useEffect(() => {
    prevCountRef.current = count;
  });
  const prevCount = prevCountRef.current;

  return <h1>Now: {count}, before: {prevCount}</h1>;
}

这或许有一点错综复杂，但你可以把它抽取成一个自定义 Hook：

function Counter() {
  const [count, setCount] = useState(0);
  const prevCount = usePrevious(count);
  return <h1>Now: {count}, before: {prevCount}</h1>;
}

function usePrevious(value) {
  const ref = useRef();
  useEffect(() => {
    ref.current = value;
  });
  return ref.current;
}

怎样使用这个自定义的hook：

function Counter() {
  const [count, setCount] = useState(0);

  const calculation = count + 100;
  const prevCalculation = usePrevious(calculation);