组件重新渲染时缓存计算的结果。

实例：count1计算斐波那契数列，count2和count1可以触发数值变化。使用memo可以使只有在count1变化时触发斐波那契数列计算函数，而count2变化时不触发斐波那契数列计算函数。

import { useMemo } from "react";
import { useState } from "react";

function fib(n) {
  console.log('计算函数执行')
  if (n < 3) {
    return 1
  }
  return fib(n - 1) + fib(n - 2)
}

function App() {
  const [count1, setCount1] = useState(0)
  const [count2, setCount2] = useState(0)
  console.log('组件重新渲染')
  const result = useMemo(() => {
    return fib(count1)
  }, [count1])
  return (
    <div className="App">
      <button onClick={() => { setCount1(count1 + 1) }}>change count1: {count1}</button>
      <button onClick={() => { setCount2(count2 + 1) }}>change count2: {count2}</button>
      result: {result}
    </div>
  )
}

export default App