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作用：在组件每次重新渲染的时候缓存计算的结果

import { useReducer, useState } from "react";

function fib(n){

  console.log('开始计算');

  if(n<3){

    return 1

  }

  return fib(n-2)+fib(n-3)

}

function App() {

  const [count1,setCount1]=useState(0)

  const [count2,setCount2]=useState(0)

  const result=fib(count1)

  return (

    <div className="App">

      this is app,

      <button onClick={()=>{setCount1(count1+1)}}>change count1:{count1}</button>

      <button onClick={()=>{setCount2(count2+1)}}>change count2:{count2}</button>

      {result}

    </div>

  );

}

export default App;

此时我们点击count1或是count2都会重新计算fib

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

function fib(n){

  console.log('开始计算');

  if(n<3){

    return 1

  }

  return fib(n-2)+fib(n-3)

}

function App() {

  const [count1,setCount1]=useState(0)

  const [count2,setCount2]=useState(0)

  const result=useMemo(()=>{

    //返回计算得到的结果

    return fib(count1)

  },[count1])

  return (

    <div className="App">

      this is app,

      <button onClick={()=>{setCount1(count1+1)}}>change count1:{count1}</button>

      <button onClick={()=>{setCount2(count2+1)}}>change count2:{count2}</button>

      {result}

    </div>

  );

}

export default App;

此时，只有count发生变化，result才会发生变化