何时用 useCallback 和 useMemo

先了解下Js数据类型方面的基础知识

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          true === true // true
false === false // true
1 === 1 // true
'a' === 'a' // true

{} === {} // false
[] === [] // false
() => {} === () => {} // false
const z = {}
z === z // true

// NOTE: React 用的是 Object.is, 和 ===非常相似

先来看看下面例子

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          function ParentComp() {
  const [count, setCount] = useState(0)
  const increment = () => setCount(count + 1)
  return (
    <div>
      <button onClick={increment}>点击次数：{count}</button>
      <SonComp />
    </div>
  );
}

function SonComp() {
  console.log('render SonComp')
  return <div>Son Comp</div>
}

export default ParentComp

这里显然父组件更新会导致子组件渲染

用上Mamo()

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          import React, { useState, memo } from 'react';
function ParentComp() {
  const [count, setCount] = useState(0)
  const increment = () => setCount(count + 1)
  return (
    <div>
      <button onClick={increment}>点击次数：{count}</button>
      <MemoSonComp />
    </div>
  );
}

const MemoSonComp = memo(function SonComp() {
  console.log('render SonComp')
  return <div>Son Comp</div>
})

此时子组件就不在被渲染

上面的例子并不能说明什么，因为SonComp是个纯组件。

接下来再来看看下面列子，我们给SonComp传个函数和一般的属性

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          import React, { useState, memo } from 'react';

function ParentComp() {
  const [count, setCount] = useState(0)
  const [name, setName] = useState('bob')
  const increment = () => setCount(count + 1)
  const changeName = (newName) => setName(newName)  // 父组件渲染时会创建一个新的函数 相当于props变化了
  return (
    <div>
      <button onClick={increment}>点击次数：{count}</button>
      <MemoSonComp name={name} onClick={changeName} />
    </div>
  );
}

const MemoSonComp = memo(function SonComp({ name, onClick }) {
  console.log('render SonComp')
  return <>
    <div>Son Comp ... {name}</div>
    <button onClick={() => onClick('allen')}>改变 name 值</button>
  </>
})

export default ParentComp

子组件又被渲染了

原因：父组件渲染时会创建一个新的函数相当于props变化了，导致子组件渲染

useCallback()

这时useCallback就派上用场了

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          import React, { useState, memo,useCallback } from 'react';

function ParentComp() {
  const [count, setCount] = useState(0)
  const [name, setName] = useState('bob')
  const increment = () => setCount(count + 1)
  const changeName = useCallback((newName) => setName(newName), [])  //包一层useCallback
  return (
    <div>
      <button onClick={increment}>点击次数：{count}</button>
      <MemoSonComp name={name} onClick={changeName} />
    </div>
  );
}

const MemoSonComp = memo(function SonComp({ name, onClick }) {
  console.log('render SonComp')
  return <>
    <div>Son Comp ... {name}</div>
    <button onClick={() => onClick('allen')}>改变 name 值</button>
  </>
})
export default ParentComp

此时点击父组件按钮，控制台不会打印子组件被渲染的信息了。

原因：useCallback() 起到了缓存的作用，即便父组件渲染了，useCallback() 包裹的函数也不会重新生成，会返回上一次的函数引用。

useMemo()

再来看useMemo用途

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          import React, { useState, memo } from 'react';

function ParentComp() {
  const [count, setCount] = useState(0)
  const [name, setName] = useState('bob')
  const [age, setAge] = useState(18)
  const increment = () => setCount(count + 1)

  const info = { name, age }  //此处对象被新建,相当于props变化了
  return (
    <div>
      <button onClick={increment}>点击次数：{count}</button>
      <MemoSonComp info={info}  /> 
    </div>
  );
}

const MemoSonComp = memo(function SonComp({ info }) {
  console.log('render SonComp')
  return <>
    <div>Son Comp ... {info.name}</div>
  </>
})
export default ParentComp

同样此处对象被新建,相当于props变化了，子组件被渲染

用上useMemo()

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          import React, { useState, memo,useMemo } from 'react';

function ParentComp() {
  const [count, setCount] = useState(0)
  const [name, setName] = useState('bob')
  const [age, setAge] = useState(18)
  const increment = () => setCount(count + 1)

  const  info=useMemo(() => ({ name, age }), [name, age]) //此处对象被新建,相当于props变化了
  return (
    <div>
      <button onClick={increment}>点击次数：{count}</button>
      <MemoSonComp info={info}  />
    </div>
  );
}

const MemoSonComp = memo(function SonComp({ info }) {
  console.log('render SonComp')
  return <>
    <div>Son Comp ... {info.name}</div>
  </>
})
export default ParentComp

子组件即可不被渲染

本文代码参考: https://www.jianshu.com/p/014ee0ebe959

——THE END——

（完）

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