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何时用 useCallback 和 useMemo

Snipaste_20200526_165641.jpg

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——

Article at   2020/05/26 23:29  Published  code  Category,viewed  304  times

Relevant tags:    React 

Address:   https://www.kedong.me/article/17

Copyright Notice: Freely reproduced for non-commercial use