学习react 笔记五 (useEffectuseRefuseContext)
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目录
一、useEffect副作用
副作用: 平常把一些 除了定义 和 dom 一些无关的代码放到effect里,(例如请求ajax)
(1)使用useEffect
import useEffect from 'react'
useEffect(() =>
console.log('useEffect1')
)
(2)执行时机(第二个参数)
情况一:不写第二个参数,页面创建 和 数据更新会执行
useEffect(() =>
console.log('useEffect1')
)
情况二:第二个参数是空数组 执行时机: 只有在组件创建时会执行一次
useEffect(() =>
console.log('useEffect2')
, [])
情况三:第二个参数 依赖项 执行时机:页面创建 和依赖数据更新了 会执行 注意写法是 [ 变量 ]
useEffect(() =>
console.log('useEffect3')
, [a])
(3)倒计时demo
import React, useState, useEffect from 'react'
import ReactDOM from 'react-dom/client'
const Ren = function ()
const [a, setA] = useState(100)
useEffect(() =>
setInterval(() =>
console.log('倒计时', a)
setA(e => e - 1)
, 1000)
, [])
return <div>倒计时100个数 a </div >
(4)请求ajax
import React, useEffect, useState from 'react'
import ReactDOM from 'react-dom/client'
import axios from 'axios'
const Ren = function ()
useEffect(() =>
axios(
method: 'GET',
url: 'https://www.xxx.cn/api/cart',
data:
).then(res =>
console.log(res)
)
)
return <div></div>
写成 async await形式
错误写法:
useEffect(async () =>
await axios(
method: 'GET',
url: 'https://www.xxx.cn/api/cart',
data:
).then(res =>
console.log(res)
)
)
正确写法:
useEffect(() =>
const getList = async () =>
const res = await axios(
method: 'GET',
url: 'https://www.xxx.cn/api/cart',
data:
)
console.log(res)
getList()
)
async 后面要跟一个promise 而useEffect并不是 所以 我们可以在useEffect里定义一个函数 直接调用
(5)useEffect的清理函数
副作用函数的返回值是可选的,也可以省略,也可以返回一个清理函数 清理副作用
执行时机:清理函数会在 组件卸载时以及下次副作用函数调用之前执行
useEffect(() =>
return () => // 做清理工作
)
demo 清除鼠标移动事件
import React, useEffect, useState from 'react'
// import ReactDOM from 'react-dom/client'
import ReactDOM from 'react-dom'
const Ren = function ()
useEffect(() =>
const f = (e) =>
console.log(e.screenX, e.screenY)
window.addEventListener('mousemove', f)
return () =>
window.removeEventListener('mousemove', f)
)
return <h1>111</h1>
export default function Use ()
const [isShow, toggle] = useState(true)
const hide = () =>
toggle(false)
return (
<div>
isShow && <Ren></Ren>
<div onClick=hide>切换</div>
</div>
)
// const root = ReactDOM.createRoot(document.getElementById('root'))
// root.render(<Use />)
ReactDOM.render(<Use></Use>, document.getElementById('root'))
在上面代码中 在组件里绑定了鼠标事件 当销毁组件时 事件并不会被销毁 这时候可以利用useEffect清理函数来清除掉 鼠标移动事件
(6)图片移动demo
import React, useEffect, useState from 'react'
// import ReactDOM from 'react-dom/client'
import ReactDOM from 'react-dom'
import img from '../images/avatar.png'
const Ren = function ()
const [X, setX] = useState(0)
const [Y, setY] = useState(0)
useEffect(() =>
const f = (e) =>
console.log(e.screenX, e.screenY)
setX(() => e.screenX)
setY(() => e.screenY)
window.addEventListener('mousemove', f)
return () =>
window.removeEventListener('mousemove', f)
)
return (
<div style= width: "50px", height: "50px" >
<img src=img style= width: "50px", height: "50px", position: "absolute", left: X + 'px', top: Y + 'px' ></img>
</div>
)
// const root = ReactDOM.createRoot(document.getElementById('root'))
// root.render(<Use />)
ReactDOM.render(<Ren></Ren>, document.getElementById('root'))
(7)图片移动demo 封装
mouseMoveXY.js
import React, useEffect, useState from 'react'
// 如果是函数组件首字母要大写 如果是hooks函数 要以use开头
// 调用要在 函数组件里面调用
export default function useMouseMove ()
const [X, setX] = useState(0)
const [Y, setY] = useState(0)
useEffect(() =>
const f = (e) =>
console.log(e.screenX, e.screenY)
setX(() => e.screenX)
setY(() => e.screenY)
window.addEventListener('mousemove', f)
return () =>
window.removeEventListener('mousemove', f)
)
return X, Y
使用
import React, useEffect, useState from 'react'
// import ReactDOM from 'react-dom/client'
import ReactDOM from 'react-dom'
import img from '../images/avatar.png'
import mousemove from './mouseMoveXY'
const Ren = function ()
const X, Y = mousemove()
return (
<div style= width: "50px", height: "50px" >
<img src=img style= width: "50px", height: "50px", position: "absolute", left: X + 'px', top: Y + 'px' ></img>
</div>
)
ReactDOM.render(<Ren></Ren>, document.getElementById('root'))
二、 useRef(引用)
(1)使用方法
1.导入 import React, useRef from 'react'
2.定义 const getInput = useRef(null)
3.使用 ref=getInput
(2)demo 获取input的值
import React, useRef, useState from 'react'
// import ReactDOM from 'react-dom/client'
import ReactDOM from 'react-dom'
const Ren = function ()
const getInput = useRef(null)
console.log(getInput)
const [text, SetText] = useState('')
const edit = (e) =>
console.log(e.target.value)
SetText(() => e.target.value)
const getText = () =>
console.log(getInput.current.value)
return (
<div>
<input value=text onChange=edit ref=getInput></input>
<button onClick=getText>获取input的值</button>
</div>
)
ReactDOM.render(<Ren></Ren>, document.getElementById('root'))
(3)demo 清除定时器
import React, useEffect, useRef, useState from 'react'
// import ReactDOM from 'react-dom/client'
import ReactDOM from 'react-dom'
const Ren = function ()
const getInput = useRef(null)
console.log(getInput)
const timer = useRef(null)
const [Num, setNum] = useState(0)
// let timer = null //相当于每次都是在赋值一个null 错误写法
useEffect(() =>
// timer = setInterval(() =>
timer.current = setInterval(() =>
setNum(() => Num + 1)
, 1000)
, [])
const edit = (e) =>
const clearTimer = () =>
// clearInterval(timer)
clearInterval(timer.current)
return (
<div>
<div>倒计时 Num </div>
<button onClick=clearTimer>清除定时器</button>
</div>
)
ReactDOM.render(<Ren></Ren>, document.getElementById('root'))
如果在外层定义一个timer 定义定时器 当每次倒计时 值发生改变时 整个组件都会重新执行 相当于每次倒计时都在定义了一个timer 如果想清除定时器 可以 用useRef 把定义的变量 使用 变量.current 来当成定时器 的变量 可以理解为 useRef 定义的 current里永远都是引用类型 不会改变
(4)demo 验证码倒计时
import React, useEffect, useRef, useState from 'react'
// import ReactDOM from 'react-dom/client'
import ReactDOM from 'react-dom'
const Ren = function ()
const timer = useRef(null)
const [Num, setNum] = useState(3)
const [Status, setStatus] = useState(false)
const countDown = () =>
timer.current = setInterval(() =>
setNum((Num) =>
setStatus(true)
return Num - 1
)
, 1000)
useEffect(() =>
if (Num == 0)
clearInterval(timer.current)
setStatus(false)
setNum(3)
console.log(Num)
, [Num])
return (
<div>
<div>倒计时 Num </div>
<button disabled=Status onClick=countDown>Status ? Num + '秒后重新获取' : '获取验证码'</button>
</div>
)
ReactDOM.render(<Ren></Ren>, document.getElementById('root'))
(5)demo 验证码倒计时 抽离hooks
import React, useEffect, useRef, useState from 'react'
// import ReactDOM from 'react-dom/client'
import ReactDOM from 'react-dom'
const useCount = (count = 10, callback = () => ) =>
const timer = useRef(null)
const [Num, setNum] = useState(null)
const start = () =>
setNum(count)
timer.current = setInterval(() =>
setNum(num => num - 1)
, 1000)
useEffect(() =>
if (Num == 0)
clearInterval(timer.current)
callback()
, [Num])
return start, Num
const Ren = function ()
const [Status, setStatus] = useState(false)
const start, Num = useCount(3, () =>
setStatus(false)
)
const countDown = () =>
setStatus(true)
start()
return (
<div>
<div>倒计时 Num </div>
<button disabled=Status onClick=countDown>获取验证码</button>
</div>
)
ReactDOM.render(<Ren></Ren>, document.getElementById('root'))
count 作为倒计时秒数、callback作为 倒计时之后的回调函数
demo 验证码倒计时封装抽离出一个组件 useCountDown.js
import useEffect, useRef, useState from 'react'
export default function useCountDown (count = 10, callback = () => )
const timer = useRef(null)
const [Num, setNum] = useState(null)
const start = () =>
setNum(count)
timer.current = setInterval(() =>
//组件清楚后仍会执行
console.log(1)
setNum(num => num - 1)
, 1000)
useEffect(() =>
if (Num == 0)
setNum(count)
clearInterval(timer.current)
callback()
, [Num])
useEffect(() =>
return () =>
clearInterval(timer.current)
, [])
return start, Num
记得通过useEffect的清理函数 清理一下 定时器事件 极端情况下 销毁组件后 延时器仍会执行
使用
import React, useState from 'react'
import ReactDOM from 'react-dom'
import useCountDown from './useCountDown'
const Ren = function ()
const [Status, setStatus] = useState(false)
const start, Num = useCountDown(3, () =>
setStatus(false)
)
const countDown = () =>
setStatus(true)
start()
return (
<div>
<div>倒计时 Num </div>
<button disabled=Status onClick=countDown>获取验证码</button>
</div>
)
ReactDOM.render(<Ren></Ren>, document.getElementById('root'))
三、useContext(跨组件传值)
一、使用方法
1.导如并调用createContext 得到 Context对象 导出
import React, useState, createContext from 'react'
export const Context = createContext()
2.使用Provider包裹根组件 通过value属性提供共享数据
return (
<Context.Provider value= a: 1, b: 2 >
<根组件内容/>
</Context.Provider>
)
3. 子代组件接收
import useContext from 'react'
const value = useContext(Context)
console.log(value)
二、传值Demo
index.js 根组件
import React, createContext from 'react'
import ReactDOM from 'react-dom'
import Father from './father'
export const Context = createContext()
const Ren = function ()
return (
<Context.Provider value= a: 1, b: 2 >
<div>
<div>根组件</div>
<div style= border: '1px solid #ccc' > <Father /></div>
</div>
</Context.Provider>
)
ReactDOM.render(<Ren></Ren>, document.getElementById('root'))
father.js 父组件
import Son from './son.js'
function Father ()
return (
<div>
<div>父组件</div>
<div style= border: '1px solid #ccc' > <Son /></div>
</div>
)
export default Father
son.js 子组件
import Context from './index'
import useContext from 'react'
function Son1 ()
const value = useContext(Context)
return (
<div>
<div> 孙子组件: value.a value.b </div>
</div>
)
export default Son1
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