学习react 笔记五 (useEffect、useRef、useContext)

目录

一、useEffect副作用

 (1)使用useEffect

(2)执行时机(第二个参数)

(3)倒计时demo

(4)请求ajax

(5)useEffect的清理函数

(6)图片移动demo

(7)图片移动demo 封装

二、 useRef(引用)

(1)使用方法

(2)demo 获取input的值

(3)demo 清除定时器

(4)demo 验证码倒计时

(5)demo 验证码倒计时 抽离hooks

三、useContext(跨组件传值)

一、使用方法

二、传值Demo


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 
倒计时100个数 {a}
}

(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 
}

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 

111

} export default function Use () { const [isShow, toggle] = useState(true) const hide = () => { toggle(false) } return (
{isShow && }
切换
) } // const root = ReactDOM.createRoot(document.getElementById('root')) // root.render() ReactDOM.render(, 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 (
    
) } // const root = ReactDOM.createRoot(document.getElementById('root')) // root.render() ReactDOM.render(, 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 (
    
) } ReactDOM.render(, 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 (
    
) } ReactDOM.render(, 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 (
    
倒计时 {Num}
) } ReactDOM.render(, 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 (
    
倒计时 {Num}
) } ReactDOM.render(, 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 (
    
倒计时 {Num}
) } ReactDOM.render(, 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 (
    
倒计时 {Num}
) } ReactDOM.render(, 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 (
    
      <根组件内容/>
    
  )

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 (
    
      
根组件
) } ReactDOM.render(, document.getElementById('root'))

father.js 父组件

import Son from './son.js'
function Father () {

  return (
    
父组件
) } export default Father

son.js 子组件

import { Context } from './index'
import { useContext } from 'react'
function Son1 () {
  const value = useContext(Context)
  return (
    
孙子组件: {value.a} {value.b}
) } export default Son1

 

 

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