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React hooks(useContext、useReducer、自定义hooks)

作者站长头像
站长
· 阅读数 43

一、hooks(useContext)

接收一个 context 对象(React.createContext 的返回值)并返回该 context 的当前值。当前的 context 值由上层组件中距离当前组件最近的 <MyContext.Provider> 的 value prop 决定。

新建useContext.js函数组件,写入如下代码:

import React, {  useEffect, useState, useContext } from 'react'
import axios from 'axios'
import '../css/middlecp.css'

const GlobalContext = React.createContext(); // 创建context对象

export default function UseContext() {

    const [filmList, setFilmList] = useState([])
    const [info, setInfo] = useState('')

    useEffect(() => {
        axios.get('/data.json').then(res => {
            console.log(res.data.data.films)
            setFilmList(res.data.data.films)
        }, err => {
            console.log(err)
        })
    }, [])
    return (
        <GlobalContext.Provider value={{
            info: info,
            changeInfo: (value) => {
                setInfo(value)
            }
        }}>
            <div>
                {
                    filmList.map(item => {
                        return <FilmItem key={item.filmId} {...item}></FilmItem>
                    })
                }
                <FilmDetail></FilmDetail>
            </div>
        </GlobalContext.Provider>
    )

}


function FilmItem(props) {
    let { name, poster, grade, synopsis } = props
    const value = useContext(GlobalContext)
    console.log(value)
    return <div className="filmitem" onClick={() => {
            value.changeInfo(synopsis)
        }}>
            <img src={poster} alt={name}></img>
            <h4>{name}</h4>
            <div>观众评分:{grade}</div>
        </div>
    
}

function FilmDetail() {
    const value = useContext(GlobalContext)
    return (
        <div className="filmdetail">
            {value.info}
        </div>
    )

}

可以看到在hooks中,我们没有像类组件那样去套了层级,直接使用useContext,可以看到效果跟类组件一样: React hooks(useContext、useReducer、自定义hooks)


二、hooks(useReducer)

useState 的替代方案。它接收一个形如 (state, action) => newState 的 reducer,并返回当前的 state 以及与其配套的 dispatch 方法。

新建useReducer.js组件,写入代码:

import React, { useReducer } from 'react'

export default function UseReducer() {

  // 处理函数
  const reducer = (prevState, action) => {
    console.log(prevState, action)
    let newState = {...prevState}
    switch(action.type) {
        case 'minus':
            newState.count--
            return newState;
        case 'add':
            newState.count++
            return newState;
        default:
            return newState;
    }
  } 

  // 外部对象
  const intialState = {
    count: 0
  }

  const [state, dispatch] = useReducer(reducer, intialState)  

  return (
    <div>
        <button onClick={() => {
            dispatch({
                type: "minus"
            })
        }}>-</button>
            {state.count}
        <button onClick={() => {
            dispatch({
                type: "add"
            })
        }}>+</button>
    </div>
  )
}

效果: React hooks(useContext、useReducer、自定义hooks)


三、hooks(useContext搭配useReducer使用)

hooks中useContext搭配useReducer使用跨级通信。(hooks中自带的,后面redux不用这么麻烦) 修改useReducer.js组件代码为如下:

import React, { useReducer, useContext } from 'react'

// 处理函数
const reducer = (prevState, action) => {
    console.log(prevState, action)
    let newState = {...prevState}
    switch(action.type) {
        case 'child2':
            newState.a = '改变后的a'
            return newState;
        case 'child3':
            newState.b = '改变后的b'
            return newState;
        default:
            return newState;
    }
  } 

// 外部对象
const intialState = {
    a: 'aaaaa',
    b: 'bbbbb'
}

const GlobalContext = React.createContext()

export default function UseReducer() {
  const [state, dispatch] = useReducer(reducer, intialState)  

  return (
    <GlobalContext.Provider value={
        {
            state,
            dispatch
        }
    }>  
        <div>
            <Child1></Child1>
            <Child2></Child2>
            <Child3></Child3>
        </div>
    </GlobalContext.Provider>
  )
}

function Child1() {
    const {dispatch} = useContext(GlobalContext)
    return (
        <div>
            <button onClick={() => {
                dispatch({
                    type: 'child2'
                })
            }}>改变child2</button>
            <button onClick={() => {
                dispatch({
                    type: 'child3'
                })
            }}>改变child3</button>
        </div>
    )
}
function Child2() {
    const {state} = useContext(GlobalContext)

    return (
        <div>
            {state.a}
        </div>
    )
}
function Child3() {
    const {state} = useContext(GlobalContext)

    return (
        <div>
            {state.b}
        </div>
    )
}

效果: React hooks(useContext、useReducer、自定义hooks)


四、自定义hooks

当我们想在两个函数之间共享逻辑时,我们会把它提取到第三个函数中。 必须使用use开头。(实际上就是将独立的逻辑函数抽离出来封装) 新建useCustom.js,写入代码:

import React, { useEffect, useMemo, useState } from 'react'
import axios from 'axios'

function useCinemaList() {
    const [cinemaList, setCinemaList] = useState([])
    useEffect(() => {
        axios({
            url: 'https://m.maizuo.com/gateway?cityId=110100&ticketFlag=1&k=3085018',
            method: 'get',
            headers: {
                'X-Client-Info':' {"a":"3000","ch":"1002","v":"5.2.0","e":"1646314068530784943341569"}',
                'X-Host': 'mall.film-ticket.cinema.list'
            }
        }).then((res) => {
            console.log(res.data)
            setCinemaList(res.data.data.cinemas)
        }).catch((err) => {
            console.log(err)
        })
      },[])
      return {
        cinemaList
      }
}

function useFliter(cinemaList, text) {
    const getCinemaList = useMemo(() => cinemaList.filter(item => item.name.toUpperCase().includes(text.toUpperCase()) || 
  item.address.toUpperCase().includes(text.toUpperCase())), [cinemaList, text]) // useMemo会执行函数并返回执行后的结果
    return {
        getCinemaList
    }
}

export default function UseCustom() {
  const [text, setText] = useState('')

  const {cinemaList} = useCinemaList()
  const {getCinemaList} = useFliter(cinemaList, text)

  return (
    <div>
        <input value={text} onChange={(event) => {
              setText(event.target.value)
            }}></input>
            {
                getCinemaList.map((item) => 
                <dl key={item.cinemaId}>
                    <dt>{item.name}</dt>
                    <dd>{item.address}</dd>
                </dl>
                )
            }
    </div>
  )
}

效果: React hooks(useContext、useReducer、自定义hooks) 可以看到我们这个影院的筛选功能还是正常,但是看代码的话,逻辑更加的清晰了。

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