接收一個(gè) context 對(duì)象（React.createContext 的返回值）并返回該 context 的當(dāng)前值。當(dāng)前的 context 值由上層組件中距離當(dāng)前組件最近的 <MyContext.Provider> 的 value prop 決定。

新建useContext.js函數(shù)組件，寫入如下代碼：

import React, {  useEffect, useState, useContext } from 'react'
import axios from 'axios'
import '../css/middlecp.css'
const GlobalContext = React.createContext(); // 創(chuàng)建context對(duì)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>觀眾評(píng)分：{grade}</div>
        </div>
}
function FilmDetail() {
    const value = useContext(GlobalContext)
    return (
        <div className="filmdetail">
            {value.info}
        </div>
    )
}

二、hooks（useReducer）

useState 的替代方案。它接收一個(gè)形如 (state, action) => newState 的 reducer，并返回當(dāng)前的 state 以及與其配套的 dispatch 方法。

新建useReducer.js組件，寫入代碼：

import React, { useReducer } from 'react'
export default function UseReducer() {
  // 處理函數(shù)
  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;
    }
  } 
  // 外部對(duì)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：

三、hooks（useContext搭配useReducer使用）

hooks中useContext搭配useReducer使用跨級(jí)通信。（hooks中自帶的，后面redux不用這么麻煩）修改useReducer.js組件代碼為如下：

import React, { useReducer, useContext } from 'react'
// 處理函數(shù)
const reducer = (prevState, action) => {
    console.log(prevState, action)
    let newState = {...prevState}
    switch(action.type) {
        case 'child2':
            newState.a = '改變后的a'
            return newState;
        case 'child3':
            newState.b = '改變后的b'
            return newState;
        default:
            return newState;
    }
  } 
// 外部對(duì)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'
                })
            }}>改變child2</button>
            <button onClick={() => {
                dispatch({
                    type: 'child3'
                })
            }}>改變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：

四、自定義hooks

當(dāng)我們想在兩個(gè)函數(shù)之間共享邏輯時(shí)，我們會(huì)把它提取到第三個(gè)函數(shù)中。必須使用use開頭。(實(shí)際上就是將獨(dú)立的邏輯函數(shù)抽離出來封裝) 新建useCustom.js，寫入代碼：

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會(huì)執(zhí)行函數(shù)并返回執(zhí)行后的結(jié)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：