新建一个example文件夹，用来存放小demo的所有js文件

开始写代码
showArea.js 上面的显示部分

import React,{useContext}  from 'react';
import {ColorContext} from './color.js'

function ShowArea(){
	const {color} = useContext(ColorContext)
	return(
		<div style={{color:color}}>
			字体颜色为{color}
		</div>
	)
	
}

export default ShowArea;

button.js下面的按钮点击部分

import React,{useContext} from 'react';
import {ColorContext,UPDATE_COLOR} from './color'


function Button(){
	const {dispatch} = useContext(ColorContext);
	return(
		<div>
			<button onClick={()=>{dispatch({type:UPDATE_COLOR,color:"yellow"})}}>黄色</button>
			<button onClick={()=>{dispatch({type:UPDATE_COLOR,color:"red"})}}>红色</button>
		</div>
	)
}

export default Button;

最后组合起来
Example7.js

import React  from 'react';
import ShowArea  from './showArea.js'
import Button from './Button.js'
import {Color} from './color.js'

function  Example7(){
	return(
		<div>
			<Color >
				<ShowArea />
				<Button />
			</Color>	
		</div>
	)
		
}

export default Example7;

具体实现功能color.js

import React,{createContext,useReducer} from 'react';

export const ColorContext = createContext()

export const UPDATE_COLOR ="update_color";
const reducer = (state,action)=>{
	switch(action.type){
		case UPDATE_COLOR:
			return action.color
		default :
			return state	
	}	
}


export const Color = props=>{
	const [color,dispatch]= useReducer(reducer,'blue');
	return(
		<ColorContext.Provider value={{color,dispatch}}>
			{props.children}
		</ColorContext.Provider>
	)
		
}