data(){
//json 解析
	todos.JSON.parse(localStorage.getItem('todos'));
}

// 存json 
watch:{
	localStorage.setItem('todos',JSON.stringify(value));
}

使用vue 的深度监视来处理。

====================================================

//保存一个数据
localStorage.setItem('msg','hello!!!') 
localStorage.setItem('msg2',666)			localStorage.setItem('person',JSON.stringify(p))

//读取一个数据
localStorage.getItem('msg')

//JSON.parse() 方法将数据转换为 JavaScript 对象。
JSON.parse(result)

//删除一个数据
localStorage.removeItem

//清空一个数据
localStorage.clear()

App.vue

<template>
	<div id="root">
		<div class="todo-container">
			<div class="todo-wrap">
				<MyHeader :addTodo="addTodo"/>
				<MyList :todos="todos" :checkTodo="checkTodo" :deleteTodo="deleteTodo"/>
				<MyFooter :todos="todos" :checkAllTodo="checkAllTodo" :clearAllTodo="clearAllTodo"/>
			</div>
		</div>
	</div>
</template>

<script>
	import MyHeader from './components/MyHeader'
	import MyList from './components/MyList'
	import MyFooter from './components/MyFooter.vue'

	export default {
		name:'App',
		components:{MyHeader,MyList,MyFooter},
		data() {
			return {
				//由于todos是MyHeader组件和MyFooter组件都在使用，所以放在App中（状态提升）
				todos:JSON.parse(localStorage.getItem('todos')) || []
			}
		},
		methods: {
			//添加一个todo
			addTodo(todoObj){
				this.todos.unshift(todoObj)
			},
			//勾选or取消勾选一个todo
			checkTodo(id){
				this.todos.forEach((todo)=>{
					if(todo.id === id) todo.done = !todo.done
				})
			},
			//删除一个todo
			deleteTodo(id){
				this.todos = this.todos.filter( todo => todo.id !== id )
			},
			//全选or取消全选
			checkAllTodo(done){
				this.todos.forEach((todo)=>{
					todo.done = done
				})
			},
			//清除所有已经完成的todo
			clearAllTodo(){
				this.todos = this.todos.filter((todo)=>{
					return !todo.done
				})
			}
		},
		watch: {
			todos:{
				deep:true,
				handler(value){
					localStorage.setItem('todos',JSON.stringify(value))
				}
			}
		},
	}
</script>

<style>
	/*base*/
	body {
		background: #fff;
	}
	.btn {
		display: inline-block;
		padding: 4px 12px;
		margin-bottom: 0;
		font-size: 14px;
		line-height: 20px;
		text-align: center;
		vertical-align: middle;
		cursor: pointer;
		box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.2), 0 1px 2px rgba(0, 0, 0, 0.05);
		border-radius: 4px;
	}
	.btn-danger {
		color: #fff;
		background-color: #da4f49;
		border: 1px solid #bd362f;
	}
	.btn-danger:hover {
		color: #fff;
		background-color: #bd362f;
	}
	.btn:focus {
		outline: none;
	}
	.todo-container {
		width: 600px;
		margin: 0 auto;
	}
	.todo-container .todo-wrap {
		padding: 10px;
		border: 1px solid #ddd;
		border-radius: 5px;
	}
</style>

main.js

//引入Vue
import Vue from 'vue'
//引入App
import App from './App.vue'
//关闭Vue的生产提示
Vue.config.productionTip = false

//创建vm
new Vue({
	el:'#app',
	render: h => h(App)
})

components/MyFooter.vue

<template>
	<div class="todo-footer" v-show="total">
		<label>
			<!-- <input type="checkbox" :checked="isAll" @change="checkAll"/> -->
			<input type="checkbox" v-model="isAll"/>
		</label>
		<span>
			<span>已完成{{doneTotal}}</span> / 全部{{total}}
		</span>
		<button class="btn btn-danger" @click="clearAll">清除已完成任务</button>
	</div>
</template>

<script>
	export default {
		name:'MyFooter',
		props:['todos','checkAllTodo','clearAllTodo'],
		computed: {
			//总数
			total(){
				return this.todos.length
			},
			//已完成数
			doneTotal(){
				//此处使用reduce方法做条件统计
				/* const x = this.todos.reduce((pre,current)=>{
					console.log('@',pre,current)
					return pre + (current.done ? 1 : 0)
				},0) */
				//简写
				return this.todos.reduce((pre,todo)=> pre + (todo.done ? 1 : 0) ,0)
			},
			//控制全选框
			isAll:{
				//全选框是否勾选
				get(){
					return this.doneTotal === this.total && this.total > 0
				},
				//isAll被修改时set被调用
				set(value){
					this.checkAllTodo(value)
				}
			}
		},
		methods: {
			/* checkAll(e){
				this.checkAllTodo(e.target.checked)
			} */
			//清空所有已完成
			clearAll(){
				this.clearAllTodo()
			}
		},
	}
</script>

<style scoped>
	/*footer*/
	.todo-footer {
		height: 40px;
		line-height: 40px;
		padding-left: 6px;
		margin-top: 5px;
	}

	.todo-footer label {
		display: inline-block;
		margin-right: 20px;
		cursor: pointer;
	}

	.todo-footer label input {
		position: relative;
		top: -1px;
		vertical-align: middle;
		margin-right: 5px;
	}

	.todo-footer button {
		float: right;
		margin-top: 5px;
	}
</style>

MyHeader.vue

<template>
	<div class="todo-header">
		<input type="text" placeholder="请输入你的任务名称，按回车键确认" v-model="title" @keyup.enter="add"/>
	</div>
</template>

<script>
	import {nanoid} from 'nanoid'
	export default {
		name:'MyHeader',
		//接收从App传递过来的addTodo
		props:['addTodo'],
		data() {
			return {
				//收集用户输入的title
				title:''
			}
		},
		methods: {
			add(){
				//校验数据
				if(!this.title.trim()) return alert('输入不能为空')
				//将用户的输入包装成一个todo对象
				const todoObj = {id:nanoid(),title:this.title,done:false}
				//通知App组件去添加一个todo对象
				this.addTodo(todoObj)
				//清空输入
				this.title = ''
			}
		},
	}
</script>

<style scoped>
	/*header*/
	.todo-header input {
		width: 560px;
		height: 28px;
		font-size: 14px;
		border: 1px solid #ccc;
		border-radius: 4px;
		padding: 4px 7px;
	}

	.todo-header input:focus {
		outline: none;
		border-color: rgba(82, 168, 236, 0.8);
		box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.075), 0 0 8px rgba(82, 168, 236, 0.6);
	}
</style>

MyItem.vue

<template>
	<li>
		<label>
			<input type="checkbox" :checked="todo.done" @change="handleCheck(todo.id)"/>
			<!-- 如下代码也能实现功能，但是不太推荐，因为有点违反原则，因为修改了props -->
			<!-- <input type="checkbox" v-model="todo.done"/> -->
			<span>{{todo.title}}</span>
		</label>
		<button class="btn btn-danger" @click="handleDelete(todo.id)">删除</button>
	</li>
</template>

<script>
	export default {
		name:'MyItem',
		//声明接收todo、checkTodo、deleteTodo
		props:['todo','checkTodo','deleteTodo'],
		methods: {
			//勾选or取消勾选
			handleCheck(id){
				//通知App组件将对应的todo对象的done值取反
				this.checkTodo(id)
			},
			//删除
			handleDelete(id){
				if(confirm('确定删除吗？')){
					//通知App组件将对应的todo对象删除
					this.deleteTodo(id)
				}
			}
		},
	}
</script>

<style scoped>
	/*item*/
	li {
		list-style: none;
		height: 36px;
		line-height: 36px;
		padding: 0 5px;
		border-bottom: 1px solid #ddd;
	}

	li label {
		float: left;
		cursor: pointer;
	}

	li label li input {
		vertical-align: middle;
		margin-right: 6px;
		position: relative;
		top: -1px;
	}

	li button {
		float: right;
		display: none;
		margin-top: 3px;
	}

	li:before {
		content: initial;
	}

	li:last-child {
		border-bottom: none;
	}

	li:hover{
		background-color: #ddd;
	}
	
	li:hover button{
		display: block;
	}
</style>

MyList.vue

<template>
	<ul class="todo-main">
		<MyItem 
			v-for="todoObj in todos"
			:key="todoObj.id" 
			:todo="todoObj" 
			:checkTodo="checkTodo"
			:deleteTodo="deleteTodo"
		/>
	</ul>
</template>

<script>
	import MyItem from './MyItem'

	export default {
		name:'MyList',
		components:{MyItem},
		//声明接收App传递过来的数据，其中todos是自己用的，checkTodo和deleteTodo是给子组件MyItem用的
		props:['todos','checkTodo','deleteTodo']
	}
</script>

<style scoped>
	/*main*/
	.todo-main {
		margin-left: 0px;
		border: 1px solid #ddd;
		border-radius: 2px;
		padding: 0px;
	}

	.todo-empty {
		height: 40px;
		line-height: 40px;
		border: 1px solid #ddd;
		border-radius: 2px;
		padding-left: 5px;
		margin-top: 10px;
	}
</style>