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                <div id="root">
  <counter :count="0" @change="handleChange"></counter> +
  <counter :count="1" @change="handleChange"></counter> = <span>{{total}}</span>




                var counter = {
  props: ['count'], // 接收父组件传来的值
  data() {
    return {
      number: this.count // 拷贝prop值的副本,用于修改
  template: '<button @click="handleClick()">{{number}}</button>',
  methods: {
    handleClick() {
      this.number++ // 由于单向数据流,不能直接修改prop的值
      this.$emit('change', 1) // 派发事件并传出值,值可以有多个
var vm = new Vue({
  el: '#root',
  data: {
   total: 1
  components: {
  methods: {
    handleChange(step) {
      // step 子组件传来的值 += + step