vue使用$emit时,父组件无法监听到子组件的事件实例

<div id="counter-event-example">
  <p>{{ total }}</p>
  <button-counter v-on:ee="incrementTotal"></button-counter>
  <button-counter v-on:eEvent="incrementTotal"></button-counter>
  <child ref="cmpSelect" v-on:ee="incrementTotal" option-api-url="/api/admin/cms/cmsCategory/getOptions.do"></child>
 </div>
 <script>
  Vue.component('button-counter', {
   template: '<button v-on:click="increment">{{ counter }}</button>',
   data: function () {
    return {
     counter: 0
    }
   },
   methods: {
    increment: function () {
     this.counter += 1
     this.$emit('ee', this.counter);//有效
     this.$emit('eEvent', this.counter);//无效，不能使用大写的驼峰规则命名
    }
   },
  })
  new Vue({
   el: '#counter-event-example',
   data: {
    total: '点击下面的按钮'
   },
   methods: {
    incrementTotal: function (b) {
     this.total = b;
    }
   }
  })
 </script>