我是 Vuejs 新手，我收到错误未知的自定义元素 -

如何注册自定义元素 - b-alert。我认为这个元素来自 bootstrapVue。

<template>
<div>
 <b-alert show>Default Alert</b-alert>



  </div> 
   </template>

    <script>
   export default {
    data () {
            return {
                 dismissSecs: 10,
                 dismissCountDown: 0,
                 showDismissibleAlert: false
            }
    },
                  methods: {
             countDownChanged (dismissCountDown) {
                     this.dismissCountDown = dismissCountDown
             },
             showAlert () {
                     this.dismissCountDown = this.dismissSecs
             }

     },

}