Vue.prototype.bus=new Vue();    Vue.component('child',{         template:`<div @click='handleClick'>{{text}}</div>`,         props:{             content:[String,Number]         },        data(){            return {                 text:this.content             }         },         methods:{             handleClick(){                this.bus.$emit('change',this.text)             }         },         mounted(){            var _this=this             console.log(_this);            this.bus.$on('change',function (msg) {                     _this.text=msg                     console.log(this);             })         }     })    var vm = new Vue({         el:"#app",     })上面代碼中的 this.bus.$on('change',function (msg) {                    _this.text=msg                     console.log(this);             })         }里面的回調函數(shù)里面的this到底是指向誰？希望大佬幫忙解答一下，謝謝