nextTick（function(){}）简单一句话理解：
nextTick（function(){}）可以理解为：在组件/内容挂载完成之前，先拿到dom元素
created () {
console.log(this.$refs.box_ref) // undefined
this.$nextTick(() => {
console.log(this.$refs.box_ref) // <div data-v-469af010 msg="Welcome to Your Vue.js App">HellWord</div>
})
}
