vue获取data数据改变前后的值方法

watch: {
a (now,old) {
console.log(now,old)
}
}

import Vue from 'vue'
/自定义指令/
/el所绑定的对象，binding指令上的参数/
Vue.directive('n',{
  /插入数据时触发/
  inserted: function (el,binding) {
    console.log('插入',binding,el)
    el.innerHTML = binding.value
  },
  update: function (el,binding) {
    console.log('更新参数',binding)
    el.dataset.oldNum = binding.oldValue
    el.innerHTML = binding.value
  },
  bind:function (el,binding) {
    console.log('绑定参数',binding)
    el.innerHTML = binding.value
  }
})

 import n from '../assets/n'
<div ref="div" v-n="a"></div>
<button @click="inc">增加</button>
inc () {
  this.a++
  var that = this
  setTimeout(function () {
    console.log(that.$refs.div.dataset.oldNum)
  },1)
}