如何实现组件props双向绑定

在vue2中props只能单向传递数据，及从父组件传递给子组件，但是子组件不能改变父组件传递过来的值，一开始不知道有这个问题的存在，效果能出来，但是项目报错，如下
Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop’s value.
然后参考https://www.cnblogs.com/xxcanghai/p/6124699.html该文章解决了问题
父组件

<template>
  <div id="combat">
      <!-- 倒计时组件 -->
      <div class="fr" >
        <my-clock  :time.sync="activity_time"  @on-result-change="onResultChange"/>// 注意
      </div>
    </div>
    </template>
   < script>
   export default {
	data(){
		return:{
			activity_time:null	// 注意	
		}
	},
	method:{
		onResultChange(val){ // 注意
        		this.time=val;//外层调用组件方注册变更方法，将组件内的数据变更，同步到组件外的数据状态中
   	 },
   	 // 获取剩余时间
      	getDate(endTime){
	          const newendTime = Date.parse(endTime)
	          const newData = Date.parse(new Date())
	          let syTime = (newendTime-newData)/1000
	          this.activity_time =  syTime
	          console.log(this.activity_time)
      }
}
}
   </script>

子组件

<template>
  <div id="clock_box">
    <span class="clock_ico">
      <i class="iconfont icon-icon-test"/>
    </span>
    <time v-text="now_time"/>
  </div>
</template>

<script>
export default {
  props: {
    time: {  // 注意	
      type: Number,
      default: null
    }
  },
  data: function() {
    return {
      now_time: '',
      son_time: this.time// 注意	
    }
  },
  created() {
    this.now_time = this.formatSeconds(this.time)
      console.log(this.time)
  },
  mounted() {
    var that = this
    var timer = setInterval(function() {
      that.son_time--
      if (that.son_time <= 0) {
        clearInterval(timer)
      }
      that.now_time = that.formatSeconds(that.son_time)
    }, 1000)
  },
    watch: {// 注意	
        time(val) {
            this.son_time = val;//新增result的watch，监听变更并同步到myResult上
        },
        son_time(val){
            this.$emit("on-result-change",val);//③组件内对myResult变更后向外部发送事件通知
        }
    },
  methods: {
    formatSeconds(s) {
      var t
      if (s > -1) {
        var hour = Math.floor(s / 3600)
        var min = Math.floor(s / 60) % 60
        var sec = s % 60
        if (hour < 10) {
          t = '0' + hour + ':'
        } else {
          t = hour + ':'
        }

        if (min < 10) {
          t += '0'
        }
        t += min + ':'
        if (sec < 10) {
          t += '0'
        }
        t += sec
      }
      return t
    }
  }
}
</script>