vue问题六：计算属性，依赖发生变化时，重新计算computed：

<el-form-item label="单价："  prop="price" >
       <el-input v-model="addform.price" type="number" placeholder="请输入单价" auto-complete="off" class="widc" ></el-input>
 </el-form-item>
<el-form-item label="数量："  prop="number" >
       <el-input v-model="addform.number" type="number" placeholder="请输入数量" auto-complete="off" class="widc" ></el-input>
</el-form-item>
<el-form-item label="总金额："  prop="sum" >
       <div v-model="addform.countNum" class="border" ><span style="margin-left: 10%">{{countNum}}</span></div>
</el-form-item>

// 计算总金额
    computed:{
      countNum:function(addform){
        let countNum=Number(this.addform.price) * Number(this.addform.number)
        return countNum
      }
    },