vue問題六：計算屬性，依賴發生變化時，重新計算computed：

<el-form-item label="單價："  prop="price" >
       <el-input v-model="addform.price" type="number" placeholder="請輸入單價" auto-complete="off" class="widc" ></el-input>
 </el-form-item>
<el-form-item label="數量："  prop="number" >
       <el-input v-model="addform.number" type="number" placeholder="請輸入數量" auto-complete="off" class="widc" ></el-input>
</el-form-item>
<el-form-item label="總金額："  prop="sum" >
       <div v-model="addform.countNum" class="border" ><span style="margin-left: 10%">{{countNum}}</span></div>
</el-form-item>

// 計算總金額
    computed:{
      countNum:function(addform){
        let countNum=Number(this.addform.price) * Number(this.addform.number)
        return countNum
      }
    },