<el-form-item label="型別">
        <el-dropdown @command="setType" trigger="click">
          <el-button type="primary">
                <span v-if='!type'>請選擇型別</span>
                <span v-else>{{type}}</span>
                <i class="el-icon-arrow-down el-icon--right"></i>
          </el-button>
          <el-dropdown-menu slot="dropdown">
            <el-dropdown-item command="1">黃金糕</el-dropdown-item>
            <el-dropdown-item command="2">獅子頭</el-dropdown-item>
            <el-dropdown-item command="3">螺螄粉</el-dropdown-item>
            <el-dropdown-item command="4">雙皮奶</el-dropdown-item>
            <el-dropdown-item command="5">蚵仔煎</el-dropdown-item>
          </el-dropdown-menu>
        </el-dropdown>
    </el-form-item>


    setType(command){
        console.log(command)
    },


   給html的options的裡寫selected="selected"