vue-element-admin - dialog彈窗form表單規則驗證，第二次打開顯示未填寫

原因：因為dialog不是銷毀組件，是隱藏組件
解決：加上v-if

<el-dialog title="添加" :visible.sync="dialogVisible" class="dialog-content" width="600px"  v-if="dialogVisible">
            <div class="dialog-content">
                <el-form :model="form" :rules="rules" label-width="120px" label-position="left" ref="form">
                    <el-form-item label="起止時間" prop="time" v-if="dialogVisible">
                        <el-date-picker @blur="changeTime" v-model="form.time" type="daterange" range-separator="至" start-placeholder="開始日期" end-placeholder="結束日期" value-format="yyyy-MM-dd" format="yyyy-MM-dd" style="width: 100%"></el-date-picker>
                    </el-form-item>
                    <el-form-item label="標題" prop="yh_title">
                        <el-input v-model="form.yh_title"></el-input>
                    </el-form-item>
                    <el-form-item label="副標題">
                        <el-input v-model="form.yh_ftitle"></el-input>
                    </el-form-item>
                    <el-row style="text-align: right">
                        <el-button type="default" @click="dialogVisible = false">取消</el-button>
                        <el-button type="primary" v-else @click="handleConfirm">確認</el-button>
                    </el-row>
                </el-form>
            </div>
        </el-dialog>