p21- 统计、清空Clear、隐藏
隐藏：没有数据时，隐藏元素（intems left 和 Clear）（v-show v-if 数据非空）
<body>
<!--列表区域-->
<ul>
<ol class="input_text color" v-for="(item,index) in arr">{{index+1}}.{{item}}
<button type="button" class="destroy" @click="remove(index)">×</button> <!--删除键-->
</ol>
</ul>
<div class="footer" v-show="arr.length!=0">
<span class="todo-count" v-if="arr.length!=0">
<strong>{{arr.length}} items left</strong> <!--统计-->
</span>
<!--<div @click="clear" class="pointer">Clear</div> -->
<botton v-show="arr.length!=0" class="clear-completed" @click="clear"> Clear </botton>
</div>
<hr >
</div> <script>....</script></body>
