vue2.0 展开收缩动画有没有源码啊 ,我使用过度效果 item 没有动画效果

Question

vue2.0 展开收缩动画有没有源码啊 ,我使用过度效果 item 没有动画效果
<template>
<div>

<div class="slide-top" @click="onSlide()">|||</div>
<ul class="slide" v-for="item in structure">
  <li v-bind:class="{ 'expanded': item.expanded}">
    <a href="JavaScript:" class="slide-nav-title" v-bind:class="{'titleColor': item.expanded}" @click="toggle(item)">
      <span>{{item.name}}</span>
    </a>
    <ul class="slide-nav-title-sub" v-for="subItem in item.subRoutes">
      <a class="slide-nav-title-sub-item" href="#"><router-link :to="{path: subItem.path}">{{subItem.name}}</router-link></a>
    </ul>
  </li>
</ul>

</div>
</template>
.slide-nav-title-sub{
display: none;
}
.expanded .slide-nav-title-sub{
display: block;
}