https://segmentfault.com/a/1190000018125564 很完善了，重点是两个过程图。
<style>
.fade-enter{
opacity: 0;
}
.fade-enter-active{
transition: opacity 3s;
}
/* .fade-enter-to{
color: red;
}
.fade-leave{
color: green;
} */
.fade-leave-to{
opacity: 0;
}
.fade-leave-active{
transition: opacity 3s;
}
</style>
</head>
<body>
<!--
过程如下：
显示 fade-enter,fade-enter-active fade-enter-active,fade-enter-to 空
隐藏 fade-leave,fade-leave-active fade-leave-active,fade-leave-to 空
-->
<div id="root">
<transition name='fade'>
<h1 v-show='show'>
最是年少时模样
</h1>
</transition>
<button @click='change'>切换</button>
</div>
<script>
var vm=new Vue({
el:'#root',
data:{
show:true
},
methods:{
change:function(){
this.show=!this.show;
}
}
})
</script>
</body>
来源：https://www.cnblogs.com/em2464/p/12332624.html