(显示的过程：在动画即将被执行的瞬间，会往div上增加两个class名：fade-enter、fade-enter-active。然后去掉fade-enter，增加fade-enter-to。接着动画继续执行，到结束的瞬间，会把最后两个class去掉。)
（隐藏的过程）
为啥style标签里的class以fade开头呢？因为我在transition取名就是fade。如果不取名，默认是v(例如：v-enter、v-enter-active)：
<!DOCTYPE html>
<html>
<head>
<title></title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script src="./vue.js"></script>
<!-- <script src="http://cdn.staticfile.org/vue/2.6.10/vue.common.dev.js"></script> -->
<style type="text/css">
.fade-enter {
opacity: 0
}
.fade-enter-active {
transition: opacity 3s;
}
.fade-leave-to {
opacity: 0
}
.fade-leave-active {
transition: opacity 3s;
}
</style>
</head>
<body>
<div id="root">
//name随便取名:
<transition name="fade">
<div v-if="show">hello</div>
</transition>
<button @click="handleClick">切换</button>
</div>
<script type="text/javascript">
var vm = new Vue({
el: "#root",
data: {
show: true
},
methods: {
handleClick: function() {
this.show = !this.show
}
}
});
</script>
</body>
</html>
