1.出现这个warn的背景，请看代码：

<router-view v-slot="{ Component }">
   <transition name="fade" mode="out-in" appear>
      <component :is="Component"></component>
   </transition>
</router-view>

2.问题原因分析：
由于:is="Component"属性会使所有的组件都渲染在这里，而外层是transition 虚拟组件，也就是说所有组件都会包裹在它里面，这是不允许的。

3.解决方案,推荐方案3：
-- 1.给每个组件分别加根节点div包裹起来：

<template>
  <div class="about">
    <h1>This is an about page</h1>
    <button @click="toPath('/test')">去测试页面看看</button>
  </div>
</template>

-- 2.

<template>
  <router-view v-slot="{ Component }">
    <transition >
     <div>
        <component :is="Component"></component>
     </div>
    </transition>
  </router-view>
</template>

-- 3.最优方案：

<router-view v-slot="{ Component, route }">
  <transition name="fade" mode="out-in">
    <div :key="route.name">  
      <component :is="Component"></component>
    </div>
  </transition>
</router-view>

end：给出最终完整版，可以实现页面切换转场动画和缓存

<template>
  <router-view v-slot="{ Component }">
    
      <transition :name="$router.customRouterData.transitionName">
        <keep-alive>
        <div :key="$route.name">
            <component
            :is="Component"
            v-if="$route.meta.keepAlive"
          />
        </div>
        </keep-alive>
      </transition>
    
      <transition :name="$router.customRouterData.transitionName">
      <div  :key="$route.name">
          <component
          :is="Component"
          v-if="!$route.meta.keepAlive"
        />
      </div>
      </transition>
    
  </router-view>
</template>

export default {
  name: "app",
  setup() {
    return {};
  },
};
</script>
<style lang="scss">
#app {
  position: relative;
  width: 100%;
  height: 100%;
}

.slide_left-enter-active,
.slide_left-leave-active,
.slide_right-enter-active,
.slide_right-leave-active {
  transition: all 0.3s;
  position: absolute !important;
  background-color: white;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  z-index: 1;
}

.slide_left-enter-from,
.slide_right-leave-to {
  opacity: 1;
  transform: translateX(100%);
}

.slide_right-enter-from,
.slide_left-leave-to {
  opacity: 1;
  transform: translateX(-100%);
}

.slide_left-leave-to,
.slide_right-leave-to {
  opacity: 0.3;
}
</style>