window.addEventListener('popstate', function (e) {
  // 用来判断是否是后退, 在判断后需要在其他地方重置
  router.isBack = true
},false)

{
   path: "/login",
   component: resolve => require(["@/pages/login"], resolve),
   meta: {
    title: "登录",
    keepAlive: false,
    index: 1
   }
  },
  {
   path: "/forward",
   name: "Forward",
   component: resolve => require(["@/pages/forward"], resolve),
   meta: {
    title: "前进",
    keepAlive: true,
    index: 2
   }
  },

 {
   path: "/login",
   component: resolve => require(["@/pages/login"], resolve),
   meta: {
    title: "登录",
    keepAlive: false, // 用来判断是否缓存, 当判断为缓存时, 则路由信息的name和组件的name选项需一致
    index: 1, // 通过比较不同的索引, 来判断是前进动画还是后退动画
   }
  },
  {
   path: "/forward",
   name: "Forward",
   component: resolve => require(["@/pages/forward"], resolve),
   meta: {
    title: "前进",
    keepAlive: true,
    index: 2
   }
  },

<template>
 <div id="project">
   <!-- 
   <keep-alive>
       <router-view v-if="$route.meta.keepAlive"></router-view>
   </keep-alive>
   <router-view v-if="!$route.meta.keepAlive"></router-view>
   这种情况下, 
   :include: 因为若是使用transition包裹两个keep-alive, vue会出现报错
        用两个transition分别包裹keep-alive, 会让transition的动画name出现问题
 --> 
  <transition :name="transitionName">
   <keep-alive :include="cachedViews">
    <router-view :key="1"></router-view>
   </keep-alive>
  </transition>
 </div>
</template>

<script>
export default {
 name: "App",
 data() {
  return {
   transitionName: "slide-right", // 初始过渡动画方向
   cachedViews: [] // 缓存组件
  };
 },
 components: {},
 created() {},
 watch: {
  $route(to, from) {
   if (to.meta.keepAlive && !this.cachedViews.includes(to.name)) {
    // 将需要缓存的组件信息, 添加进其中, 其中to.name的值应该和匹配组件的name选项一致
    this.cachedViews.push(to.name);
   }
   //如果to索引大于from索引,判断为前进状态,反之则为后退状态
   if (to.meta.index > from.meta.index) {
    //设置动画名称
    this.transitionName = "slide-left";
   } else {
    this.transitionName = "slide-right";
   }
  }
 },
 methods: {}
};
</script>


<style lang="scss" scoped>
.slide-right-enter-active,
.slide-right-leave-active,
.slide-left-enter-active,
.slide-left-leave-active {
 will-change: transform;
 transition: all 0.5s;
 <!-- 
  这个是必须的, 是为了让页面脱离文档流, 不然的话, 后进入的页面会从页面底部出来
  这个定位会直接添加到路由匹配的组件根元素上, 所以页面根组件最好设定其宽度为100vw
 -->
 width: 100vw;
 position: absolute;
}
.slide-right-enter {
 opacity: 0;
 transform: translate3d(-100%, 0, 0);
}
.slide-right-leave-active {
 opacity: 0;
 transform: translate3d(100%, 0, 0);
}
.slide-left-enter {
 opacity: 0;
 transform: translate3d(100%, 0, 0);
}
.slide-left-leave-active {
 opacity: 0;
 transform: translate3d(-100%, 0, 0);
}
</style>