beforeRouteEnter(to, from, next) {
   //判断从index页面进入，将list的isBack设置为true
   //这样就可以请求数据了
     if (from.name == 'index') {
      to.meta.isBack = true;
     }
     next();
   },
   activated: function () {
     if (this.$route.meta.isBack || this.isFirstEnter) {
      //清理已有商品列表的数据，重新请求数据，如果不清除的话就会有之前的商品缓存，延迟显示最新的商品
      this.proData = [];
      //请求数据
      this.fetchData();
     }
     //重新设置当前路由的isBack
     this.$route.meta.isBack = false;
     //重新设置是否第一次进入
     this.isFirstEnter = false;
   },
   mounted: function () {
    //如果是第一次进入，或者刷新操作的话，也请求数据
     this.isFirstEnter = true;
   },

const appRouter = {
 mode: "history",
 base: "/m/",
 routes: [
  {
   path: "",
   redirect: "/index"
  },
  {
   path: "/index",
   name: "index",
   component: Index,
   meta: {
    keepAlive: true
   }
  },
    {
   path: "/list",
   name: "list",
   component: List,
   meta: {
    keepAlive: true,
    isBack: false //isback是true的时候请求数据，或者第一次进入的时候请求数据
   }
  },
  {
   path: "/product/:id",
   name: "product",
   component: Product,
   meta: {
    keepAlive: false
   }
  }
  
 ]
};

Vue.use(Router);
export default new Router(appRouter);