beforeRouteEnter(to, from, next) {
   //判斷從index頁面進入，將list的isBack設置為true
   //這樣就可以請求數據了
     if (from.name == 'index') {
      to.meta.isBack = true;
     }
     next();
   },
   activated: function () {
     if (this.$route.meta.isBack || this.isFirstEnter) {
      //清理已有商品列表的數據，重新請求數據，如果不清除的話就會有之前的商品緩存，延遲顯示最新的商品
      this.proData = [];
      //請求數據
      this.fetchData();
     }
     //重新設置當前路由的isBack
     this.$route.meta.isBack = false;
     //重新設置是否第一次進入
     this.isFirstEnter = false;
   },
   mounted: function () {
    //如果是第一次進入，或者刷新操作的話，也請求數據
     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的時候請求數據，或者第一次進入的時候請求數據
   }
  },
  {
   path: "/product/:id",
   name: "product",
   component: Product,
   meta: {
    keepAlive: false
   }
  }
  
 ]
};

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