documentation/.vuepress/theme/Sidebar.vue

<template>
  <div class="sidebar">
    <NavLinks class="block md:hidden" />
    <slot name="top" />
    <ul class="sidebar-links" v-if="items.length">
      <li v-for="(item, i) in items" :key="item.path">
        <SidebarGroup
          v-if="item.type === 'group'"
          :item="item"
          :first="i === 0"
          :open="i === openGroupIndex"
          :collapsable="item.collapsable"
          @toggle="toggleGroup(i)"
        />
        <SidebarLink v-else :item="item" />
      </li>
    </ul>
    <slot name="bottom" />
  </div>
</template>

<script>
import SidebarGroup from "./SidebarGroup.vue";
import SidebarLink from "./SidebarLink.vue";
import NavLinks from "./NavLinks.vue";
import { isActive } from "./util";

export default {
  components: { SidebarGroup, SidebarLink, NavLinks },
  props: ["items"],
  data() {
    return {
      openGroupIndex: 0
    };
  },
  created() {
    this.refreshIndex();
  },
  watch: {
    $route() {
      this.refreshIndex();
    }
  },
  methods: {
    refreshIndex() {
      const index = resolveOpenGroupIndex(this.$route, this.items);
      if (index > -1) {
        this.openGroupIndex = index;
      }
    },
    toggleGroup(index) {
      this.openGroupIndex = index === this.openGroupIndex ? -1 : index;
    },
    isActive(page) {
      return isActive(this.$route, page.path);
    }
  }
};

function resolveOpenGroupIndex(route, items) {
  for (let i = 0; i < items.length; i++) {
    const item = items[i];
    if (
      item.type === "group" &&
      item.children.some(c => isActive(route, c.path))
    ) {
      return i;
    }
  }
  return -1;
}
</script>