处理每切换一次页面就增加一条sidebarItems数据的问题

[xiyueW]

问题：
在使用useSidebarItems时，watch监听sidebarItems，发现每切换一次页面，sidebarItems打印的数量就加一
watch(sidebarItems,(n)=>{ console.log(n) })
处理：
可能原因是VPPage组件中调用setupHeaders函数中，此函数每调用一次会给router添加一次beforeEach，导致headersRef改变，从而影响provide的sidebarItems的改变。

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[Mister-Hope]

I don't understand why this fix is valid and why it should be "fixed this way". Detailed explanation would help.

[xiyueW]

问题回显：

1. 新启一个项目，在config.js中添加两个sidebar配置作为测试：
   { sidebar: { '/get-started/': ['started.md', 'started2.md'] } }
2. 在源码lib/client/layouts/Layout.vue文件监听sidebarItems：
   watch(sidebarItems,(n)=>{ console.log(n) })
   

问题分析：
页面切换VPPage.vue会执行setupHeaders函数，router.beforeEach就会多添加一次执行函数，导致每切换一次路由，router.beforeEach的函数会多执行一个，从而多影响一次sidebarItems。

[Mister-Hope]

Great thanks for the explanation, I decide to fix in another way.

The other way adds a new alias @theme/useHeaders for people to override the default logic if necessary easily while decoupling from sidebar.