vue.use中发生了什么

import Router from 'vue-router'
// 入口文件全局注入vue-router, 从而可以在全局使用this.$route
Vue.use(Router)

// a.vue
import Router from 'vue-router'
// b.vue
import Router from 'vue-router'

Vue.use = function (plugin: Function | Object) {
const installedPlugins = (this._installedPlugins || (this._installedPlugins = []))
if (installedPlugins.indexOf(plugin) > -1) {
return this
}
// additional parameters
const args = toArray(arguments, 1)
args.unshift(this)
if (typeof plugin.install === ‘function’) {
plugin.install.apply(plugin, args)
} else if (typeof plugin === ‘function’) {
plugin.apply(null, args)
}
installedPlugins.push(plugin)
return this
}