if (installedPlugins.indexOf(plugin) > -1) {
 return this
}

if (typeof plugin.install === 'function') {
 plugin.install.apply(plugin, args)
} else if (typeof plugin === 'function') {
 plugin.apply(null, args)
}

// src/install.js
Vue.mixin({
 beforeCreate () {
  if (isDef(this.$options.router)) {
   this._routerRoot = this
   this._router = this.$options.router
   this._router.init(this)
   Vue.util.defineReactive(this, '_route', this._router.history.current)
  } else {
   this._routerRoot = (this.$parent && this.$parent._routerRoot) || this
  }
  registerInstance(this, this)
 },
 destroyed () {
  registerInstance(this)
 }
})
...
// 全局注冊(cè) `router-view` 和 `router-link`組件
Vue.component('RouterView', View)
Vue.component('RouterLink', Link)

// src/index.js
let mode = options.mode || 'hash'
this.fallback = mode === 'history' && !supportsPushState && options.fallback !== false
if (this.fallback) {
 mode = 'hash'
}
if (!inBrowser) {
 mode = 'abstract'
}

// src/index.js
switch (mode) {
 case 'history':
  this.history = new HTML5History(this, options.base)
  break
 case 'hash':
  this.history = new HashHistory(this, options.base, this.fallback)
  break
 case 'abstract':
  this.history = new AbstractHistory(this, options.base)
  break
 default:
  if (process.env.NODE_ENV !== 'production') {
   assert(false, `invalid mode: ${mode}`)
  }
}

// src/create-route-map.js
function addRouteRecord (
 pathList: Array<string>,
 pathMap: Dictionary<RouteRecord>,
 nameMap: Dictionary<RouteRecord>,
 route: RouteConfig,
 parent?: RouteRecord,
 matchAs?: string
) {
 ...
 route.children.forEach(child => {
  const childMatchAs = matchAs
   ? cleanPath(`${matchAs}/${child.path}`)
   : undefined
  addRouteRecord(pathList, pathMap, nameMap, child, record, childMatchAs)
 })
 ...
}

// src/create-route-map.js
if (!pathMap[record.path]) {
 pathList.push(record.path)
 pathMap[record.path] = record
}

const routes = [
 { path: '/foo', component: Foo },
 { path: '/bar', component: Bar1 },
 { path: '/bar', component: Bar2 },
];

// src/util/route.js
const route: Route = {
 name: location.name || (record && record.name),
 meta: (record && record.meta) || {},
 path: location.path || '/',
 hash: location.hash || '',
 query,
 params: location.params || {},
 fullPath: getFullPath(location, stringifyQuery),
 matched: record ? formatMatch(record) : []
}

transitionTo (location: RawLocation, onComplete?: Function, onAbort?: Function) {
 const route = this.router.match(location, this.current)
 this.confirmTransition(route, () => {
 ...
}

// src/history/hash.js
push (location: RawLocation, onComplete?: Function, onAbort?: Function) {
 const { current: fromRoute } = this
 // 利用了 transitionTo 方法
 this.transitionTo(location, route => {
  pushHash(route.fullPath)
  handleScroll(this.router, route, fromRoute, false)
  onComplete && onComplete(route)
 }, onAbort)
}

// src/history/base.js
const queue: Array<?NavigationGuard> = [].concat(
 // in-component leave guards
 extractLeaveGuards(deactivated),
 // global before hooks
 this.router.beforeHooks,
 // in-component update hooks
 extractUpdateHooks(updated),
 // in-config enter guards
 activated.map(m => m.beforeEnter),
 // async components
 resolveAsyncComponents(activated)
)

// src/history/base.js
// 異步回調(diào)函數(shù)
runQueue(queue, iterator, () => {
 const postEnterCbs = []
 const isValid = () => this.current === route
 // wait until async components are resolved before
 // extracting in-component enter guards
 const enterGuards = extractEnterGuards(activated, postEnterCbs, isValid)
 const queue = enterGuards.concat(this.router.resolveHooks)
 // 遞歸執(zhí)行
 runQueue(queue, iterator, () => {
  if (this.pending !== route) {
   return abort()
  }
  this.pending = null
  onComplete(route)
  if (this.router.app) {
   this.router.app.$nextTick(() => {
    postEnterCbs.forEach(cb => { cb() })
   })
  }
 })
})

// src/history/base.js
const iterator = (hook: NavigationGuard, next) => {
 ...
 hook(route, current, (to: any) => {
  ...
  } else {
   // confirm transition and pass on the value
   next(to)
  }
 })
...
}

// src/history/hash.js
function pushHash (path) {
 if (supportsPushState) {
  pushState(getUrl(path))
 } else {
  window.location.hash = path
 }
}

// src/history/hash.js
setupListeners () {
 ...
 window.addEventListener(supportsPushState ? 'popstate' : 'hashchange', () => {
  const current = this.current
  if (!ensureSlash()) {
   return
  }
  this.transitionTo(getHash(), route => {
   if (supportsScroll) {
    handleScroll(this.router, route, current, true)
   }
   if (!supportsPushState) {
    replaceHash(route.fullPath)
   }
  })
 })
}

// src/history/html5.js
window.addEventListener('popstate', e => {
 const current = this.current

 // Avoiding first `popstate` event dispatched in some browsers but first
 // history route not updated since async guard at the same time.
 const location = getLocation(this.base)
 if (this.current === START && location === initLocation) {
  return
 }

 this.transitionTo(location, route => {
  if (supportsScroll) {
   handleScroll(router, route, current, true)
  }
 })
})

// src/history/hash.js
function ensureSlash (): boolean {
 const path = getHash()
 if (path.charAt(0) === '/') {
  return true
 }
 replaceHash('/' + path)
 return false
}

// src/history/hash.js
setupListeners () {
 ...
 if (supportsScroll) {
  // 進(jìn)行瀏覽器滾動(dòng)條的事件控制
  setupScroll()
 }
 ...
}

// src/util/scroll.js
window.addEventListener('popstate', e => {
 saveScrollPosition()
 if (e.state && e.state.key) {
  setStateKey(e.state.key)
 }
})

// vue-router/src/components/view.js
...
const matched = route.matched[depth]
// render empty node if no matched route
if (!matched) {
 cache[name] = null
 return h()
}
const component = cache[name] = matched.components[name]
...
return h(component, data, children)

// src/install.js
Vue.mixin({
 beforeCreate () {
  if (isDef(this.$options.router)) {
   this._routerRoot = this
   this._router = this.$options.router
   this._router.init(this)
   // 觸發(fā) router-view重渲染
   Vue.util.defineReactive(this, '_route', this._router.history.current)
   ...
})

Object.defineProperty(obj, key, {
  enumerable: true,
  configurable: true,
  get: function reactiveGetter () {
   const value = getter ? getter.call(obj) : val
   if (Dep.target) {
    // 進(jìn)行依賴收集
    dep.depend()
    if (childOb) {
     childOb.dep.depend()
     if (Array.isArray(value)) {
      dependArray(value)
     }
    }
   }
   return value
  },
  set: function reactiveSetter (newVal) {
   ...
   // 通知訂閱當(dāng)前數(shù)據(jù) watcher的觀察者進(jìn)行響應(yīng)
   dep.notify()
  }

// vue-router/src/components/view.js
render (_, { props, children, parent, data }) {
 // used by devtools to display a router-view badge
 data.routerView = true

 // directly use parent context's createElement() function
 // so that components rendered by router-view can resolve named slots
 const h = parent.$createElement
 const name = props.name
 // 觸發(fā)依賴收集，建立 render watcher
 const route = parent.$route
 ...
}

history.listen(route => {
 this.apps.forEach((app) => {
  // 觸發(fā) setter
  app._route = route
 })
})

// src/components/link.js
render (h: Function) {
 ...
 const globalActiveClass = router.options.linkActiveClass
 const globalExactActiveClass = router.options.linkExactActiveClass
 // Support global empty active class
 const activeClassFallback = globalActiveClass == null
  ? 'router-link-active'
  : globalActiveClass
 const exactActiveClassFallback = globalExactActiveClass == null
  ? 'router-link-exact-active'
  : globalExactActiveClass
  ...
}

// src/components/link.js
data.on = on
data.attrs = { href }

if (this.tag === 'a') {
  data.on = on
  data.attrs = { href }
 } else {
  // find the first <a> child and apply listener and href
  // findAnchor即為遞歸遍歷子元素的方法
  const a = findAnchor(this.$slots.default)
  ...
 }
}

// src/components/link.js
const handler = e => {
 if (guardEvent(e)) {
  if (this.replace) {
   // replace路由
   router.replace(location)
  } else {
   // push 路由
   router.push(location)
  }
 }
}

// vue/src/core/vdom/create-functional-component.js
(clone.data || (clone.data = {})).slot = data.slot

if (clone.data) {
 clone.data.slot = data.slot
} else {
 clone.data = {
  slot: data.slot
 }
}