router: Router; 表示VueRouter实例。实例化History类时的第一个参数
 base: string;  表示基路径。会用normalizeBase进行规范化。实例化History类时的第二个参数。
 current: Route; 表示当前路由(route)。
 pending: ?Route; 描述阻塞状态。
 cb: (r: Route) => void; 监听时的回调函数。
 ready: boolean; 描述就绪状态。
 readyCbs: Array; 就绪状态的回调数组。
 readyErrorCbs: Array; 就绪时产生错误的回调数组。
 errorCbs: Array; 错误的回调数组

 // implemented by sub-classes
 
 +go: (n: number) => void;
 +push: (loc: RawLocation) => void;
 +replace: (loc: RawLocation) => void;
 +ensureURL: (push?: boolean) => void;
 +getCurrentLocation: () => string;

transitionTo (location: RawLocation, onComplete?: Function, onAbort?: Function) {
  const route = this.router.match(location, this.current) // 解析成每一个location需要的route
  this.confirmTransition(route, () => {
   this.updateRoute(route)
   onComplete && onComplete(route)
   this.ensureURL()

   // fire ready cbs once
   if (!this.ready) {
    this.ready = true
    this.readyCbs.forEach(cb => { cb(route) })
   }
  }, err => {
   if (onAbort) {
    onAbort(err)
   }
   if (err && !this.ready) {
    this.ready = true
    this.readyErrorCbs.forEach(cb => { cb(err) })
   }
  })
 }

confirmTransition(route: Route, onComplete: Function, onAbort?: Function) {
    const current = this.current
    const abort = err => { // 异常处理函数
      if (isError(err)) {
        if (this.errorCbs.length) {
          this.errorCbs.forEach(cb => { cb(err) })
        } else {
          warn(false, 'uncaught error during route navigation:')
          console.error(err)
        }
      }
      onAbort && onAbort(err)
    }
    if (
      isSameRoute(route, current) &&
      // in the case the route map has been dynamically appended to
      route.matched.length === current.matched.length
    ) {
      this.ensureURL()
      return abort()
    }
    
    const {
      updated,
      deactivated,
      activated
    } = resolveQueue(this.current.matched, route.matched)
    
    const queue: Array<?NavigationGuard> = [].concat(
      // beforeRouteLeave 钩子函数
      extractLeaveGuards(deactivated),
      // 全局的beforeHooks勾子
      this.router.beforeHooks,
      // beforeRouteUpdate 钩子函数调用
      extractUpdateHooks(updated),
      // config里的勾子
      activated.map(m => m.beforeEnter),
      // async components
      resolveAsyncComponents(activated)
    )
    
    this.pending = route
    
    const iterator = (hook: NavigationGuard, next) => {
      if (this.pending !== route) {
        return abort()
      }
      try {
        hook(route, current, (to: any) => {
          if (to === false || isError(to)) {
            // next(false) -> abort navigation, ensure current URL
            this.ensureURL(true)
            abort(to)
          } else if (
            typeof to === 'string' ||
            (typeof to === 'object' && (
              typeof to.path === 'string' ||
              typeof to.name === 'string'
            ))
          ) {
            // next('/') or next({ path: '/' }) -> redirect
            abort()
            if (typeof to === 'object' && to.replace) {
              this.replace(to)
            } else {
              this.push(to)
            }
          } else {
            // confirm transition and pass on the value
            next(to)
          }
        })
      } catch (e) {
        abort(e)
      }
    }
    
    runQueue(queue, iterator, () => {
      const postEnterCbs = []
      const isValid = () => this.current === route
      
      const enterGuards = extractEnterGuards(activated, postEnterCbs, isValid)
      const queue = enterGuards.concat(this.router.resolveHooks)
      
      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() })
          })
        }
      })
    })
  }