export class History {
  router: Router; //router对象
  base: string;  //基准路径
  current: Route;  //当前的路由
  pending: ?Route;
  cb: (r: Route) => void;  //回调
  ready: boolean; 
  readyCbs: Array<Function>;
  readyErrorCbs: Array<Function>;
  errorCbs: Array<Function>;

  // 子类实现
  +go: (n: number) => void;
  +push: (loc: RawLocation) => void;
  +replace: (loc: RawLocation) => void;
  +ensureURL: (push?: boolean) => void;
  +getCurrentLocation: () => string;

  constructor (router: Router, base: ?string) {
    this.router = router
    this.base = normalizeBase(base) //返回基准路径
    this.current = START //route 设置当前route
    this.pending = null
    this.ready = false
    this.readyCbs = []
    this.readyErrorCbs = []
    this.errorCbs = []
  }

  listen (cb: Function) {
    this.cb = cb
  }

  onReady (cb: Function, errorCb: ?Function) {
   //*****
  }

  onError (errorCb: Function) {
    //*****
  }
  
  //路由转化操作
  transitionTo (location: RawLocation, onComplete?: Function, onAbort?: Function) {
    const route = this.router.match(location, this.current) //找到匹配路由
    this.confirmTransition(route, () => { //确认是否转化
      this.updateRoute(route) //更新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) })
      }
    })
  }
  //确认是否转化路由
  confirmTransition (route: Route, onComplete: Function, onAbort?: Function) {
    const current = this.current
    const abort = err => {
      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)
    }
    //判断如果前后是同一个路由，不进行操作
    if (
      isSameRoute(route, current) &&
      route.matched.length === current.matched.length
    ) {
      this.ensureURL()
      return abort()
    }
    //下面是各类钩子函数的处理
    //*********************
    })
  }
 
  //更新路由
  updateRoute (route: Route) {
    const prev = this.current //跳转前路由
    this.current = route //装备跳转路由
    this.cb && this.cb(route) //回调函数，这一步很重要，这个回调函数在index文件中注册，会更新被劫持的数据 _router
    this.router.afterHooks.forEach(hook => {
      hook && hook(route, prev)
    })
  }
}

//push方法
 push (location: RawLocation, onComplete?: Function, onAbort?: Function) {
   this.transitionTo(location, route => {
     pushHash(route.fullPath)
     onComplete && onComplete(route)
   }, onAbort)
 }
 //replace方法
 replace (location: RawLocation, onComplete?: Function, onAbort?: Function) {
   this.transitionTo(location, route => {
     replaceHash(route.fullPath)
     onComplete && onComplete(route)
   }, onAbort)
 }

//push
 push (location: RawLocation, onComplete?: Function, onAbort?: Function) {
  const { current: fromRoute } = this
  this.transitionTo(location, route => {
    pushState(cleanPath(this.base + route.fullPath)) //保存当前的位置信息，用于返回时候复位
    handleScroll(this.router, route, fromRoute, false)
    onComplete && onComplete(route)
  }, onAbort)
}

replace (location: RawLocation, onComplete?: Function, onAbort?: Function) {
  const { current: fromRoute } = this
  this.transitionTo(location, route => {
    replaceState(cleanPath(this.base + route.fullPath)) //保存当前的位置信息，用于返回时候复位
    handleScroll(this.router, route, fromRoute, false)
    onComplete && onComplete(route)
  }, onAbort)
}

push (location: RawLocation, onComplete?: Function, onAbort?: Function) {
  this.transitionTo(location, route => {
    this.stack = this.stack.slice(0, this.index + 1).concat(route)
    this.index++
    onComplete && onComplete(route)
  }, onAbort)
}

replace (location: RawLocation, onComplete?: Function, onAbort?: Function) {
  this.transitionTo(location, route => {
    this.stack = this.stack.slice(0, this.index).concat(route)
    onComplete && onComplete(route)
  }, onAbort)
}