text: /* @flow */
  
  import type Router from '../index'
  import { assert } from './warn'
  import { getStateKey, setStateKey } from './push-state'
  
  const positionStore = Object.create(null)
  
  export function setupScroll () {
    // Fix for #1585 for Firefox
    // 监听 popstate 事件
    window.history.replaceState({ key: getStateKey() }, '')
    window.addEventListener('popstate', e => {
      // 在 positionStore 对象中以 key: value 形式保存当前页面滚动位置
      saveScrollPosition()
      // 将 key 储存 返回时使用
      if (e.state && e.state.key) {
        setStateKey(e.state.key)
      }
    })
  }
  
  export function handleScroll (
    router: Router,
    to: Route,
    from: Route,
    isPop: boolean
  ) {
    if (!router.app) {
      return
    }
  
    const behavior = router.options.scrollBehavior
    if (!behavior) {
      return
    }
  
    if (process.env.NODE_ENV !== 'production') {
      assert(typeof behavior === 'function', `scrollBehavior must be a function`)
    }
  
    // wait until re-render finishes before scrolling
    router.app.$nextTick(() => {
      const position = getScrollPosition()
      const shouldScroll = behavior.call(router, to, from, isPop ? position : null)
  
      if (!shouldScroll) {
        return
      }
  
      if (typeof shouldScroll.then === 'function') {
        shouldScroll.then(shouldScroll => {
          scrollToPosition((shouldScroll: any), position)
        }).catch(err => {
          if (process.env.NODE_ENV !== 'production') {
            assert(false, err.toString())
          }
        })
      } else {
        scrollToPosition(shouldScroll, position)
      }
    })
  }
  
  export function saveScrollPosition () {
    const key = getStateKey()
    if (key) {
      positionStore[key] = {
        x: window.pageXOffset,
        y: window.pageYOffset
      }
    }
  }
  
  function getScrollPosition (): ?Object {
    const key = getStateKey()
    if (key) {
      return positionStore[key]
    }
  }
  
  function getElementPosition (el: Element, offset: Object): Object {
    const docEl: any = document.documentElement
    const docRect = docEl.getBoundingClientRect()
    const elRect = el.getBoundingClientRect()
    return {
      x: elRect.left - docRect.left - offset.x,
      y: elRect.top - docRect.top - offset.y
    }
  }
  
  function isValidPosition (obj: Object): boolean {
    return isNumber(obj.x) || isNumber(obj.y)
  }
  
  function normalizePosition (obj: Object): Object {
    return {
      x: isNumber(obj.x) ? obj.x : window.pageXOffset,
      y: isNumber(obj.y) ? obj.y : window.pageYOffset
    }
  }
  
  function normalizeOffset (obj: Object): Object {
    return {
      x: isNumber(obj.x) ? obj.x : 0,
      y: isNumber(obj.y) ? obj.y : 0
    }
  }
  
  function isNumber (v: any): boolean {
    return typeof v === 'number'
  }
  
  function scrollToPosition (shouldScroll, position) {
    const isObject = typeof shouldScroll === 'object'
    if (isObject && typeof shouldScroll.selector === 'string') {
      const el = document.querySelector(shouldScroll.selector)
      if (el) {
        let offset = shouldScroll.offset && typeof shouldScroll.offset === 'object' ? shouldScroll.offset : {}
        offset = normalizeOffset(offset)
        position = getElementPosition(el, offset)
      } else if (isValidPosition(shouldScroll)) {
        position = normalizePosition(shouldScroll)
      }
    } else if (isObject && isValidPosition(shouldScroll)) {
      position = normalizePosition(shouldScroll)
    }
  
    if (position) {
      window.scrollTo(position.x, position.y)
    }
  }
  