Vue Router：如何实现一个前端路由

在我们创建 router 对象的时候，会创建一个 history 对象，前面提到 Vue Router 支持三种模式，这里我们重点分析 HTML5 的 history 的模式：

function createWebHistory(base) {
  base = normalizeBase(base)
  const historyNavigation = useHistoryStateNavigation(base)
  const historyListeners = useHistoryListeners(base, historyNavigation.state, historyNavigation.location, historyNavigation.replace)
  function go(delta, triggerListeners = true) {
    if (!triggerListeners)
      historyListeners.pauseListeners()
    history.go(delta)
  }
  const routerHistory = assign({
    // it's overridden right after
    location: '',
    base,
    go,
    createHref: createHref.bind(null, base),
  }, historyNavigation, historyListeners)
  Object.defineProperty(routerHistory, 'location', {
    get: () => historyNavigation.location.value,
  })
  Object.defineProperty(routerHistory, 'state', {
    get: () => historyNavigation.state.value,
  })
  return routerHistory
}

对于 routerHistory 对象而言，它有两个重要的作用，一个是路径的切换，一个是监听路径的变化。

其中，路径切换主要通过 historyNavigation 来完成的，它是 useHistoryStateNavigation 函数的返回值，我们来看它的实现：

function useHistoryStateNavigation(base) {
  const { history, location } = window
  let currentLocation = {
    value: createCurrentLocation(base, location),
  }
  let historyState = { value: history.state }
  if (!historyState.value) {
    changeLocation(currentLocation.value, {
      back: null,
      current: currentLocation.value,
      forward: null,
      position: history.length - 1,
      replaced: true,
      scroll: null,
    }, true)
  }
  function changeLocation(to, state, replace) {
    const url = createBaseLocation() +
      // preserve any existing query when base has a hash
      (base.indexOf('#') > -1 && location.search
        ? location.pathname + location.search + '#'
        : base) +
      to
    try {
      history[replace ? 'replaceState' : 'pushState'](state, '', url)
      historyState.value = state
    }
    catch (err) {
      warn('Error with push/replace State', err)
      location[replace ? 'replace' : 'assign'](url)
    }
  }
  function replace(to, data) {
    const state = assign({}, history.state, buildState(historyState.value.back,
      // keep back and forward entries but override current position
      to, historyState.value.forward, true), data, { position: historyState.value.position })
    changeLocation(to, state, true)
    currentLocation.value = to
  }
  function push(to, data) {
    const currentState = assign({},
      historyState.value, history.state, {
        forward: to,
        scroll: computeScrollPosition(),
      })
    if ( !history.state) {
      warn(`history.state seems to have been manually replaced without preserving the necessary values. Make sure to preserve existing history state if you are manually calling history.replaceState:\n\n` +
        `history.replaceState(history.state, '', url)\n\n` +
        `You can find more information at https://next.router.vuejs.org/guide/migration/#usage-of-history-state.`)
    }
    changeLocation(currentState.current, currentState, true)
    const state = assign({}, buildState(currentLocation.value, to, null), { position: currentState.position + 1 }, data)
    changeLocation(to, state, false)
    currentLocation.value = to
  }
  return {
    location: currentLocation,
    state: historyState,
    push,
    replace
  }
}

该函数返回的 push 和 replace 函数，会添加给 routerHistory 对象上，因此当我们调用 routerHistory.push 或者是 routerHistory.replace 方法的时候实际上就是在执行这两个函数。

push 和 replace 方法内部都是执行了 changeLocation 方法，该函数内部执行了浏览器底层的 history.pushState 或者 history.replaceState 方法，会向当前浏览器会话的历史堆栈中添加一个状态，这样就在不刷新页面的情况下修改了页面的 URL。

我们使用这种方法修改了路径，这个时候假设我们点击浏览器的回退按钮回到上一个 URL，这需要恢复到上一个路径以及更新路由视图，因此我们还需要监听这种 history 变化的行为，做一些相应的处理。