前端路由简介以及vue-router实现原理

后端路由简介

路由这个概念最先是后端出现的。在以前用模板引擎开发页面时，经常会看到这样

http://www.xxx.com/login

http://www.xxx.com/#/login

function matchAndUpdate () {
   // todo 匹配 hash 做 dom 更新操作
}

window.addEventListener('hashchange', matchAndUpdate)

function matchAndUpdate () {
   // todo 匹配路径 做 dom 更新操作
}

window.addEventListener('popstate', matchAndUpdate)

import VueRouter from 'vue-router'
Vue.use(VueRouter)

const router = new VueRouter({
  mode: 'history',
  routes: [...]
})

new Vue({
  router
  ...
})

new Router({
  id: 'router-view', // 容器视图
  mode: 'hash', // 模式
  routes: [
    {
      path: '/',
      name: 'home',
      component: '<div>Home</div>',
      beforeEnter: (next) => {
        console.log('before enter home')
        next()
      },
      afterEnter: (next) => {
        console.log('enter home')
        next()
      },
      beforeLeave: (next) => {
        console.log('start leave home')
        next()
      }
    },
    {
      path: '/bar',
      name: 'bar',
      component: '<div>Bar</div>',
      beforeEnter: (next) => {
        console.log('before enter bar')
        next()
      },
      afterEnter: (next) => {
        console.log('enter bar')
        next()
      },
      beforeLeave: (next) => {
        console.log('start leave bar')
        next()
      }
    },
    {
      path: '/foo',
      name: 'foo',
      component: '<div>Foo</div>'
    }
  ]
})

思路整理

首先是数据驱动，所以我们可以通过一个route对象来表述当前路由状态，比如：

current = {
    path: '/', // 路径
    query: {}, // query
    params: {}, // params
    name: '', // 路由名
    fullPath: '/', // 完整路径
    route: {} // 记录当前路由属性
}

class Observer {
  constructor (value) {
    this.walk(value)
  }

  walk (obj) {
    Object.keys(obj).forEach((key) => {
      // 如果是对象，则递归调用walk，保证每个属性都可以被defineReactive
      if (typeof obj[key] === 'object') {
        this.walk(obj[key])
      }
      defineReactive(obj, key, obj[key])
    })
  }
}

function defineReactive(obj, key, value) {
  let dep = new Dep()
  Object.defineProperty(obj, key, {
    get: () => {
      if (Dep.target) {
        // 依赖收集
        dep.add()
      }
      return value
    },
    set: (newValue) => {
      value = newValue
      // 通知更新，对应的更新视图
      dep.notify()
    }
  })
}

export function observer(value) {
  return new Observer(value)
}

export class Dep {
  constructor () {
    this.deppend = []
  }
  add () {
    // 收集watcher
    this.deppend.push(Dep.target)
  }
  notify () {
    this.deppend.forEach((target) => {
      // 调用watcher的更新函数
      target.update()
    })
  }
}

Dep.target = null

export function setTarget (target) {
  Dep.target = target
}

export function cleanTarget() {
  Dep.target = null
}

// Watcher
export class Watcher {
  constructor (vm, expression, callback) {
    this.vm = vm
    this.callbacks = []
    this.expression = expression
    this.callbacks.push(callback)
    this.value = this.getVal()

  }
  getVal () {
    setTarget(this)
    // 触发 get 方法，完成对 watcher 的收集
    let val = this.vm
    this.expression.split('.').forEach((key) => {
      val = val[key]
    })
    cleanTarget()
    return val
  }

  // 更新动作
  update () {
    this.callbacks.forEach((cb) => {
      cb()
    })
  }
}

  // 响应式数据劫持
  observer(this.current)

  // 对 current.route 对象进行依赖收集，变化时通过 render 来更新
  new Watcher(this.current, 'route', this.render.bind(this))

  render() {
    let i
    if ((i = this.history.current) && (i = i.route) && (i = i.component)) {
      document.getElementById(this.container).innerHTML = i
    }
  }

hash 和 history

接下来是hash和history模式的实现，这里我们可以沿用vue-router的思想，建立不同的处理模型便可。来看一下我实现的核心代码：

this.history = this.mode === 'history' ? new HTML5History(this) : new HashHistory(this)

  /**
   * 路由转换
   * @param target 目标路径
   * @param cb 成功后的回调
   */
  transitionTo(target, cb) {
    // 通过对比传入的 routes 获取匹配到的 targetRoute 对象
    const targetRoute = match(target, this.router.routes)
    this.confirmTransition(targetRoute, () => {
      // 这里会触发视图更新
      this.current.route = targetRoute
      this.current.name = targetRoute.name
      this.current.path = targetRoute.path
      this.current.query = targetRoute.query || getQuery()
      this.current.fullPath = getFullPath(this.current)
      cb && cb()
    })
  }

  /**
   * 确认跳转
   * @param route
   * @param cb
   */
  confirmTransition (route, cb) {
    // 钩子函数执行队列
    let queue = [].concat(
      this.router.beforeEach,
      this.current.route.beforeLeave,
      route.beforeEnter,
      route.afterEnter
    )
    
    // 通过 step 调度执行
    let i = -1
    const step = () => {
      i ++
      if (i > queue.length) {
        cb()
      } else if (queue[i]) {
        queue[i](step)
      } else {
        step()
      }

    }
    step(i)
  }
}

 /**
   * 跳转，添加历史记录
   * @param location 
   * @example this.push({name: 'home'})
   * @example this.push('/')
   */
  push (location) {
    const targetRoute = match(location, this.router.routes)

    this.transitionTo(targetRoute, () => {
      changeUrl(this.router.base, this.current.fullPath)
    })
  }

  /**
   * 跳转，添加历史记录
   * @param location
   * @example this.replaceState({name: 'home'})
   * @example this.replaceState('/')
   */
  replaceState(location) {
    const targetRoute = match(location, this.router.routes)

    this.transitionTo(targetRoute, () => {
      changeUrl(this.router.base, this.current.fullPath, true)
    })
  }

  go (n) {
    window.history.go(n)
  }

  function changeUrl(path, replace) {
    const href = window.location.href
    const i = href.indexOf('#')
    const base = i >= 0 ? href.slice(0, i) : href
    if (replace) {
      window.history.replaceState({}, '', `${base}#/${path}`)
    } else {
      window.history.pushState({}, '', `${base}#/${path}`)
    }
  }