深入理解Vue.js源碼之事件機(jī)制

{
  eventName: [func1, func2, func3]
}

/*初始化事件*/
export function initEvents (vm: Component) {
 /*在vm上創(chuàng)建一個(gè)_events對象，用來存放事件。*/
 vm._events = Object.create(null)
 /*這個(gè)bool標(biāo)志位來表明是否存在鉤子，而不需要通過哈希表的方法來查找是否有鉤子，這樣做可以減少不必要的開銷，優(yōu)化性能。*/
 vm._hasHookEvent = false
 // init parent attached events
 /*初始化父組件attach的事件*/
 const listeners = vm.$options._parentListeners
 if (listeners) {
  updateComponentListeners(vm, listeners)
 }
}

 Vue.prototype.$on = function (event: string | Array<string>, fn: Function): Component {
  const vm: Component = this

  /*如果是數(shù)組的時(shí)候，則遞歸$on，為每一個(gè)成員都綁定上方法*/
  if (Array.isArray(event)) {
   for (let i = 0, l = event.length; i < l; i++) {
    this.$on(event[i], fn)
   }
  } else {
   (vm._events[event] || (vm._events[event] = [])).push(fn)
   // optimize hook:event cost by using a boolean flag marked at registration
   // instead of a hash lookup
   /*這里在注冊事件的時(shí)候標(biāo)記bool值也就是個(gè)標(biāo)志位來表明存在鉤子，而不需要通過哈希表的方法來查找是否有鉤子，這樣做可以減少不必要的開銷，優(yōu)化性能。*/
   if (hookRE.test(event)) {
    vm._hasHookEvent = true
   }
  }
  return vm
 }

 Vue.prototype.$once = function (event: string, fn: Function): Component {
  const vm: Component = this
  function on () {
   /*在第一次執(zhí)行的時(shí)候?qū)⒃撌录N毀*/
   vm.$off(event, on)
   /*執(zhí)行注冊的方法*/
   fn.apply(vm, arguments)
  }
  on.fn = fn
  vm.$on(event, on)
  return vm
 }

Vue.prototype.$off = function (event?: string | Array<string>, fn?: Function): Component {
  const vm: Component = this
  // all
  /*如果不傳參數(shù)則注銷所有事件*/
  if (!arguments.length) {
   vm._events = Object.create(null)
   return vm
  }
  // array of events
  /*如果event是數(shù)組則遞歸注銷事件*/
  if (Array.isArray(event)) {
   for (let i = 0, l = event.length; i < l; i++) {
    this.$off(event[i], fn)
   }
   return vm
  }
  // specific event
  const cbs = vm._events[event]
  /*Github:https://github.com/answershuto*/
  /*本身不存在該事件則直接返回*/
  if (!cbs) {
   return vm
  }
  /*如果只傳了event參數(shù)則注銷該event方法下的所有方法*/
  if (arguments.length === 1) {
   vm._events[event] = null
   return vm
  }
  // specific handler
  /*遍歷尋找對應(yīng)方法并刪除*/
  let cb
  let i = cbs.length
  while (i--) {
   cb = cbs[i]
   if (cb === fn || cb.fn === fn) {
    cbs.splice(i, 1)
    break
   }
  }
  return vm
 }

Vue.prototype.$emit = function (event: string): Component {
  const vm: Component = this
  if (process.env.NODE_ENV !== 'production') {
   const lowerCaseEvent = event.toLowerCase()
   if (lowerCaseEvent !== event && vm._events[lowerCaseEvent]) {
    tip(
     `Event "${lowerCaseEvent}" is emitted in component ` +
     `${formatComponentName(vm)} but the handler is registered for "${event}". ` +
     `Note that HTML attributes are case-insensitive and you cannot use ` +
     `v-on to listen to camelCase events when using in-DOM templates. ` +
     `You should probably use "${hyphenate(event)}" instead of "${event}".`
    )
   }
  }
  let cbs = vm._events[event]
  if (cbs) {
   /*將類數(shù)組的對象轉(zhuǎn)換成數(shù)組*/
   cbs = cbs.length > 1 ? toArray(cbs) : cbs
   const args = toArray(arguments, 1)
   /*遍歷執(zhí)行*/
   for (let i = 0, l = cbs.length; i < l; i++) {
    cbs[i].apply(vm, args)
   }
  }
  return vm
 }