Vue.js event mechanism source code analysis

Because I am very interested in Vue.js, and the technology stack I usually work on is also Vue.js, I spent some time studying the Vue.js source code in the past few months, and made a summary and output. This article mainly introduces the event mechanism of Vue.js source code. The editor thinks it is quite good, so I will share it with you now and give it as a reference. Let’s follow the editor to take a look, I hope it can help everyone.

During the learning process, Chinese comments were added to Vue https://github.com/answershuto/learnVue/tree/master/vue-src. I hope it can help other friends who want to learn Vue source code. Helps.
There may be deviations in understanding. Welcome to raise issues to learn and make progress together.

Vue Event API

As we all know, Vue.js provides us with four event APIs, namely $on](https://cn. vuejs.org/v2/api/#vm-on-event-callback), [$once, $off](https://cn.vuejs.org/v2/api/#vm-off-event-callback), [$emit.

Initialization event

The initialization event creates an _events object on the vm to store events. The content of _events is as follows:

{
  eventName: [func1, func2, func3]
}

stores event names and corresponding execution methods.

/*初始化事件*/
export function initEvents (vm: Component) {
 /*在vm上创建一个_events对象，用来存放事件。*/
 vm._events = Object.create(null)
 /*这个bool标志位来表明是否存在钩子，而不需要通过哈希表的方法来查找是否有钩子，这样做可以减少不必要的开销，优化性能。*/
 vm._hasHookEvent = false
 // init parent attached events
 /*初始化父组件attach的事件*/
 const listeners = vm.$options._parentListeners
 if (listeners) {
  updateComponentListeners(vm, listeners)
 }
}

$on

$on method is used to listen to a custom event on the vm instance. The event Can be triggered by $emit.

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

  /*如果是数组的时候，则递归$on，为每一个成员都绑定上方法*/
  if (Array.isArray(event)) {
   for (let i = 0, l = event.length; i < l; i++) {
    this.$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
   /*这里在注册事件的时候标记bool值也就是个标志位来表明存在钩子，而不需要通过哈希表的方法来查找是否有钩子，这样做可以减少不必要的开销，优化性能。*/
   if (hookRE.test(event)) {
    vm._hasHookEvent = true
   }
  }
  return vm
 }

$once

##$once listens for an event that can only be triggered once, and will automatically move after it is triggered. Except this event.

 Vue.prototype.$once = function (event: string, fn: Function): Component {
  const vm: Component = this
  function on () {
   /*在第一次执行的时候将该事件销毁*/
   vm.$off(event, on)
   /*执行注册的方法*/
   fn.apply(vm, arguments)
  }
  on.fn = fn
  vm.$on(event, on)
  return vm
 }

$off

$off is used to remove custom events

Vue.prototype.$off = function (event?: string | Array<string>, fn?: Function): Component {
  const vm: Component = this
  // all
  /*如果不传参数则注销所有事件*/
  if (!arguments.length) {
   vm._events = Object.create(null)
   return vm
  }
  // array of events
  /*如果event是数组则递归注销事件*/
  if (Array.isArray(event)) {
   for (let i = 0, l = event.length; i < l; i++) {
    this.$off(event[i], fn)
   }
   return vm
  }
  // specific event
  const cbs = vm._events[event]
  /*Github:https://github.com/answershuto*/
  /*本身不存在该事件则直接返回*/
  if (!cbs) {
   return vm
  }
  /*如果只传了event参数则注销该event方法下的所有方法*/
  if (arguments.length === 1) {
   vm._events[event] = null
   return vm
  }
  // specific handler
  /*遍历寻找对应方法并删除*/
  let cb
  let i = cbs.length
  while (i--) {
   cb = cbs[i]
   if (cb === fn || cb.fn === fn) {
    cbs.splice(i, 1)
    break
   }
  }
  return vm
 }

$emit

$emit is used to trigger specified custom events.

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) {
   /*将类数组的对象转换成数组*/
   cbs = cbs.length > 1 ? toArray(cbs) : cbs
   const args = toArray(arguments, 1)
   /*遍历执行*/
   for (let i = 0, l = cbs.length; i < l; i++) {
    cbs[i].apply(vm, args)
   }
  }
  return vm
 }

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