深入理解Vue.js源码之事件机制

{  eventName: [func1, func2, func3]}

/*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) }}

 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 }

 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 }

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 }

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 }