深入理解Vue.js源码之事件机制

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用来移除自定义事件

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用来触发指定的自定义事件。

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
 }