能否让$emit方法返回一个Promise对象。

[LastHeaven]

What problem does this feature solve?

能否让$emit方法返回一个Promise对象？这可以知道分发的事件在什么时候完成，方便在对这个事件进行监听的所有方法完成后进行一些额外的操作。
比如说，一个按钮组件，在点击之后变成加载中状态，在点击操作（这个操作可能是向服务器请求数据，也有可能仅仅只是个同步操作）完成之后，还原成之前的状态。这样的话，可以让按钮的状态变化与事件逻辑分离开。

What does the proposed API look like?

可以像下面一样的使用

{
  async onClick () {
     this.loading = true
     await this.$emit('click')
     this.loading = false
  }
}

[posva]

you would need to pass some callback or resolve function as the emit payload. Otherwise, there is no way for the parent to tell that we are done:
Duplicate of #5443

You could implement a user land solution:

Vue.prototype.$asyncEmit = function (name, payload) {
  return new Promise((resolve, reject) => {
    this.$emit(name, { payload, resolve, reject })
  })
}

Then you could do something like this:

this.loading = true
await this.$asyncEmit('click')
this.loading = false

but the parent needs to resolve when it's done

[LastHeaven]

@posva
Can I use like this?

Vue.prototype.$asyncEmit = function (event) {
      const vm = this
      let cbs = vm._events[event]
      if (cbs) {
        cbs = cbs.length > 1 ? [...cbs] : cbs
        const args = [...arguments].slice(1)
        return cbs.map(cb => {
          const res = args ? cb.apply(vm, args) : cb.call(vm)
          if (isPromiseLike(res)) {
            return res
          } else {
            return new Promise(resolve => {
              resolve(res)
            })
          }
        })
      }
      return vm
    }

Then

async onClick () {
      this.loading = true
      const [res] = await Promise.all(this.$asyncEmit('click'))
      console.log('res', res)
      this.loading = false
}