详解vuex中action何时完成以及如何正确调用dispat

 dispatch (_type, _payload) {
  // check object-style dispatch
  const {
   type,
   payload
  } = unifyObjectStyle(_type, _payload)

  const action = { type, payload }
  const entry = this._actions[type]
  if (!entry) {
   if (process.env.NODE_ENV !== 'production') {
    console.error(`[vuex] unknown action type: ${type}`)
   }
   return
  }

  try {
   this._actionSubscribers
    .filter(sub => sub.before)
    .forEach(sub => sub.before(action, this.state))
  } catch (e) {
   if (process.env.NODE_ENV !== 'production') {
    console.warn(`[vuex] error in before action subscribers: `)
    console.error(e)
   }
  }

  const result = entry.length > 1
   ? Promise.all(entry.map(handler => handler(payload)))
   : entry[0](payload)

  return result.then(res => {
   try {
    this._actionSubscribers
     .filter(sub => sub.after)
     .forEach(sub => sub.after(action, this.state))
   } catch (e) {
    if (process.env.NODE_ENV !== 'production') {
     console.warn(`[vuex] error in after action subscribers: `)
     console.error(e)
    }
   }
   return res
  })
 }