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

  const mutation = { type, payload }
  const entry = this._mutations[type]
  if (!entry) {
   if (process.env.NODE_ENV !== 'production') {
    console.error(`[vuex] unknown mutation type: ${type}`)
   }
   return
  }
  // 用来修改state
  this._withCommit(() => {
   entry.forEach(function commitIterator (handler) {
    handler(payload)
   })
  })
  this._subscribers.forEach(sub => sub(mutation, this.state))

  if (
   process.env.NODE_ENV !== 'production' &&
   options && options.silent
  ) {
   console.warn(
    `[vuex] mutation type: ${type}. Silent option has been removed. ` +
    'Use the filter functionality in the vue-devtools'
   )
  }
 }

_withCommit (fn) {
 const committing = this._committing
 this._committing = true
 fn()
 this._committing = committing
}

if (store.strict) {
 enableStrictMode(store)
}

function enableStrictMode (store) {
 // $watch 函数来观察 state的变化
 store._vm.$watch(function () { return this._data.$$state }, () => {
 // tate变化时,调用 assert函数
 if (process.env.NODE_ENV !== 'production') {
 // 判断 store._committing
  assert(store._committing, `do not mutate vuex store state outside mutation handlers.`)
 }
 }, { deep: true, sync: true })
}