class Store{
  constructor (options = {}) {
  // 1.部分2個‘?dāng)嘌院瘮?shù)'判斷條件
  assert(Vue, `must call Vue.use(Vuex) before creating a store 
  instance.`) // 在Store實例化之前一定要確保Vue的存在
  assert(typeof Promise !== 'undefined', `vuex requires a Promise polyfill in this browser.`)
  //確保promise存在
  
  // 2.結(jié)構(gòu)賦值拿到options里面的state，plugins和strict
  const {
  state = {}, //rootState
  plugins = [], // 插件
  strict = false //是否嚴格模式
   } = options
   
  // 3.Store internal state創(chuàng)建store內(nèi)部屬性
  this._options = options //存儲參數(shù)
  this._committing = false //標(biāo)識提交狀態(tài)，保證修改state只能在mutation里面，不能在外部隨意修改
  this._actions = Object.create(null) //存儲用戶定義的actions
  this._mutations = Object.create(null) //存儲用戶定義的mutations
  this._wrappedGetters = Object.create(null) //存儲用戶定義的getters
  this._runtimeModules = Object.create(null) //存儲運行時的modules
  this._subscribers = [] //存儲所有堵mutation變化的訂閱者
  this._watcherVM = new Vue() //借用Vue實例的方法，$watch來觀測變化
  
  // 4.將dispatch和commit的this指向當(dāng)前store實例
  const store = this
  const { dispatch, commit } = this
  this.dispatch = function boundDispatch (type, payload) {
  return dispatch.call(store, type, payload)}
  this.commit = function boundCommit (type, payload, options) {
  return commit.call(store, type, payload, options)}}