class Store{
  constructor (options = {}) {
  // 1.部分2个‘断言函数'判断条件
  assert(Vue, `must call Vue.use(Vuex) before creating a store 
  instance.`) // 在Store实例化之前一定要确保Vue的存在
  assert(typeof Promise !== 'undefined', `vuex requires a Promise polyfill in this browser.`)
  //确保promise存在
  
  // 2.结构赋值拿到options里面的state，plugins和strict
  const {
  state = {}, //rootState
  plugins = [], // 插件
  strict = false //是否严格模式
   } = options
   
  // 3.Store internal state创建store内部属性
  this._options = options //存储参数
  this._committing = false //标识提交状态，保证修改state只能在mutation里面，不能在外部随意修改
  this._actions = Object.create(null) //存储用户定义的actions
  this._mutations = Object.create(null) //存储用户定义的mutations
  this._wrappedGetters = Object.create(null) //存储用户定义的getters
  this._runtimeModules = Object.create(null) //存储运行时的modules
  this._subscribers = [] //存储所有堵mutation变化的订阅者
  this._watcherVM = new Vue() //借用Vue实例的方法，$watch来观测变化
  
  // 4.将dispatch和commit的this指向当前store实例
  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)}}