constructor (options = {}) {
  // Auto install if it is not done yet and `window` has `Vue`.
  // To allow users to avoid auto-installation in some cases,
  // this code should be placed here. See #731
  if (!Vue && typeof window !== 'undefined' && window.Vue) {
    //通过浏览器script标签使用vuex时自动安装
    install(window.Vue)
  }

  if (process.env.NODE_ENV !== 'production') {
    //断言一些使用环境，必须使用vue.use(vuex)，必须存在Promise，必须使用new操作符
    assert(Vue, `must call Vue.use(Vuex) before creating a store instance.`)
    assert(typeof Promise !== 'undefined', `vuex requires a Promise polyfill in this browser.`)
    assert(this instanceof Store, `store must be called with the new operator.`)
  }

  const {
    plugins = [],
    strict = false
  } = options

  // store internal state
  //初始化需要使用到的变量
  this._committing = false
  this._actions = Object.create(null)
  this._actionSubscribers = []
  this._mutations = Object.create(null)
  this._wrappedGetters = Object.create(null)
  this._modules = new ModuleCollection(options) //这里会对模块module进行收集
  this._modulesNamespaceMap = Object.create(null)
  this._subscribers = []
  this._watcherVM = new Vue()

  // bind commit and dispatch to self
  //将dispatch和commit方法的this指针绑定到store上，防止被修改
  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)
  }

  // strict mode
  this.strict = strict

  const state = this._modules.root.state

  // init root module.
  // this also recursively registers all sub-modules
  // and collects all module getters inside this._wrappedGetters
  //处理的核心，包括处理根module、action、mutation、getters和递归注册子module
  installModule(this, state, [], this._modules.root)

  // initialize the store vm, which is responsible for the reactivity
  // (also registers _wrappedGetters as computed properties)
  //也是重点，使用vue实例来保存state和getter
  resetStoreVM(this, state)

  // apply plugins
  //插件注册和使用，内置的有devtool和logger
  plugins.forEach(plugin => plugin(this))

  const useDevtools = options.devtools !== undefined ? options.devtools : Vue.config.devtools
  if (useDevtools) {
    devtoolPlugin(this)
  }
}

assert(Vue, `must call Vue.use(Vuex) before creating a store instance.`)
assert(typeof Promise !== 'undefined', `vuex requires a Promise polyfill in this browser.`)
assert(this instanceof Store, `store must be called with the new operator.`)

this._committing = false
this._actions = Object.create(null)
this._actionSubscribers = []
this._mutations = Object.create(null)
this._wrappedGetters = Object.create(null)
this._modules = new ModuleCollection(options) //这里会对模块module进行收集
this._modulesNamespaceMap = Object.create(null)
this._subscribers = []
this._watcherVM = new Vue()

//module目录下的moduleCollection.js
export default class ModuleCollection {
  constructor (rawRootModule) {
    // register root module (Vuex.Store options)
    this.register([], rawRootModule, false)
  }
  /** 省略 **/
}

//module目录下的module.js
export default class Module {
  constructor (rawModule, runtime) {
    this.runtime = runtime
    // Store some children item
    this._children = Object.create(null)
    // Store the origin module object which passed by programmer
    this._rawModule = rawModule
    const rawState = rawModule.state

    // Store the origin module's state
    this.state = (typeof rawState === 'function' ? rawState() : rawState) || {}
  }
  /** 省略 **/
}

function installModule (store, rootState, path, module, hot) {
  const isRoot = !path.length
  const namespace = store._modules.getNamespace(path)

  // register in namespace map
  //保存namespaced模块
  if (module.namespaced) {
    store._modulesNamespaceMap[namespace] = module
  }

  // set state
  //非根组件设置state，根据path获取父state
  if (!isRoot && !hot) {
    const parentState = getNestedState(rootState, path.slice(0, -1))
    const moduleName = path[path.length - 1]
    store._withCommit(() => {
      Vue.set(parentState, moduleName, module.state)
    })
  }

  //设置module的上下文，从而保证mutation和action的第一个参数能拿到对应的state getter等
  const local = module.context = makeLocalContext(store, namespace, path)

  //逐一注册mutation
  module.forEachMutation((mutation, key) => {
    const namespacedType = namespace + key
    registerMutation(store, namespacedType, mutation, local)
  })

  //逐一注册action
  module.forEachAction((action, key) => {
    const type = action.root ? key : namespace + key
    const handler = action.handler || action
    registerAction(store, type, handler, local)
  })

  //逐一注册getter
  module.forEachGetter((getter, key) => {
    const namespacedType = namespace + key
    registerGetter(store, namespacedType, getter, local)
  })

  //逐一注册子module
  module.forEachChild((child, key) => {
    installModule(store, rootState, path.concat(key), child, hot)
  })
}