subscribe(listener)

添加一个变化监听器，每当 dispatch action 的时候就会执行。

该方法主要有如下操作：
1、将listener推入listeners队列，等待dispatch action时调用。
2、修改isSubscribe的值。
3、返回unsubscribe函数。（函数内操作：1、修改isSubscribe值；1、listeners队列删除listener）

代码如下：

function subscribe(listener: () => void) {
    if (typeof listener !== 'function') {
      throw new Error('Expected the listener to be a function.')
    }

    if (isDispatching) {
      throw new Error(
        'You may not call store.subscribe() while the reducer is executing. ' +
          'If you would like to be notified after the store has been updated, subscribe from a ' +
          'component and invoke store.getState() in the callback to access the latest state. ' +
          'See https://redux.js.org/api/store#subscribelistener for more details.'
      )
    }

    let isSubscribed = true

    ensureCanMutateNextListeners()
    nextListeners.push(listener)

    return function unsubscribe() {
      if (!isSubscribed) {
        return
      }

      if (isDispatching) {
        throw new Error(
          'You may not unsubscribe from a store listener while the reducer is executing. ' +
            'See https://redux.js.org/api/store#subscribelistener for more details.'
        )
      }

      isSubscribed = false

      ensureCanMutateNextListeners()
      const index = nextListeners.indexOf(listener)
      nextListeners.splice(index, 1)
      currentListeners = null
    }
  }

replateReducer(nextReducer)

这个方法没怎么用到过，代码写法也有地方没看懂。。。

主要操作：
1、将当前reducer替换为nextReducer。
2、dispatch一个replace action。（dispatch的操作就是上面那个函数里的，修改属性状态啦，执行所有listener啦，返回action啦）
3、返回store。

代码如下：

  function replaceReducer<NewState, NewActions extends A>(
    nextReducer: Reducer<NewState, NewActions>
  ): Store<ExtendState<NewState, StateExt>, NewActions, StateExt, Ext> & Ext {
    if (typeof nextReducer !== 'function') {
      throw new Error('Expected the nextReducer to be a function.')
    }

    ;((currentReducer as unknown) as Reducer<
      NewState,
      NewActions
    >) = nextReducer

    dispatch({ type: ActionTypes.REPLACE } as A)
    // change the type of the store by casting it to the new store
    return (store as unknown) as Store<
      ExtendState<NewState, StateExt>,
      NewActions,
      StateExt,
      Ext
    > &
      Ext
  }

createStore文件的主要内容就是这些，还有其他一些类型检测以及工具函数的代码未放上来，这个源码里都有。