为什么combinireducer可以传入很多reducer呢？
因为在经过一系列判断异常之后（第一步 浅拷贝reducer, 第二步检测reducer默认值）
用一个for循环， 获取全部的nextState,再返回， hasChange就是判定是否state发生变化
let hasChanged = false const nextState = {} for (let i = 0; i < finalReducerKeys.length; i++) { const key = finalReducerKeys[i] const reducer = finalReducers[key] const previousStateForKey = state[key] const nextStateForKey = reducer(previousStateForKey, action) if (typeof nextStateForKey === 'undefined') { const errorMessage = getUndefinedStateErrorMessage(key, action) throw new Error(errorMessage) } nextState[key] = nextStateForKey hasChanged = hasChanged || nextStateForKey !== previousStateForKey } return hasChanged ? nextState : state