http://coding.imooc.com/learn/questiondetail/GzLgVXkW3vLXkWxb.html

老师，我看了你在这个问题下的回复，但是如果按照这个方法来的话，是不是也就意味着，不能在根state下定义变量了呀。
老师我是这么写的，如果不在根state下定义是没问题的，可能是因为用了InjectionKey的原因吧。如果在RootState下定义app，跟state就会报错，因为没有定义app。是不是不能共存呀

// store/index.ts

import { InjectionKey } from 'vue’
import { createStore, useStore as baseUseStore, Store } from 'vuex’
import app from ‘./modules/app’

export interface RootState {
name: string;
age: number;
}

export const key: InjectionKey<Store> = Symbol(‘store-key’)

const store = createStore({
state: {
name: ‘wang’,
age: 29
},
mutations: {
},
actions: {
},
modules: {
app
}
})

export default store

export function useStore () {
return baseUseStore(key)
}

// store/modules/app.ts

import { Module } from 'vuex’
import { RootState } from ‘…/index’

export interface AppState {
count: number;
}

const state: AppState = {
count: 1
}

const mutations = {
updateCount (state: AppState, num: number) {
state.count += num
}
}

export default {
namespaced: true,
state,
mutations
} as Module<AppState, RootState>