動作未確認

with-local-storage.js

const store = {
  state: {},
  
  load() {
    this.state = JSON.parse(localStorage.getItem('state'))
  },
  
  watch(vm, key) {
    return vm.$watch(key, (value) => {
      localStorage.setItem('state', JSON.stringify(value))
    }, { deep: true })
  }
}

store.load()

Vue.mixin({
  data() {
    return { sharedState: store.state }
  }
})

// App.vue とかで
export default {
  mounted() {
    this._unwatch = store.watch(this, 'sharedState')
  },
  
  beforeDestroy() {
    this._unwatch()
  }
}