Computed property "xxx" was assigned to but it has no setter.

vue使用computed属性计算某些值时，在页面上重新赋值时，报错Computed property "xxxx" was assigned to but it has no setter，翻译成中文就是“计算属性“xxxx”已经分配了，但它没有setter”，理解一下，就是属性xxxx获取了值，但是没有设置值的方法，报错代码示例贴一下：

computed: {
  cityOptions() {
    return this.$store.state.common.cityOptions
  }
},
created() {
  this.getCityOptions()
},
methods: {
  getCityOptions() {
    getCitys().then(res => {
      this.cityOptions = res.data
    })
  }
}

此时我们的代码需要修改为：

computed: {
  cityOptions: {
    get() {
      return this.$store.state.common.cityOptions
    },
    set(val) {
      return val
    }
  }
},
created() {
  this.getCityOptions()
},
methods: {
  getCityOptions() {
    getCitys().then(res => {
      this.cityOptions = res.data
    })
  }
}