elementui checkbox 省市，有全选，省，市三个级别，当市没有全选时，省为半选状态，当省为半选时或不选时，全部为半选状态

以下是实现elementui checkbox 省市三级联动的代码示例：

<template>
  <div>
    <el-checkbox v-model="isCheckAll" @change="handleCheckAllChange">全选</el-checkbox>
    <el-checkbox-group v-model="checkedCities" @change="handleCheckedCitiesChange">
      <el-checkbox v-for="city in cities" :label="city.value" :key="city.value">{{ city.label }}</el-checkbox>
    </el-checkbox-group>
    <el-checkbox-group v-model="checkedProvinces" @change="handleCheckedProvincesChange">
      <el-checkbox v-for="province in provinces" :label="province.value" :key="province.value">{{ province.label }}</el-checkbox>
    </el-checkbox-group>
  </div>
</template>

<script>
export default {
  data() {
    return {
      isCheckAll: false,
      checkedCities: [],
      checkedProvinces: [],
      cities: [
        { value: 'Shanghai', label: '上海' },
        { value: 'Beijing', label: '北京' },
        { value: 'Guangzhou', label: '广州' },
        { value: 'Shenzhen', label: '深圳' }
      ],
      provinces: [
        { value: 'Jiangsu', label: '江苏' },
        { value: 'Zhejiang', label: '浙江' },
        { value: 'Fujian', label: '福建' },
        { value: 'Guangdong', label: '广东' }
      ]
    }
  },
  methods: {
    handleCheckAllChange(val) {
      this.checkedCities = val ? this.cities.map(city => city.value) : []
      this.handleCheckedCitiesChange()
    },
    handleCheckedCitiesChange() {
      const checkedCount = this.checkedCities.length
      this.isCheckAll = checkedCount === this.cities.length
      this.checkedProvinces = []
      if (checkedCount > 0 && checkedCount < this.cities.length) {
        this.checkedProvinces = this.provinces.filter(province =>
          this.checkedCities.includes(province.value)
        ).map(province => province.value)
      }
    },
    handleCheckedProvincesChange() {
      const checkedCount = this.checkedProvinces.length
      if (checkedCount === 0) {
        this.isCheckAll = false
        this.checkedCities = []
      } else if (checkedCount > 0 && checkedCount < this.provinces.length) {
        this.isCheckAll = false
        this.checkedCities = this.cities.filter(city =>
          this.checkedProvinces.includes(city.value)
        ).map(city => city.value)
      } else {
        this.isCheckAll = true
        this.checkedCities = this.cities.map(city => city.value)
      }
    }
  }
}
</script>

实现思路：

1. 使用两个 `el-checkbox-group` 分别绑定省和市的选中状态。
2. 使用一个 `el-checkbox` 实现全选功能，并绑定 `isCheckAll` 变量。
3. 监听全选框的变化，根据选中状态设置市的选中状态，并调用 `handleCheckedCitiesChange` 方法。
4. 监听市的选中状态变化，根据选中状态设置省的选中状态，并调用 `handleCheckedProvincesChange` 方法。
5. 监听省的选中状态变化，根据选中状态设置全选框的选中状态，并调用 `handleCheckedCitiesChange` 方法。

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