vue三级联动，不发ajax请求

<template>
    <div class="city-select" id="city">
        <select v-model="selectedProvince" name="province">
            <option v-for="(item, index) in provinces"
                    v-if="item.level === 1"
                    :value="item">
                {{ item.name }}
      </option>
        </select>
        <select v-model="selectedCity" name="city">
            <option
                    v-for="(item, index) in cities"
                    :value="item">
                {{ item.name }}
      </option>
        </select>
        <select v-model="selectedBlock" name="block">
            <option
                    v-for="(item, index) in blocks"
                    :value="item">
                {{ item.name }}
      </option>
        </select>
    </div>
</template>
<script>
    import provinces from './../../assets/js/city'
    import Vue from 'vue'
    export default {
        name: 'city',
        created() {
            // 数据初始化,默认选中北京市,默认选中第一个;北京市数据为总数据的前18个
            let beijing = this.provinces.slice(0, 19);
            this.cities = beijing.filter(item => {
                if (item.level === 2) {
                    return true
                }
            });
            this.selectedCity = this.cities[0];
            this.blocks = beijing.filter(item => {
                if (item.level === 3) {
                    return true
                }
            });
            this.selectedBlock = this.blocks[0]
        },
        watch: {
            selectedProvince(newVal, oldVal) {
                // 港澳台数据只有一级,特殊处理
                if (newVal.sheng === '71' || newVal.sheng === '81' || newVal.sheng === '82') {
                    this.cities = [newVal];
                    this.blocks = [newVal]
                } else {
                    this.cities = this.provinces.filter(item => {
                        if (item.level === 2 && item.sheng && newVal.sheng === item.sheng) {
                            return true
                        }
                    })
                }
                var _this = this;
                // 此时在渲染DOM,渲染结束之后再选中第一个
                Vue.nextTick(() => {
                    _this.selectedCity = _this.cities[0];
                    _this.$emit('input', _this.info)
                })
            },
            selectedBlock() {
                var _this = this;
                Vue.nextTick(() => {
                    _this.$emit('input', _this.info)
                })
            },
            selectedCity(newVal) {
                // 选择了一个市,要选择区了 di是城市的代表,sheng
                if (newVal.sheng === '71' || newVal.sheng === '81' || newVal.sheng === '82') {
                    this.blocks = [newVal];
                    this.cities = [newVal]
                } else {
                    this.blocks = this.provinces.filter(item => {
                        if (item.level === 3 && item.sheng && item.sheng == newVal.sheng && item.di === newVal.di && item.name !== '市辖区') {
                            return true
                        }
                    })
                }
                var _this = this;
                Vue.nextTick(() => {
                    _this.selectedBlock = _this.blocks[0];
                    // 触发与 v-model相关的 input事件
                    _this.$emit('input', _this.info)
                })
            }
        },
        computed: {
            info() {
                return {
                    province: this.selectedProvince,
                    city: this.selectedCity,
                    block: this.selectedBlock
                }
            }
        },
        data() {
            return {
                selectedProvince: provinces[0],
                selectedCity: 0,
                selectedBlock: 0,
                cities: 0,
                provinces,
                blocks: 0
            }
        }
    }
</script>
city.js部分格式