实现场景：试卷答题，点击开始考试，出现倒计时，刷新倒计时就没有了

实现思路：

 关键点在于进来以后获取当前时间计算出结束时间

假设接口是20分钟以后倒计时结束，下面就算出了都是秒的时候倒计时结束了

new Date().getTime() + 20*60*1000

结束时间减去当前时间，就算出了时间差毫秒数

let time = new Date().getTime() + 20*60*1000 - new Date().getTime()

uview组件的使用办法

<u-count-down :time="time " format="HH:mm:ss"></u-count-down>

下面是是实现代码相关逻辑

<template>
    <u-count-down :time="form.countdown" format="HH:mm:ss"></u-count-down>
</template>

<script>
import { reactive, toRefs, defineComponent, computed } from 'vue';
export default defineComponent({
    components: {},
    setup() {
        const state = reactive({
            loading: true,
            form: {},
        });
        onLoad((options) => {
            state.loading = true;
            if (options.examId) {
                start({ examId: options.examId }).then((response) => {
                    const { data } = response;
                    let countdown = getStorage('countDow');
                    let countTime = 0;
                    let currentTime = new Date().getTime();
                    if (!countdown) {
                        let endTime = new Date().getTime() + data.examTime*60*1000;
                        setStorage(endTime,'countDow')
                        countTime = endTime - currentTime; 
                    }else{
                        countTime = countdown - currentTime;
                    }
                    data.countdown = countTime;
                    state.loading = false;
                    state.form = data;
                }).catch(() => {
                    state.loading = false;
                })
            }
        })
        return {
            ...toRefs(state)
        };
    },
    methods: {}
});
</script>

<style scoped lang="scss">
</style>