微信公众号开发，完成倒计时的一个技巧（纯代码）

开始的思路没有考虑页面在后台运行以及锁屏等情况。代码如下：

let interval = setInterval(() => {
            let {staticTime} = this.state;
            staticTime = staticTime - 1;
            if (staticTime <= 0) {
                clearInterval(interval);
                this.setState({
                    tip:'支付超时',
                    staticTime:0
                });
                return;
            }
            let minutes = parseInt(staticTime/60);
            let Seconds = staticTime%60;
            let tip = '订单已提交，请在'+minutes+'分'+Seconds+'秒内完成支付';
            this.setState({
                tip:tip,
                staticTime:staticTime 
            });
        }, 1000);
后来测试发现锁屏或者把页面留在后台，计算就不对，于是把代码进行了如下改造。



  let interval = setInterval(() => {
            let {backGroundTime, staticTime} = this.state;
            this.setState({
                backGroundTime:0
            });
            staticTime = staticTime - backGroundTime - 1; 
            if (staticTime <= 0) {
                clearInterval(interval);
                this.setState({
                    tip:'支付超时',
                    staticTime:0,
                });
                return;
            }
            let minutes = parseInt(staticTime/60);
            let Seconds = staticTime%60;
            let tip = '订单已提交，请在'+minutes+'分'+Seconds+'秒内完成支付';
            this.setState({
                tip:tip,
                staticTime:staticTime,
            });
        }, 1000);
        this.listenPageShowHideHandle();
    
     //计算页面在后台的时间

listenPageShowHideHandle = () =>{

let {backGroundTime} = this.state;
let start, end;
let self = this;
document.addEventListener("visibilitychange", function() {
    if(document.visibilityState == 'hidden'){
        start = new Date().getTime();
    }else if(document.visibilityState == 'visible'){
        end = new Date().getTime();
        backGroundTime = Math.floor((end - start)/1000);
        self.setState({backGroundTime});
        console.log('时间差：', backGroundTime);
    }
    console.log( document.visibilityState );
});

}

改造之后发先问题依然存在。原因是：
You cannot continue to run javascript while the iPhone is sleeping using setTimeout(), however.When the phone is put to sleep, Safari will kill any running javascript processes using setTimeout(). Check out this answer here for some reasons why this is done.

**解决方案：**
订单生成的时候我们记录下这个时间为A， 时间间隔为B（3分钟内需要付款，B为3*60*1000），C为现在的时间。我们使用setInterval 每个1秒读取一下时间。那么倒计时时间 == A+B-C，代码如下

 let interval = setInterval(()=>{
        let {orderTime, staticTime} = this.state;
        let nowTime = Date.now();
        let sub = Math.floor((orderTime + staticTime - nowTime)/1000);
        console.log('sub',sub);
        if(sub<=0){
            clearInterval(interval);
            this.setState({
                tip:'支付超时',
                isFalse:true
            });
            return;
        }
        let minutes = parseInt(sub/60);
        let Seconds = sub%60;
        let tip = '订单已提交，请在'+minutes+'分'+Seconds+'秒内完成支付';
        console.log(tip);
        this.setState({
            tip:tip,
            isFalse:false
        });
    },1000);

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