微信公眾號(hào)開發(fā)，完成倒計(jì)時(shí)的一個(gè)技巧（純代碼）

開始的思路沒有考慮頁(yè)面在后臺(tái)運(yùn)行以及鎖屏等情況。代碼如下：

let interval = setInterval(() => {
            let {staticTime} = this.state;
            staticTime = staticTime - 1;
            if (staticTime <= 0) {
                clearInterval(interval);
                this.setState({
                    tip:'支付超時(shí)',
                    staticTime:0
                });
                return;
            }
            let minutes = parseInt(staticTime/60);
            let Seconds = staticTime%60;
            let tip = '訂單已提交，請(qǐng)?jiān)?#39;+minutes+'分'+Seconds+'秒內(nèi)完成支付';
            this.setState({
                tip:tip,
                staticTime:staticTime 
            });
        }, 1000);
后來(lái)測(cè)試發(fā)現(xiàn)鎖屏或者把頁(yè)面留在后臺(tái)，計(jì)算就不對(duì)，于是把代碼進(jìn)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:'支付超時(shí)',
                    staticTime:0,
                });
                return;
            }
            let minutes = parseInt(staticTime/60);
            let Seconds = staticTime%60;
            let tip = '訂單已提交，請(qǐng)?jiān)?#39;+minutes+'分'+Seconds+'秒內(nèi)完成支付';
            this.setState({
                tip:tip,
                staticTime:staticTime,
            });
        }, 1000);
        this.listenPageShowHideHandle();
    
     //計(jì)算頁(yè)面在后臺(tái)的時(shí)間

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('時(shí)間差：', backGroundTime);
    }
    console.log( document.visibilityState );
});

}

改造之后發(fā)先問題依然存在。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.

**解決方案：**
訂單生成的時(shí)候我們記錄下這個(gè)時(shí)間為A， 時(shí)間間隔為B（3分鐘內(nèi)需要付款，B為3*60*1000），C為現(xiàn)在的時(shí)間。我們使用setInterval 每個(gè)1秒讀取一下時(shí)間。那么倒計(jì)時(shí)時(shí)間 == A+B-C，代碼如下

 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:'支付超時(shí)',
                isFalse:true
            });
            return;
        }
        let minutes = parseInt(sub/60);
        let Seconds = sub%60;
        let tip = '訂單已提交，請(qǐng)?jiān)?#39;+minutes+'分'+Seconds+'秒內(nèi)完成支付';
        console.log(tip);
        this.setState({
            tip:tip,
            isFalse:false
        });
    },1000);

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