思路：使用setInterval一直调用获取当前时间方法

setInterval(getCurrentTime, 1000)getCurrentTime () {    let now = new Date ();    let hh = now.getHours();    let mm = now.getMinutes() > 10 ? now.getMinutes() : '0'+now.getMinutes();    let ss = now.getSeconds() > 10 ? now.getSeconds() : '0'+ now.getSeconds();       this.currentTime = hh + ':' + mm + ':' + ss       }