setCache(){
    if(this.judgeTime()){ //判斷緩存是否過期，過期就重新添加一個
    wx.setStorageSync('oldTime',this.oldTime());
    }
}
judgeTime(){ //判斷緩存是否過期
      let nowTime = Date.now();
      let oldTime = wx.getStorageSync('oldTime');
      if(oldTime && nowTime < oldTime){
       return false;
      }
      return true;
    }
 oldTime(){ //返回以現在為準的3天后的時間
      return Date.now() + 259200000; //毫秒(72小時)
    }
    this.setCache();