微信小程序进度条

<button class='down-img' bindtap='downImg' data-scr='{{imageUrl}}'>下载图片{{progress}}</button>

downImg: function (e) {
    var _this = this;

    // 获取图片地址(http://www.playsort.cn/...)
    //var img = e.currentTarget.dataset.src;
    //var img = '../pic/U1513P28T52D3414F872DT20081124143718.jpg?id=' + Math.floor(Math.random() * 100000);;
    var img = 'http://i0.sinaimg.cn/ent/y/w/2008-11-24/U1513P28T52D3414F872DT20081124143718.jpg?id=' + Math.floor(Math.random() * 100000);
    // 下载监听进度
    const downloadTask = wx.downloadFile({
      url: img,
      success: function (res) {
        // 只要服务器有响应数据，就会把响应内容写入文件并进入 success 回调，业务需要自行判断是否下载到了想要的内容
        console.log(res)
        if (res.statusCode === 200) {
          // wx.saveImageToPhotosAlbum({
          //   filePath: res.tempFilePath,
          //   success: function (res) {
          //     wx.showToast({
          //       title: '保存图片成功!~',
          //     });
          //   },
          //   fail: function (res) {
          //     wx.showToast({
          //       title: '保存图片失败!~',
          //     });
          //   }
          // })
          console.log('code:' + res.statusCode )
        }else{
          console.log('code:' + res.statusCode )
        }
      }
    });
    downloadTask.onProgressUpdate((res) => {
      if (res.progress === 100) {
        this.setData({
          progress: '100%'
        });
        var st = getApp().globalData.st
        var et = new Date()
        console.log(et-st)
        console.log(410.45 * 1000 / (et - st))
      }else if(res.progress === 1){
        getApp().globalData.st = new Date()
        console.log(getApp().globalData.st);
      } 
      else {
        this.setData({
          progress: res.progress + '%'
        });
      }
    });
  }