开发者工具上正常显示， 安卓手机上也正常显示，就ios设备上显示不了

按照小程序开发文档上写的，找不出bug原因，求大佬指点一下

//小程序js
downloadfile () {
    const downloadTask = wx.downloadFile({
      url: 'test.mp4',
      success: function (res) {
        console.log(res)
        if (res.statusCode === 200) {
          wx.saveVideoToPhotosAlbum({
            filePath: res.tempFilePath,
            success: function (res) {
              wx.showToast({
                title: '保存成功'
              })
            },
            fail: function (res) {
              console.log(res)
              wx.showToast({
                title: '保存失败'
              })
            }
          })
        }
      }
    })
    downloadTask.onProgressUpdate((res) => {
      this.setData({
        progress: res.progress
      })
    })
  },

//小程序wxml

<progress wx:if="{{progress}}" percent="{{progress}}" backgroundColor="white" color="#0081FF" duration="30" border-radius='50' bindtap="progress" show-info></progress>
<button bindtap='downloadfile'>保存视频{{progress}}</button>