private void OnDownloadCompleted(object sender, DownloadFileCompletedEventArgs e)
        {
            var downloadedPicture =
                DownloadablePictures.FirstOrDefault(elem => elem.PictureName == e.DownloadedFileName);

            if (downloadedPicture != null)
            {
                downloadedPicture.IsDownloaded = true;
            }
        }
        private void OnDownloadStarted(object sender, DownloadFileStartedEventArgs e)
        {
            if (DownloadablePictures == null || DownloadablePictures.Count == 0)
            {
                return;
            }

            var downloadablePicture =
                DownloadablePictures.FirstOrDefault(elem => elem.PictureName == e.DownloadedFileName);

            if (downloadablePicture != null)
            {
                _dropBoxCloudStorageManager.DownloadProgressChangedCallback = downloadablePicture.OnDownloadProgressChanged;
            }
        }
        private async void Download(object parameter)
        {
            DownloadablePictureViewModel.Cancel = false;

            foreach (var downloadablePicture in DownloadablePictures)
            {
                downloadablePicture.DownloadProgress = 0;
                downloadablePicture.IsDownloaded     = false;
            }

            IsDownloading = true;

            var downloadedCount = await _dropBoxCloudStorageManager
                                  .DownloadFilesAsync(DownloadablePictures
                                                      .Where(elem => elem.ReadyForDownload)
                                                      .Select(p => p.PictureName), _targetFolder);

            IsDownloading = false;
        }