private async void Query_ContentsChanged(IStorageQueryResultBase sender, object args)
        {
            // Run on UI thread.
            await App.Current.NavigationService.Frame.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, async() =>
            {
                var tracker = _folder.TryGetChangeTracker();
                if (tracker != null)
                {
                    var changeReader = tracker.GetChangeReader();
                    var changes      = await changeReader.ReadBatchAsync();

                    foreach (var change in changes)
                    {
                        if (change.ChangeType == StorageLibraryChangeType.ChangeTrackingLost)
                        {
                            // Change tracker is in an invalid state and must be reset
                            // This should be a very rare case, but must be handled
                            tracker.Reset();
                            return;
                        }
                        if (change.IsOfType(StorageItemTypes.File))
                        {
                            // Process file change on UI thread.
                            await ProcessFileChange(change);
                        }
                        else if (change.IsOfType(StorageItemTypes.Folder))
                        {
                            // No-op; not interested in folders
                        }
                        else
                        {
                            if (change.ChangeType == StorageLibraryChangeType.Deleted)
                            {
                                // TODO: The application does not have to support FAT drives at the moment. NTFS is enough.
                                //UnknownItemRemoved(change.Path);
                            }
                        }
                    }

                    // If any changes were recorded, save database.
                    if (changes.Count > 0)
                    {
                        // Log query contens changed saving.
                        GalleryLog.QueryContentsChangedSaving();
                        await DatabaseUtils.SaveAllMetadatasAsync(MediaDatas);
                    }

                    await changeReader.AcceptChangesAsync();
                }
            });
        }