private void GroupItems(ArrayList items)
        {
            var castedItems = items.OfType <FolderItemViewModel>().ToArray();
            var fromDate    = castedItems.Min(x => x.FromDate);
            var toDate      = castedItems.Max(x => x.ToDate);

            var newFolder = Folder.AddFolder("", fromDate, toDate);

            foreach (var item in castedItems)
            {
                item._item.Move(Folder, newFolder);
                Items.Remove(item);
            }

            var newFolderVm = new FolderItemViewModel(newFolder);

            Items.Add(newFolderVm);
        }
 public void Rename(FolderItemViewModel item, string newName)
 {
     Folder.RenameItem(item._item, newName);
     item.RaisePropertyChanged(() => Name);
 }