来自异步调用的WPF组合框类别分组

在呈现{Initialization}时，我可以通过如下代码设置将数据按类别分组到Combobox

    _cboItem = new List<string>();
_cboitem=service.GetData();
    CollectionView cvs =     CollectionView)CollectionViewSource.GetDefaultView(_cboItem);
cvs.GroupDescriptions.Add(new  PropertyGroupDescription("Category"));

    BindingOperations.EnableCollectionSynchronization(_cboDocType, _lock);
Task.Factory.StartNew(() => GetDocTypeAsync());

    CollectionView cv=(CollectionView)CollectionViewSource.GetDefaultView(_cboDocType);
      cv.GroupDescriptions.Add(new PropertyGroupDescription("Category"));
      OnPropertyChanged("DocType");