title: Collapse All DataGrid groups programatically without using PagedCollectionView
body: <p>I'm trying to do as much as possible in the Xaml (rather than code-behind) for a relatively simple application.  I have the DataGrid bound to a DomainDataSource in Silverlight 4 and I'm binding the DomainDataSource's GroupDescriptors to ComboBoxes allowing the user to group the rows in the DataGrid according to values they select.  I'd like to give them the ability to click a button to collapse/expand <strong>all</strong> groups.  I know this can be done using a PagedCollectionView, but then I end up doing the grouping, etc. in code-behind.  Is there a way to accomplish this <em>without</em> using a PagedCollectionView?</p>
  
  <p>I'm aware of the DataGrid.CollapseRowGroup(CollectionViewGroup collectionViewGroup, bool collapseAllSubgroups) method, but I haven't found a way to iterate through the top-level groups.  </p>