Two-way databinding to ObservableCollection on Silverlight TreeView using DragDropTarget - silverlight

Question

Here's the question at its most basic: how do I listen for an update of what is changing in a TreeView control modified via a DragDropTarget?
So here's my deal: I have a TreeView that holds agenda items. All are of the same data type (WCFAgendaItem), and are loaded into a hierarchy with children expressed as a property ChildItems. The whole thing is wrapped up in an ObservableCollection and bound to the TreeView using MVVM Light. Works great to view. I also want users to be able to use drag and drop to reorder, reorganize and add new items to this agenda coming from a variety of other sources (one example is a ListView of image slides). All new items would also have the same data type of WCFAgendaItem, for consistency's sake and easy serialization.
Here's my issue: dragging and dropping works beautifully on the UI using the Toolkit's drag drop functionality. But I have no idea how to get the ViewModel to understand changes to the contents of the TreeView.
Code from the view (Agenda.xaml):
(up top)
<UserControl.Resources>
<AHHSTeam_SLClassroomManagerMVVM_Helpers_Converters:BooleanVisibilityConverter x:Key="BooleanVisibilityConverter"/>
<sdk:HierarchicalDataTemplate x:Key="hdtAgenda" ItemsSource="{Binding ChildItems, Mode=TwoWay}" >
<Grid HorizontalAlignment="Left">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="{Binding ImageThumbnailWidth}" />
<ColumnDefinition Width="250" />
</Grid.ColumnDefinitions>
<Image Grid.Column="0" Source="{Binding ThumbnailURL}" Width="{Binding ImageThumbnailWidth}" Height="{Binding ImageThumbnailHeight}" Visibility="{Binding HasImage, Converter={StaticResource BooleanVisibilityConverter}}" >
<ToolTipService.ToolTip>
<Image Source="{Binding ResizedImageURL}" />
</ToolTipService.ToolTip>
</Image>
<TextBlock Grid.Column="1" Text="{Binding Title}" TextWrapping="Wrap" />
</Grid>
</sdk:HierarchicalDataTemplate>
<Style TargetType="sdk:TreeViewItem" >
<Setter Property="IsExpanded" Value="True" />
</Style>
</UserControl.Resources>
(later on)
<controlsToolkit:TreeViewDragDropTarget Grid.Row="1" Grid.Column="0" x:Name="ddtAgenda" AllowDrop="True"
HorizontalContentAlignment="Stretch" VerticalContentAlignment="Stretch" >
<sdk:TreeView Width="375" ScrollViewer.HorizontalScrollBarVisibility="Auto"
ScrollViewer.VerticalScrollBarVisibility="Visible" ItemsSource="{Binding DailyAgenda, Mode=TwoWay}" ItemTemplate="{StaticResource hdtAgenda}">
</sdk:TreeView>
</controlsToolkit:TreeViewDragDropTarget>
ViewModel code (AgendaViewModel.cs) --> I tried listening for CollectionChanged, so far that doesn't seem to work
(in constructor)
//add notification of agenda changes
DailyAgenda.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(DailyAgenda_CollectionChanged);
(event)
void DailyAgenda_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
System.Windows.MessageBox.Show("Daily agenda updated, now has " + e.NewItems.Count.ToString() + " top-level elements.");
}
Code from model (WCFAgendaItem.cs)
[ContentProperty("ChildItems")]
public partial class WCFAgendaItem: INotifyPropertyChanged
{
private ObservableCollection<WCFAgendaItem> _childItems = new ObservableCollection<WCFAgendaItem>();
public ObservableCollection<WCFAgendaItem> ChildItems
{
get
{
return _childItems;
}
set
{
_childItems = value;
}
}
...
I am pretty sure that I get that listening for CollectionChanged isn't right in any case, given that this data doesn't just change at the top level. I looked at EventToCommand in Blend (MVVM Light, remember) but the only TreeView-specific event appears to be SelectionChanged, which doesn't seem right either. I looked at putting an EventToCommand trigger on the TreeViewDragDropTarget, but aren't those methods about overriding how the UI interactions happen? I don't think INotifyPropertyChanged on WCFAgendaItem is right for this either: although I'm going to want that later for editing item titles, it doesn't seem like it'll help me when items get moved around.
Maybe what I'm looking for is a stretch, but what I really want to have happen is for Silverlight to understand that the databinding works both ways on the ordering and contents of the WCFAgendaItem collection, and do all the collection reworking itself based on UI interactions. Then I could just listen for an update event after the collection is reworked - after that I can just crawl the modified ObservableCollection bound to the TreeView, and flatten/serialize/update via WCF.
Failing the ideal situation: I'm willing to crawl TreeViewItems if need be, but even if that's what I need to do I'm stuck on when to do it. Plus I need a way to pass all that back to the ViewModel so I'm not writing code behind. Do I need to attach to Drop() and rework the dropping logic? I found several old articles about custom drag drop implementations starting from the Toolkit, but nobody mentions how to save out the modified TreeView, especially in an MVVM situation.
finally {
While typing this out I found this article which may be useful, though that's a fair amount of work in the ViewModel. This is promising and I'll investigate, but I'm still holding out hope for something simpler. Also it looks like the Toolkit events have changed a little since the article was written.
}

Answer 1

I also had issues implementing this type of DragDrop functionality. The root cause seemed to be that neither the ItemDragCompleted event (EventHandler) nor ItemDroppedOnSource (DragEventHandler) pass the index at which the item was dropped.
I ended up subclassing the DragDropTarget in order to expose the protected method:
int GetDropTargetInsertionIndex(TItemsControlType dropTarget, DragEventArgs args)
I then used an attached behavior to assume responsibility for inserting items at the specified index into to the underlying collections.
I'm afraid the underlying code is too expansive to include in a StackOverflow answer (mainly due to extensive decoupling) but it's on my list of subjects to blog about. In the mean time, I hope the information above helps, it was certainly the key to the solution for me.
Ian