Listbox Custom DragDropBehavior

[Roman]

Hi!

I'm trying to use binding in DragDropBehavior of RadListBox like this

<UserControl xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation" x:Class="MyProject.Views.MyView"
                     ....
                     xmlns:behaviors="clr-namespace:MyProject.Behaviors"
                     mc:Ignorable="d"
                     d:DesignHeight="300"
                     d:DesignWidth="300">
<Grid>
          ...
           <telerik:RadListBox ItemsSource="{Binding Items}"
                         <telerik:RadListBox.DragDropBehavior>
                                       <behaviors:MyDragDropBehavior AllowReorder="True" DropCommand="{Binding RelativeSource={RelativeSource AncestorType={x:Type UserControl}}, Path=DataContext.DropCommand}"/>
                         </telerik:RadListBox.DragDropBehavior>
           </telerik:RadListBox>
</Grid>
</UserControl>

View gets viewmodel via injection

public partial class MyView : UserControl
{
                   public MyView (ViewModels.MyViewModel viewModel)
                   {
                                InitializeComponent();
                                DataContext = viewModel;
                    }
}

Behavior code:

public class MyDragDropBehavior : Telerik.Windows.DragDrop.Behaviors.ListBoxDragDropBehavior
{
                   public override bool CanDrop(Telerik.Windows.DragDrop.Behaviors.DragDropState state)
                   {
                                    return state.IsSameControl;
                   }
                 
                 public override void Drop(Telerik.Windows.DragDrop.Behaviors.DragDropState state)
                  {
                                  base.Drop(state);
                                  DropCommand.Execute(null);
                  }
                 
                 public ICommand DropCommand
                 {
                               get { return (ICommand)GetValue(DropCommandProperty); }
                               set { SetValue(DropCommandProperty, value); }
                 }
 
                  public static readonly DependencyProperty DropCommandProperty =DependencyProperty.Register("DropCommand", typeof(ICommand), typeof(MyDragDropBehavior), new PropertyMetadata(null));
}

Items binding is working well. Behavior is working but binding to DropCommand is not. I obtain binding error:

Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='System.Windows.Controls.UserControl', AncestorLevel='1''. BindingExpression:Path=DataContext.DropCommand; DataItem=null; target element is 'MyDragDropBehavior' (HashCode=25707777); target property is 'DropCommand' (type 'ICommand')

ViewModel is

public class MyViewModel
{
                     public MyViewModel()
                     {
                                      DropCommand = new DelegateCommand(OnDrop);
                                      Items = new ObservableCollection<MyItem>();
                      }
                     public ObservableCollection<MyItem> Items { get; set; }
                     public DelegateCommand DropCommand { get; set; }

                     private void OnDrop(){}
}

What is wrong?

[Nasko]

Hi Roman,

In order to achieve the desired binding for that concrete scenario you need to add the DataContext inside the Resources of the UserControl and get access to the Command from that StaticResource.

Please, check the attached sample that demonstrates that approach.

Hope this helps.

Regards,
Nasko
Telerik

[Mehdi]

Is there any way to do this without newing up the viewmodel in the code behind? We are using Caliburn and the viewmodels have numerous dependencies so its not practical to new them up.

[Martin]

Hi Trevor,

I prepared an example to demonstrate how to achieve the desired behavior. Please take a look at the implementation and consider how this approach fits your scenario.

I hope that this helps.

Regards,
Martin Vatev
Progress Telerik

[Sathya]

Hi Martin,

Still you have used StaticResource in your sample, how we can implement if we used AutoWireViewModel through prism?

Regards,

Sathya

[Martin]

Hello Sathya,

Can you take a look at this article where you can find three ways to simulate having an inheritance context for DependencyObjects external to an element tree? I think that the third way will help you to create a static resource from your ViewModel and use it in the binding of the command.

I hope that this helps.

Regards,
Martin Vatev
Progress Telerik