WPF Drag & drop from ListBox with SelectionMode Multiple

Question

I've almost got this working apart from one little annoying thing...

Because the ListBox selection happens on mouse down, if you start the drag with the mouse down when selecting the last item to drag it works fine, but if you select all the items to drag first and then click on the selection to start dragging it, the one you click on gets unselected and left behind after the drag.

Any thoughts on the best way to get around this?

Cheers, IanR

<DockPanel LastChildFill="True">
    <ListBox ItemsSource="{Binding SourceItems}" SelectionMode="Multiple" PreviewMouseLeftButtonDown="HandleLeftButtonDown" PreviewMouseLeftButtonUp="HandleLeftButtonUp" PreviewMouseMove="HandleMouseMove" MultiSelectListboxDragDrop:ListBoxExtension.SelectedItemsSource="{Binding SelectedItems}"/>
    <ListBox ItemsSource="{Binding DestinationItems}" AllowDrop="True" Drop="DropOnToDestination"/>
<DockPanel>

...

public partial class Window1
{
 private bool clickedOnSourceItem;

 public Window1()
 {
  InitializeComponent();

  DataContext = new WindowViewModel();
 }

 private void DropOnToDestination(object sender, DragEventArgs e)
 {
  var viewModel = (WindowViewModel)e.Data.GetData(typeof(WindowViewModel));
  viewModel.CopySelectedItems();
 }

 private void HandleLeftButtonDown(object sender, MouseButtonEventArgs e)
 {
  var sourceElement = (FrameworkElement)sender;
  var hitItem = sourceElement.InputHitTest(e.GetPosition(sourceElement)) as FrameworkElement;

  if(hitItem != null)
  {
   clickedOnSourceItem = true;
  }
 }

 private void HandleLeftButtonUp(object sender, MouseButtonEventArgs e)
 {
  clickedOnSourceItem = false;
 }

 private void HandleMouseMove(object sender, MouseEventArgs e)
 {
  if(clickedOnSourceItem)
  {
   var sourceItems = (FrameworkElement)sender;
   var viewModel = (WindowViewModel)DataContext;

   DragDrop.DoDragDrop(sourceItems, viewModel, DragDropEffects.Move);
   clickedOnSourceItem = false;
  }
 }
}

Answer 1

So...having become the proud owner of a tumbleweed badge, I've got back on to this to try & find a way around it. ;-)

I'm not sure I like the solution so I'm still very much open to any better approaches.

Basically, what I ended up doing is remember what ListBoxItem was last clicked on & then make sure that gets added to the selected items before a drag. This also meant looking at how far the mouse moves before starting a drag - because clicking on a selected item to unselect it could sometimes result in it getting selected again if mouse bounce started a little drag operation.

Finally, I added some hot tracking to the listbox items so, if you mouse down on a selected item it'll get unselected but you still get some feedback to indicate that it will get included in the drag operation.

private void HandleLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    var sourceElement = (FrameworkElement)sender;
    var hitItem = sourceElement.InputHitTest(e.GetPosition(sourceElement)) as FrameworkElement;
    hitListBoxItem = hitItem.FindVisualParent<ListBoxItem>();
    mouseDownPosition = e.GetPosition(null);
}

private void HandleLeftButtonUp(object sender, MouseButtonEventArgs e)
{
    hitListBoxItem = null;
}

private void HandleMouseMove(object sender, MouseEventArgs e)
{
    if (ShouldStartDrag(e))
    {
     hitListBoxItem.IsSelected = true;

     var sourceItems = (FrameworkElement)sender;
     var viewModel = (WindowViewModel)DataContext;

     DragDrop.DoDragDrop(sourceItems, viewModel, DragDropEffects.Move);
     hitListBoxItem = null;
    }
}

private bool ShouldStartDrag(MouseEventArgs e)
{
    if (hitListBoxItem == null)
    {
     return false;
    }

    var currentMousePosition = e.GetPosition(null);
    return Math.Abs(currentMousePosition.X - mouseDownPosition.X) > SystemParameters.MinimumHorizontalDragDistance
     || Math.Abs(currentMousePosition.Y - mouseDownPosition.Y) > SystemParameters.MinimumVerticalDragDistance;
}

XAML changes to include hot tracking...

<Style TargetType="ListBoxItem">
    <Setter Property="Margin" Value="1"/>
    <Setter Property="Template">
     <Setter.Value>
      <ControlTemplate TargetType="{x:Type ListBoxItem}">
       <Grid>
        <Border Background="{TemplateBinding Background}" />
        <Border Background="#BEFFFFFF" Margin="1">
         <Grid>
          <Grid.RowDefinitions>
           <RowDefinition />
           <RowDefinition />
          </Grid.RowDefinitions>
          <Border Margin="1" Grid.Row="0" Background="#57FFFFFF" />
         </Grid>
        </Border>
        <ContentPresenter Margin="8,5" />
       </Grid>
       <ControlTemplate.Triggers>
        <Trigger Property="IsSelected" Value="True">
         <Setter Property="Background" Value="PowderBlue" />
        </Trigger>
        <MultiTrigger>
         <MultiTrigger.Conditions>
          <Condition Property="IsMouseOver" Value="True" />
          <Condition Property="IsSelected" Value="False"/>
         </MultiTrigger.Conditions>
         <Setter Property="Background" Value="#5FB0E0E6" />
        </MultiTrigger>
       </ControlTemplate.Triggers>
      </ControlTemplate>
     </Setter.Value>
    </Setter>
</Style>