Drop into empty collection with gong-wpf

Question

I am using the wpf component https://github.com/punker76/gong-wpf-dragdrop in order to do drag and drop, but I cannot seem to be able to drop in a empty collection. If I initialize the collection with one element, it works.

I also created my own drop handler to see what happens, but it is never called for the empty collection. Is there any way of enabling drop to an empty collection?

Sample xaml:

<UserControl.Resources>
    <wf:MyDefaultDropHandler x:Key="myDND" />                                 

    <HierarchicalDataTemplate ItemsSource="{Binding Instructions}" DataType="{x:Type wf:WhileBlock}">
        <TextBlock Text="While" />
    </HierarchicalDataTemplate>

    <DataTemplate DataType="{x:Type wf:InstructionsList}">
        <ItemsControl ItemsSource="{Binding}"
                      dd:DragDrop.IsDragSource="True"
                      dd:DragDrop.IsDropTarget="True"
                      dd:DragDrop.UseDefaultDragAdorner="True"
                      dd:DragDrop.DropHandler="{StaticResource myDND}" />
    </DataTemplate>

    <DataTemplate DataType="{x:Type wf:IfElseBlock}">
        <StackPanel>
            <TextBlock Text="If" />
            <ContentControl Content="{Binding IfInstructions}" />
            <TextBlock Text="Else" />
            <ContentControl Content="{Binding ElseInstructions}" />
        </StackPanel>
    </DataTemplate>
</UserControl.Resources>
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="100" />
        <ColumnDefinition />
    </Grid.ColumnDefinitions>

    <Border Grid.Column="0">
        <ContentControl Content="{Binding AvailableComponents}" />
    </Border>
    <Border Grid.Column="1"
            dd:DragDrop.IsDropTarget="True"
            dd:DragDrop.DropHandler="{StaticResource myDND}">
        <ContentControl Content="{Binding Instructions}" />
    </Border>
</Grid>

My view models. If I uncomment the line //Instructions.Add(new IfElseBlock()); drop works as expected

public abstract class AInstruction
{
}

public abstract class ACondition
{
}

public class InstructionsList : ObservableCollection<AInstruction>
{

}

public class WhileBlock : AInstruction
{
    private readonly InstructionsList _instructions = new InstructionsList();

    public ACondition ExecuteIf { get; set; }
    public InstructionsList Instructions { get { return _instructions; } }
}

public class IfElseBlock : AInstruction
{
    private readonly InstructionsList _ifInstructions = new InstructionsList();
    private readonly InstructionsList _elseInstructions = new InstructionsList();

    public ACondition Condition { get; set; }

    public InstructionsList IfInstructions { get { return _ifInstructions; } }
    public InstructionsList ElseInstructions { get { return _elseInstructions; } }
}

public class Script
{
    private readonly InstructionsList _instructions = new InstructionsList();
    private readonly InstructionsList _availableComponents = new InstructionsList();

    public Script()
    {
        AvailableComponents.Add(new IfElseBlock());
        AvailableComponents.Add(new IfElseBlock());
        AvailableComponents.Add(new IfElseBlock());
        AvailableComponents.Add(new WhileBlock());
        AvailableComponents.Add(new WhileBlock());
        AvailableComponents.Add(new WhileBlock());

        //Instructions.Add(new IfElseBlock());
    }

    public InstructionsList Instructions { get { return _instructions; } }

    public InstructionsList AvailableComponents { get { return _availableComponents; } }
}

my handler, just to do some debugging

public class MyDefaultDropHandler : DefaultDropHandler
{
    public override void DragOver(IDropInfo dropInfo)
    {
        Debug.WriteLine("DragOver " + dropInfo.TargetItem);
        base.DragOver(dropInfo);
    }

    public override void Drop(IDropInfo dropInfo)
    {
        Debug.WriteLine("Drop     " + dropInfo.TargetItem);
        base.Drop(dropInfo);
    }
}

Just in case anyone stumbles over this question in the future: Another potential cause for the inability of not being able to drop something into an ItemsControl (e.g. a ListBox) occurs when nesting multiple ItemsControls within each other. Doing this can lead to this bug: github.com/punker76/gong-wpf-dragdrop/issues/168 — Hauke P.

Robot Mess Robot Mess · Accepted Answer · 2015-04-17T08:09:06

It seems there is no hit target because of the transparent background and the size of the control.

I just added Padding="1" MinHeight="10" Background="White" to my ItemsControl and now drop works for empty collections.

In my sample xaml it would look like this:

<ItemsControl ItemsSource="{Binding}" Padding="1" BorderThickness="5,0,0,0"
                      MinHeight="10"
                      Background="White"
                      dd:DragDrop.IsDragSource="True"
                      dd:DragDrop.IsDropTarget="True"
                      dd:DragDrop.UseDefaultDragAdorner="True"
                      dd:DragDrop.DropHandler="{StaticResource myDND}" />

Drop into empty collection with gong-wpf

1 Answers