c# - ItemsControl Drag and Drop

Question

I have an ItemsControl with a DataTemplate that is bound to an ObservableCollection of integers.

<ItemsControl Name="DimsContainer" ItemTemplate="{StaticResource DimensionsTemplate}">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <StackPanel Orientation="Horizontal"/>
        </ItemsPanelTemplate>
   </ItemsControl.ItemsPanel>
</ItemsControl>

And in the Windows Resources:

<Window.Resources>
    <DataTemplate x:Key="DimensionsTemplate" >
        <TextBlock Text="{Binding}"
                       Padding="5"
                       VerticalAlignment="Center"
                       FontSize="32"/>
    </DataTemplate>
</Window.Resources>

I'm trying to implement the ability to drag and drop items within the ItemsControl (i.e. to be able to reorder the integers). Does anyone have a simple example of how to do this? I hooked up the PreviewMouseMove, DragEnter and Drop events. The problem is that I can't figure out how to determine which item is being dragged and where it is dragged to. It seems that the entire ItemsControl gets passed into the events.

Answer 1

Here is an example how I've done it.

XAML:

<Window.DataContext>
    <local:MyViewModel />
</Window.DataContext>

<Grid>
    <ScrollViewer>
        <ListView ItemsSource="{Binding MyData}" HorizontalAlignment="Stretch" Name="listview" ScrollViewer.PanningMode="VerticalOnly">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <Button Content="{Binding}"
                        Command="{Binding DataContext.MyCommand, RelativeSource={RelativeSource AncestorType=ItemsControl}}" 
                        CommandParameter="{Binding}"
                        Margin="5 2" Width="150" Height="50"
                        FontSize="30" />
                </DataTemplate>
            </ListView.ItemTemplate>
            <ListView.Resources>
                <Style TargetType="Button">
                    <EventSetter Event="PreviewMouseMove" Handler="PreviewMouseMove" />                        
                    <EventSetter Event="Drop" Handler="Drop" />                       
                    <Setter Property="AllowDrop" Value="True" />                        
                </Style>
            </ListView.Resources>
        </ListView>
    </ScrollViewer>
</Grid>

ViewModel:

 class MyViewModel
{
    public MyViewModel()
    {
        MyCommand = new ICommandImplementation();
    }

    public ObservableCollection<string> MyData
    {
        get
        {
            return new ObservableCollection<string>(new string[]{
            "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", 
            "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen", "twenty"
            });
        }
    }

    public ICommand MyCommand { get; private set; }

    private class ICommandImplementation : ICommand
    {
        public bool CanExecute(object parameter) { return true; }
        public event EventHandler CanExecuteChanged;
        public void Execute(object parameter) { System.Windows.MessageBox.Show("Button clicked! " + (parameter ?? "").ToString()); }
    }
}

Events:

 private void Drop(object sender, DragEventArgs e)
    {
        var source = e.Data.GetData("Source") as string;
        if (source != null)
        {
            int newIndex = listview.Items.IndexOf((sender as Button).Content);
            var list = listview.ItemsSource as ObservableCollection<string>;
            list.RemoveAt(list.IndexOf(source));
            list.Insert(newIndex, source);
        }
    }

    private void PreviewMouseMove(object sender, MouseEventArgs e)
    {
        if (e.LeftButton == MouseButtonState.Pressed)
        {
            Task.Factory.StartNew(new Action(() =>
                {
                    Thread.Sleep(500);
                    App.Current.Dispatcher.BeginInvoke(new Action(() =>
                        {
                            if (e.LeftButton == MouseButtonState.Pressed)
                            {                                    
                                var data = new DataObject();
                                data.SetData("Source", (sender as Button).Content);
                                DragDrop.DoDragDrop(sender as DependencyObject, data, DragDropEffects.Move);
                                e.Handled = true;
                            }
                        }), null);
                }), CancellationToken.None);
        }           
    }

Above example is a little complex cause every item of list is a Button and on Button click I also have to do some action. Your case is relatively easy.

Drag & Drop can be confusing for many developers. But below are the some key points how to do it:

1. Use PreviewMouseMove event to actually start a drag and in handler use DragDrop.DoDragDrop event to raise DragDrop related events and Cursors. sender argument is the element that has captured the mouse currently in this case the UIElement that is being dragged.

2. Use DragEnter & DragOver event if want to change the visual of element over which the Mouse is currently dragging. sender argument is the element that has currently dragged over / that just ended drag over situation.

3. Use Drop event to handle the dropped element. sender argument is the element on which the Drop happened.

4. Use DataObject object to pass info between these events. SetData method of the class is used to add data in this. This method has two arguments, and they work like key-value pair. Once set you can get this data in next called event of DragDrop by using GetData method by passing the key as argument. (i.e. e.Data.GetData("Source"))

Here is a relative post.