龚解决方案拖放以对列表进行排序，从列表中删除项目

Question

I am trying to get my Gong Solutions drag and drop to sort a single list. When I drag an item and drop it, it removes the item from the list and I can see it disappear from the interface. It appears that the DragOver method is not moving the object first, so when the Drop method fires off, it simply removes the item.

When I remove the attribute dd:DragDrop.DropHandler="{Binding}" , the drag and drop on the interface works fine. However, I have to have an event fire so i know when the list has been re-ordered.

XAML:

<Window x:Class="Reorder_item_WPF.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525" 
        xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
        xmlns:dd="clr-namespace:GongSolutions.Wpf.DragDrop;assembly=GongSolutions.Wpf.DragDrop">
    <Grid>
        <ListBox Grid.Column="1" SelectionMode="Extended" ItemsSource="{Binding MSPCollection}"
                 dd:DragDrop.IsDragSource="True" Width="300" Margin="0,0,5,0" 
                 dd:DragDrop.IsDropTarget="True"
                 dd:DragDrop.DropHandler="{Binding}">

            <ListBox.ItemTemplate>
                <DataTemplate>
                    <Grid Background="#2ba3d5" Height="50" Width="280">
                        <TextBlock Drop="TextBlock_Drop" Text="{Binding Name}" 
                                   Foreground="White" 
                                   HorizontalAlignment="Center" 
                                   VerticalAlignment="Center" 
                                   FontSize="40"/>
                    </Grid>                    
                </DataTemplate>             
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
</Window>

C#:

public class MSP {
    public int Id { get; set; }
    public string Name { get; set; }        
}

class MainViewModel : IDropTarget
{
    public ObservableCollection<MSP> MSPCollection { get; set; }

    public MainViewModel() {
        MSPCollection = new ObservableCollection<MSP>();

        MSPCollection.Add(new MSP() { 
            Id = 1,
            Name = "Anis Derbel"
        });

        MSPCollection.Add(new MSP()
        {
            Id = 2,
            Name = "Firas Mdimagh"
        });

        MSPCollection.Add(new MSP()
        {
            Id = 3,
            Name = "Khaled Jemni"
        });

        MSPCollection.Add(new MSP()
        {
            Id = 4,
            Name = "Sahbouch"
        });        
    }

    public void DragOver(IDropInfo dropInfo) {
        if (dropInfo.Data is MSP) {
            dropInfo.DropTargetAdorner = DropTargetAdorners.Highlight;
            dropInfo.Effects = DragDropEffects.Move;
        }
    }

    public void Drop(IDropInfo dropInfo) {
        MSP msp = (MSP)dropInfo.Data;
        ((IList)dropInfo.DragInfo.SourceCollection).Remove(msp);
    }
}

Answer 1

DragOver does not remove any items. You're removing the items in the Drop method. You should not be doing any removing or adding in the DragOver method, that's what the Drop method is for. DragOver should only be used to set the target adorner and effects. I used ILSpy to look at their code (see below), and that helped shed some light on what's happening here. They have a class called DefaultDropHandler : IDropTarget , which, as the name implies, is the default drop handler if one is not assigned using the attached property dd:DragDrop.DropHandler . So when you delete the line dd:DragDrop.DropHandler="{Binding}" , that is what is used for the drop operation. Looking at their code, everything is handled for your, that is the item is added and removed, with a whole slew of error checking and index control along the way.

When you do indeed set the drop handler to be your ViewModel, none of the code in the default handler executes, as you replaced that handler with your ViewModel. Thus, you must fully handle the drop. In other words, you have to do all the error and type checking, removing items and adding items (if you have more than one list), and keeping the correct order. You may also want to check that the source collection is not the same as the target collection before you do any of the removing and adding, but this will still not add them into the correct positions.

Since you have only one list, do not remove the item in Drop method. But, nothing will happen for the reasons I stated above. If you do have more than one list, here's how you would make the items move from one list to another:

public void Drop(IDropInfo dropInfo) 
{
    MSP msp = (MSP)dropInfo.Data;
    if(dropInfo.DragInfo.SourceCollection != dropInfo.TargetCollection)
    {
        ((IList)dropInfo.DragInfo.SourceCollection).Remove(msp);
        ((IList)dropInfo.TargetCollection).Add(msp);
    }
}

You will have to do the work of keeping the items in the correct order, if that is what you require. As an alternative to doing all this work, you can leverage and extend their default handler. They made Drop method virtual , so you can make your view model inherit from DefaultDropHandler
(ie class MainViewModel : DefaultDropHandler ) rather than implementing the IDropTarget interface. Then, simply override the Drop method, and call to the base method. Something like this:

public override void Drop(IDropInfo dropInfo)
{
    base.Drop(dropInfo);
    //do other stuff
}

You can also override the DragOver method in the same way if you need to, but if you don't, it will just use the default behaviour.

Extra Info

If you're curious, here's the default handler that Gong uses when you don't assign one:

public virtual void Drop(IDropInfo dropInfo)
{
    if (dropInfo != null && dropInfo.DragInfo != null)
    {
        int insertIndex = (dropInfo.InsertIndex != dropInfo.UnfilteredInsertIndex) ? dropInfo.UnfilteredInsertIndex : dropInfo.InsertIndex;
        ItemsControl itemsControl = dropInfo.VisualTarget as ItemsControl;
        if (itemsControl != null)
        {
            IEditableCollectionView editableItems = itemsControl.Items;
            if (editableItems != null)
            {
                NewItemPlaceholderPosition newItemPlaceholderPosition = editableItems.NewItemPlaceholderPosition;
                if (newItemPlaceholderPosition == NewItemPlaceholderPosition.AtBeginning && insertIndex == 0)
                {
                    insertIndex++;
                }
                else if (newItemPlaceholderPosition == NewItemPlaceholderPosition.AtEnd && insertIndex == itemsControl.Items.Count)
                {
                    insertIndex--;
                }
            }
        }
        IList destinationList = dropInfo.TargetCollection.TryGetList();
        List<object> data = ExtractData(dropInfo.Data).OfType<object>().ToList();
        List<object>.Enumerator enumerator;
        if (!ShouldCopyData(dropInfo))
        {
            IList sourceList = dropInfo.DragInfo.SourceCollection.TryGetList();
            if (sourceList != null)
            {
                enumerator = data.GetEnumerator();
                try
                {
                    while (enumerator.MoveNext())
                    {
                        object o2 = enumerator.Current;
                        int index = sourceList.IndexOf(o2);
                        if (index != -1)
                        {
                            sourceList.RemoveAt(index);
                            if (destinationList != null && object.Equals(sourceList, destinationList) && index < insertIndex)
                            {
                                insertIndex--;
                            }
                        }
                    }
                }
                finally
                {
                    ((IDisposable)enumerator).Dispose();
                }
            }
        }
        if (destinationList != null)
        {
            TabControl tabControl = dropInfo.VisualTarget as TabControl;
            bool cloneData = dropInfo.Effects.HasFlag(DragDropEffects.Copy) || dropInfo.Effects.HasFlag(DragDropEffects.Link);
            enumerator = data.GetEnumerator();
            try
            {
                while (enumerator.MoveNext())
                {
                    object o = enumerator.Current;
                    object obj2Insert = o;
                    if (cloneData)
                    {
                        ICloneable cloneable = o as ICloneable;
                        if (cloneable != null)
                        {
                            obj2Insert = cloneable.Clone();
                        }
                    }
                    destinationList.Insert(insertIndex++, obj2Insert);
                    if (tabControl != null)
                    {
                        TabItem obj = tabControl.ItemContainerGenerator.ContainerFromItem(obj2Insert) as TabItem;
                        if (obj != null)
                        {
                            obj.ApplyTemplate();
                        }
                        tabControl.SetSelectedItem(obj2Insert);
                    }
                }
            }
            finally
            {
                ((IDisposable)enumerator).Dispose();
            }
        }
    }
}