如果在某些控件上按下鼠标，如何防止拖动列表框项目

Question

I would like some advice on drag and drop operations of listbox items. Each of my items has a ComboBox, TextBox, CheckBox and Button, as shown below.

I use my drag/drop to reorder these and it is almost working correctly.

The main issue is that a drag operation on an item is occurring when a PreviewMouseLeftButtonDown and PreviewMouseMove event happens on one of the controls noted above.

My question is given the code below what's good way I can prevent this drag occuring when one of the controls is clicked on?

XAML:

<DataTemplate DataType="{x:Type helpers:Filter}">
    <Border>
        <Border>
            <Grid>
                <ComboBox />
                <TextBox />
                <CheckBox />
                <Button />
            </Grid>
        </Border>
    </Border>
</DataTemplate>

<ListBox x:Name="FilterList"
         ItemsSource="{Binding Filters}"
         helpers:DragDropHelper.IsDragSource="true" 
         helpers:DragDropHelper.IsDropTarget="true" />

C#:

public static readonly DependencyProperty IsDragSourceProperty =
    DependencyProperty.RegisterAttached("IsDragSource", typeof(bool), typeof(DragDropHelper), new UIPropertyMetadata(false, IsDragSourceChanged));


private void DragSource_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    this.sourceItemsControl = (ItemsControl)sender;
    Visual visual = e.OriginalSource as Visual;

    this.topWindow = Window.GetWindow(this.sourceItemsControl);
    this.initialMousePosition = e.GetPosition(this.topWindow);

    this.sourceItemContainer = sourceItemsControl.ContainerFromElement(visual) as FrameworkElement;

    if (this.sourceItemContainer != null)
    {
        this.draggedData = this.sourceItemContainer.DataContext;
    }
}

// Drag = mouse down + move by a certain amount
private void DragSource_PreviewMouseMove(object sender, MouseEventArgs e)
{
    if (this.draggedData != null)
    {
        // Only drag when user moved the mouse by a reasonable amount.
        if (Utilities.IsMovementBigEnough(this.initialMousePosition, e.GetPosition(this.topWindow)))
        {
            this.initialMouseOffset = this.initialMousePosition - this.sourceItemContainer.TranslatePoint(new Point(0, 0), this.topWindow);

            DataObject data = new DataObject(this.format.Name, this.draggedData);

            // Adding events to the window to make sure dragged adorner comes up when mouse is not over a drop target.
            bool previousAllowDrop = this.topWindow.AllowDrop;
            this.topWindow.AllowDrop = true;
            this.topWindow.DragEnter += TopWindow_DragEnter;
            this.topWindow.DragOver += TopWindow_DragOver;
            this.topWindow.DragLeave += TopWindow_DragLeave;

            DragDropEffects effects = DragDrop.DoDragDrop((DependencyObject)sender, data, DragDropEffects.Move);

            RemoveDraggedAdorner();

            this.topWindow.AllowDrop = previousAllowDrop;
            this.topWindow.DragEnter -= TopWindow_DragEnter;
            this.topWindow.DragOver -= TopWindow_DragOver;
            this.topWindow.DragLeave -= TopWindow_DragLeave;

            this.draggedData = null;
        }
    }
}

Answer 1

I tend to initialise my Drag and Drop properties in a PreviewMouseDown event handler, but it's unwise to perform any other Drag and Drop operations in that handler, because the user might not be dragging... they might just have clicked.

Instead, it's better to handle the PreviewMouseMove event to initiate a Drag and Drop operation. Here is a simplified example:

private void DragSourcePreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    isMouseDown = true;
}

private void DragSourcePreviewMouseMove(object sender, MouseEventArgs e)
{
    if (isMouseDown && IsConfirmedDrag(e.GetPosition(sender as ListBox)))
    {
        isMouseDown = false;
        ...
        DragDrop.DoDragDrop(dragSource, data, DragDropEffects);
    }
}

It is in the PreviewMouseMove event handler that you could check which UI element has been clicked on and determine whether you start the Drag and Drop operation or not. Try something like this:

private void DragSourcePreviewMouseMove(object sender, MouseEventArgs e)
{
    ListBox dragSourceControl = (ListBox)sender;
    HitTestResult result = VisualTreeHelper.HitTest(dragSourceControl, 
        Mouse.GetPosition(dragSourceControl));
    UIElement draggedUIElement = result.VisualHit.GetParentOfType<ListBoxItem>();
    bool isViable = AddYourViabilityConditionHere(draggedUIElement);
    if (isMouseDown && IsConfirmedDrag(e.GetPosition(sender as ListBox)) && isViable)
    {
        isMouseDown = false;
        ...
        DragDrop.DoDragDrop(dragSource, data, DragDropEffects);
    }
}