WPF ListBox拖拽时自动滚动

Question

我有一个WPF应用程序，其中包含一个ListBox。拖动机制已经实现，但是当列表太长并且我想将项目移动到不可见的位置时，我就无法操作。

例如，屏幕显示10个项目。我有20个项目。如果我想将最后一个项目拖到第一个位置，我必须拖到顶部并释放。然后再向上滚动并再次拖动。

如何使ListBox自动滚动？

- Artur Carvalho

Answer 1

懂了。使用了 ListBox 的 DragOver 事件，使用在这里找到的函数获取了列表框的 scrollviewer，然后就是对位置进行一些调整。

private void ItemsList_DragOver(object sender, System.Windows.DragEventArgs e)
{
    ListBox li = sender as ListBox;
    ScrollViewer sv = FindVisualChild<ScrollViewer>(ItemsList);

    double tolerance = 10;
    double verticalPos = e.GetPosition(li).Y;
    double offset = 3;

    if (verticalPos < tolerance) // Top of visible list?
    {
        sv.ScrollToVerticalOffset(sv.VerticalOffset - offset); //Scroll up.
    }
    else if (verticalPos > li.ActualHeight - tolerance) //Bottom of visible list?
    {
        sv.ScrollToVerticalOffset(sv.VerticalOffset + offset); //Scroll down.    
    }
}

public static childItem FindVisualChild<childItem>(DependencyObject obj) where childItem : DependencyObject
{
    // Search immediate children first (breadth-first)
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
    {
        DependencyObject child = VisualTreeHelper.GetChild(obj, i);

        if (child != null && child is childItem)
            return (childItem)child;

        else
        {
            childItem childOfChild = FindVisualChild<childItem>(child);

            if (childOfChild != null)
                return childOfChild;
        }
    }

    return null;
}

- Artur Carvalho

---

我尝试了你的方法，它有效。但是，在将对象拖动到同一列表中时，放下后，它会返回到原始对象，而我希望看到已放置的项目。你有这个问题吗？你解决了吗？ - David Brunelle

@DavidBrunelle 我不记得了，抱歉。 - Artur Carvalho

+1 很棒的答案，尽管它是深度优先搜索，而不是广度优先搜索。 - Cameron

我的“FindVisualChild”从未返回ScrollViewer，但我在XAML中将ListBox放在ScrollViewer中并使用“FindVisualParent”代替，这样就解决了问题。 - miriyo

@miriyo你好。我在尝试类似的东西。你能发一下代码吗？ - Juan Pablo Gomez

嗨，我刚刚将这个从原始建议更改了：（抱歉它是在VB中）`Dim lListBoxItem As DragDropListBoxItem = FindVisualParent(Of DragDropListBoxItem)((DirectCast(e.OriginalSource, DependencyObject)))如果（lListBoxItem不是空的）Then     DragDrop.DoDragDrop(lListBoxItem，lListBoxItem.DataContext，DragDropEffects.Move) End If` - miriyo

Answer 2

基于此，我已经创建了一个附加行为，可以像这样轻松使用 -

<ListView
   xmlns:WpfExtensions="clr-namespace:WpfExtensions" 
   WpfExtensions:DragDropExtension.ScrollOnDragDrop="True"

以下是所附行为的代码 -

/// <summary>
/// Provides extended support for drag drop operation
/// </summary>
public static class DragDropExtension
{
    public static readonly DependencyProperty ScrollOnDragDropProperty =
        DependencyProperty.RegisterAttached("ScrollOnDragDrop",
            typeof(bool),
            typeof(DragDropExtension),
            new PropertyMetadata(false, HandleScrollOnDragDropChanged));

    public static bool GetScrollOnDragDrop(DependencyObject element)
    {
        if (element == null)
        {
            throw new ArgumentNullException("element");
        }

        return (bool)element.GetValue(ScrollOnDragDropProperty);
    }

    public static void SetScrollOnDragDrop(DependencyObject element, bool value)
    {
        if (element == null)
        {
            throw new ArgumentNullException("element");
        }

        element.SetValue(ScrollOnDragDropProperty, value);
    }

    private static void HandleScrollOnDragDropChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        FrameworkElement container = d as FrameworkElement;

        if (d == null)
        {
            Debug.Fail("Invalid type!");
            return;
        }

        Unsubscribe(container);

        if (true.Equals(e.NewValue))
        {
            Subscribe(container);
        }
    }

    private static void Subscribe(FrameworkElement container)
    {
        container.PreviewDragOver += OnContainerPreviewDragOver;
    }

    private static void OnContainerPreviewDragOver(object sender, DragEventArgs e)
    {
        FrameworkElement container = sender as FrameworkElement;

        if (container == null)
        {
            return;
        }

        ScrollViewer scrollViewer = GetFirstVisualChild<ScrollViewer>(container);

        if (scrollViewer == null)
        {
            return;
        }

        double tolerance = 60;
        double verticalPos = e.GetPosition(container).Y;
        double offset = 20;

        if (verticalPos < tolerance) // Top of visible list? 
        {
            scrollViewer.ScrollToVerticalOffset(scrollViewer.VerticalOffset - offset); //Scroll up. 
        }
        else if (verticalPos > container.ActualHeight - tolerance) //Bottom of visible list? 
        {
            scrollViewer.ScrollToVerticalOffset(scrollViewer.VerticalOffset + offset); //Scroll down.     
        }
    }

    private static void Unsubscribe(FrameworkElement container)
    {
        container.PreviewDragOver -= OnContainerPreviewDragOver;
    }

    private static T GetFirstVisualChild<T>(DependencyObject depObj) where T : DependencyObject
    {
        if (depObj != null)
        {
            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
            {
                DependencyObject child = VisualTreeHelper.GetChild(depObj, i);
                if (child != null && child is T)
                {
                    return (T)child;
                }

                T childItem = GetFirstVisualChild<T>(child);
                if (childItem != null)
                {
                    return childItem;
                }
            }
        }

        return null;
    }
}

- akjoshi

---

非常好的解决方案。如果您想要平滑滚动，不要忘记在您的ListBox/ListView上添加“ScrollViewer.CanContentScroll="False"”。 - Pak

@Pak 值得一提的是，当您将 CanContentScroll 设置为 false 时，同时也会禁用虚拟化。 - pjrki

Answer 3

Artur的回答很好，让我找到了正确的方向，但我发现了一些问题。

首先，代码在这里有一个错误：

ScrollViewer sv = FindVisualChild<ScrollViewer>(ItemsList);

应该传入li。

此外，上面的代码没有正确考虑ListBox是否有水平滚动条。我已经进行了修改，并进行了两个修复，如下所示：

private void MainTreeView_DragOver(object sender, DragEventArgs e)
{
    ListBox li = sender as ListBox;
    ScrollViewer sv = FindVisualChild<ScrollViewer>(li);

    double tolerance = 24;
    double verticalPos = e.GetPosition(li).Y;

    double topMargin = tolerance;
    var bottomMargin = li.ActualHeight - tolerance;
    if(sv.ComputedHorizontalScrollBarVisibility == Visibility.Visible)
    {
        var horizontalScrollBar = sv.Template.FindName("PART_HorizontalScrollBar", sv) as System.Windows.Controls.Primitives.ScrollBar;

        if(horizontalScrollBar != null)
        {
            bottomMargin -= horizontalScrollBar.ActualHeight;
        }
    }

    double distanceToScroll = 3;
    if (verticalPos < topMargin) // Top of visible list?
    {
        sv.ScrollToVerticalOffset(sv.VerticalOffset - distanceToScroll); //Scroll up.
    }
    else if (verticalPos > bottomMargin) //Bottom of visible list?
    {
        sv.ScrollToVerticalOffset(sv.VerticalOffset + distanceToScroll); //Scroll down.    
    }
}

public static childItem FindVisualChild<childItem>(DependencyObject obj) where childItem : DependencyObject
{
    // Search immediate children first (breadth-first)
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
    {
        DependencyObject child = VisualTreeHelper.GetChild(obj, i);

        if (child != null && child is childItem)
            return (childItem)child;

        else
        {
            childItem childOfChild = FindVisualChild<childItem>(child);

            if (childOfChild != null)
                return childOfChild;
        }
    }

    return null;

- Victor Chelaru