WPF列表框在拖动时自动滚动

Question

我有一个带有ListBox 的 WPF 应用程序。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，我不能。

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，我必须拖到顶部并放下。向上滚动并再次拖动。

如何使ListBox 自动滚动？

Answer 1

知道了。使用ListBox 的事件DragOver，使用找到的函数here 来获取列表框的scrollviewer，然后它只是有点与位置的杂耍。

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;
}

Answer 2

基于此，我创建了一个Attached Behavior，可以像这样轻松使用 -

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

这是附加行为的代码 -

/// <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;
    }
}