如何修改MouseDragPopupBehavior以限制Popup拖拽范围在应用窗口内

修改思路 #

现有代码仅实现了基础拖拽功能，我们通过以下调整增加范围限制：

• 新增ParentContainer依赖属性，支持绑定你要限制的父容器（比如应用主窗口）
• 拖拽计算偏移后，先获取父容器和Popup的屏幕坐标、实际尺寸，做边界校验
• 仅允许Popup的所有边缘都在父容器范围内，超出时自动校正偏移量

修改后完整代码 #

public class MouseDragPopupBehavior : Behavior<Popup>
{
    private bool mouseDown;
    private Point oldMousePosition;

    // 可绑定的父容器属性，指定拖拽限制的边界容器
    public static readonly DependencyProperty ParentContainerProperty =
        DependencyProperty.Register(nameof(ParentContainer), typeof(UIElement), typeof(MouseDragPopupBehavior), new PropertyMetadata(null));

    public UIElement ParentContainer
    {
        get => (UIElement)GetValue(ParentContainerProperty);
        set => SetValue(ParentContainerProperty, value);
    }

    protected override void OnAttached()
    {
        AssociatedObject.MouseLeftButtonDown += (s, e) =>
        {
            mouseDown = true;
            oldMousePosition = AssociatedObject.PointToScreen(e.GetPosition(AssociatedObject));
            AssociatedObject.Child.CaptureMouse();
        };
        AssociatedObject.MouseMove += (s, e) =>
        {
            if (!mouseDown || ParentContainer == null) return;
            
            var newMousePosition = AssociatedObject.PointToScreen(e.GetPosition(AssociatedObject));
            var offset = newMousePosition - oldMousePosition;
            oldMousePosition = newMousePosition;

            // 计算边界参数
            var popupCurrentLeft = AssociatedObject.PointToScreen(new Point(0, 0)).X;
            var popupCurrentTop = AssociatedObject.PointToScreen(new Point(0, 0)).Y;
            var popupWidth = AssociatedObject.ActualWidth;
            var popupHeight = AssociatedObject.ActualHeight;

            var parentLeft = ParentContainer.PointToScreen(new Point(0, 0)).X;
            var parentTop = ParentContainer.PointToScreen(new Point(0, 0)).Y;
            var parentRight = parentLeft + ParentContainer.RenderSize.Width;
            var parentBottom = parentTop + ParentContainer.RenderSize.Height;

            // 校正水平偏移
            var expectedLeft = popupCurrentLeft + offset.X;
            var expectedRight = expectedLeft + popupWidth;
            if (expectedLeft < parentLeft)
            {
                offset.X = parentLeft - popupCurrentLeft;
            }
            else if (expectedRight > parentRight)
            {
                offset.X = parentRight - popupCurrentLeft - popupWidth;
            }

            // 校正垂直偏移
            var expectedTop = popupCurrentTop + offset.Y;
            var expectedBottom = expectedTop + popupHeight;
            if (expectedTop < parentTop)
            {
                offset.Y = parentTop - popupCurrentTop;
            }
            else if (expectedBottom > parentBottom)
            {
                offset.Y = parentBottom - popupCurrentTop - popupHeight;
            }

            // 应用校正后的偏移
            AssociatedObject.HorizontalOffset += offset.X;
            AssociatedObject.VerticalOffset += offset.Y;
        };
        AssociatedObject.MouseLeftButtonUp += (s, e) =>
        {
            mouseDown = false;
            AssociatedObject.Child.ReleaseMouseCapture();
        };
    }
}

使用方式 #

在XAML中给Behavior绑定父容器即可，比如要限制在当前窗口内，写法如下：

<Popup>
    <i:Interaction.Behaviors>
        <local:MouseDragPopupBehavior ParentContainer="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=Window}}" />
    </i:Interaction.Behaviors>
    <!-- Popup内容 -->
</Popup>

如果要限制在其他容器比如Grid里，只需要把AncestorType改成对应容器类型即可。

内容的提问来源于stack exchange，提问作者Julien