虽然这已经是一个老问题了，但我也有同样的需求——我需要能够将 Popup 与其放置目标对齐。对转换器解决方案不满意，我想出了自己的解决方案，使用附加的依赖属性，我在这里与您和任何有相同需求的人分享。
注意：此解决方案不包括如何在鼠标上显示Popup
悬停。它只涵盖了最棘手的部分——Popup 与其放置目标的对齐。有几种方法可以在鼠标悬停时显示 Popup，例如使用 Triggers 或 Bindings，这两种方法都在 StackOverflow 上广泛介绍。
附加依赖属性解决方案
此解决方案使用单个静态类公开一些附加的依赖属性。使用这些属性，您可以将Popup 与其PlacementTarget 或其PlacementRectangle 水平或垂直对齐。仅当Popup 的Placement 属性的值表示一条边（Left、Top、Right 或Bottom）时，才会发生对齐。
实施
弹出属性.cs
using System;
using System.Windows;
using System.Windows.Controls.Primitives;
using System.Windows.Media;
namespace MyProjectName.Ui
{
/// <summary>
/// Exposes attached dependency properties that provide
/// additional functionality for <see cref="Popup"/> controls.
/// </summary>
/// <seealso cref="Popup"/>
/// <seealso cref="DependencyProperty"/>
public static class PopupProperties
{
#region Properties
#region IsMonitoringState attached dependency property
/// <summary>
/// Attached <see cref="DependencyProperty"/>. This property
/// registers (<b>true</b>) or unregisters (<b>false</b>) a
/// <see cref="Popup"/> from the popup monitoring mechanism
/// used internally by <see cref="PopupProperties"/> to keep
/// the <see cref="Popup"/> in synchrony with the
/// <see cref="PopupProperties"/>' attached properties. A
/// <see cref="Popup"/> will be automatically unregistered from
/// this mechanism after it is unloaded.
/// </summary>
/// <seealso cref="Popup"/>
private static readonly DependencyProperty IsMonitoringStateProperty
= DependencyProperty.RegisterAttached("IsMonitoringState",
typeof(bool), typeof(PopupProperties),
new FrameworkPropertyMetadata(false,
FrameworkPropertyMetadataOptions.None,
new PropertyChangedCallback(IsMonitoringStatePropertyChanged)));
private static void IsMonitoringStatePropertyChanged(
DependencyObject dObject, DependencyPropertyChangedEventArgs e)
{
Popup popup = (Popup)dObject;
bool value = (bool)e.NewValue;
if (value)
{
// Attach popup.
popup.Opened += Popup_Opened;
popup.Unloaded += Popup_Unloaded;
// Update popup.
UpdateLocation(popup);
}
else
{
// Detach popup.
popup.Opened -= Popup_Opened;
popup.Unloaded -= Popup_Unloaded;
}
}
private static bool GetIsMonitoringState(Popup popup)
{
if (popup is null)
throw new ArgumentNullException(nameof(popup));
return (bool)popup.GetValue(IsMonitoringStateProperty);
}
private static void SetIsMonitoringState(Popup popup, bool isMonitoringState)
{
if (popup is null)
throw new ArgumentNullException(nameof(popup));
popup.SetValue(IsMonitoringStateProperty, isMonitoringState);
}
#endregion
#region HorizontalPlacementAlignment attached dependency property
public static readonly DependencyProperty HorizontalPlacementAlignmentProperty
= DependencyProperty.RegisterAttached("HorizontalPlacementAlignment",
typeof(AlignmentX), typeof(PopupProperties),
new FrameworkPropertyMetadata(AlignmentX.Left,
FrameworkPropertyMetadataOptions.None,
new PropertyChangedCallback(HorizontalPlacementAlignmentPropertyChanged)),
new ValidateValueCallback(HorizontalPlacementAlignmentPropertyValidate));
private static void HorizontalPlacementAlignmentPropertyChanged(
DependencyObject dObject, DependencyPropertyChangedEventArgs e)
{
Popup popup = (Popup)dObject;
SetIsMonitoringState(popup, true);
UpdateLocation(popup);
}
private static bool HorizontalPlacementAlignmentPropertyValidate(object obj)
{
return Enum.IsDefined(typeof(AlignmentX), obj);
}
public static AlignmentX GetHorizontalPlacementAlignment(Popup popup)
{
if (popup is null)
throw new ArgumentNullException(nameof(popup));
return (AlignmentX)popup.GetValue(HorizontalPlacementAlignmentProperty);
}
public static void SetHorizontalPlacementAlignment(Popup popup, AlignmentX alignment)
{
if (popup is null)
throw new ArgumentNullException(nameof(popup));
popup.SetValue(HorizontalPlacementAlignmentProperty, alignment);
}
#endregion
#region VerticalPlacementAlignment attached dependency property
public static readonly DependencyProperty VerticalPlacementAlignmentProperty
= DependencyProperty.RegisterAttached("VerticalPlacementAlignment",
typeof(AlignmentY), typeof(PopupProperties),
new FrameworkPropertyMetadata(AlignmentY.Top,
FrameworkPropertyMetadataOptions.None,
new PropertyChangedCallback(VerticalPlacementAlignmentPropertyChanged)),
new ValidateValueCallback(VerticalPlacementAlignmentPropertyValidate));
private static void VerticalPlacementAlignmentPropertyChanged(
DependencyObject dObject, DependencyPropertyChangedEventArgs e)
{
Popup popup = (Popup)dObject;
SetIsMonitoringState(popup, true);
UpdateLocation(popup);
}
private static bool VerticalPlacementAlignmentPropertyValidate(object obj)
{
return Enum.IsDefined(typeof(AlignmentY), obj);
}
public static AlignmentY GetVerticalPlacementAlignment(Popup popup)
{
if (popup is null)
throw new ArgumentNullException(nameof(popup));
return (AlignmentY)popup.GetValue(VerticalPlacementAlignmentProperty);
}
public static void SetVerticalPlacementAlignment(Popup popup, AlignmentY alignment)
{
if (popup is null)
throw new ArgumentNullException(nameof(popup));
popup.SetValue(VerticalPlacementAlignmentProperty, alignment);
}
#endregion
#region HorizontalOffset attached dependency property
public static readonly DependencyProperty HorizontalOffsetProperty
= DependencyProperty.RegisterAttached("HorizontalOffset",
typeof(double), typeof(PopupProperties),
new FrameworkPropertyMetadata(0d,
FrameworkPropertyMetadataOptions.None,
new PropertyChangedCallback(HorizontalOffsetPropertyChanged)),
new ValidateValueCallback(HorizontalOffsetPropertyValidate));
private static void HorizontalOffsetPropertyChanged(
DependencyObject dObject, DependencyPropertyChangedEventArgs e)
{
Popup popup = (Popup)dObject;
SetIsMonitoringState(popup, true);
UpdateLocation(popup);
}
private static bool HorizontalOffsetPropertyValidate(object obj)
{
double value = (double)obj;
return !double.IsNaN(value) && !double.IsInfinity(value);
}
public static double GetHorizontalOffset(Popup popup)
{
if (popup is null)
throw new ArgumentNullException(nameof(popup));
return (double)popup.GetValue(HorizontalOffsetProperty);
}
public static void SetHorizontalOffset(Popup popup, double offset)
{
if (popup is null)
throw new ArgumentNullException(nameof(offset));
popup.SetValue(HorizontalOffsetProperty, offset);
}
#endregion
#region VerticalOffset attached dependency property
public static readonly DependencyProperty VerticalOffsetProperty
= DependencyProperty.RegisterAttached("VerticalOffset",
typeof(double), typeof(PopupProperties),
new FrameworkPropertyMetadata(0d,
FrameworkPropertyMetadataOptions.None,
new PropertyChangedCallback(VerticalOffsetPropertyChanged)),
new ValidateValueCallback(VerticalOffsetPropertyValidate));
private static void VerticalOffsetPropertyChanged(
DependencyObject dObject, DependencyPropertyChangedEventArgs e)
{
Popup popup = (Popup)dObject;
SetIsMonitoringState(popup, true);
UpdateLocation(popup);
}
private static bool VerticalOffsetPropertyValidate(object obj)
{
double value = (double)obj;
return !double.IsNaN(value) && !double.IsInfinity(value);
}
public static double GetVerticalOffset(Popup popup)
{
if (popup is null)
throw new ArgumentNullException(nameof(popup));
return (double)popup.GetValue(VerticalOffsetProperty);
}
public static void SetVerticalOffset(Popup popup, double offset)
{
if (popup is null)
throw new ArgumentNullException(nameof(offset));
popup.SetValue(VerticalOffsetProperty, offset);
}
#endregion
#endregion Properties
#region Methods
private static void OnMonitorState(Popup popup)
{
if (popup is null)
throw new ArgumentNullException(nameof(popup));
UpdateLocation(popup);
}
private static void UpdateLocation(Popup popup)
{
// Validate parameters.
if (popup is null)
throw new ArgumentNullException(nameof(popup));
// If the popup is not open, we don't need to update its position yet.
if (!popup.IsOpen)
return;
// Setup initial variables.
double offsetX = 0d;
double offsetY = 0d;
PlacementMode placement = popup.Placement;
UIElement placementTarget = popup.PlacementTarget;
Rect placementRect = popup.PlacementRectangle;
// If the popup placement mode is an edge of the placement target,
// determine the alignment offset.
if (placement == PlacementMode.Top || placement == PlacementMode.Bottom
|| placement == PlacementMode.Left || placement == PlacementMode.Right)
{
// Try to get the popup size. If its size is empty, use the size
// of its child, if any child exists.
Size popupSize = GetElementSize(popup);
UIElement child = popup.Child;
if ((popupSize.IsEmpty || popupSize.Width <= 0d || popupSize.Height <= 0d)
&& child != null)
{
popupSize = GetElementSize(child);
}
// Get the placement rectangle size. If it's empty, get the
// placement target's size, if a target is set.
Size targetSize;
if (placementRect.Width > 0d && placementRect.Height > 0d)
targetSize = placementRect.Size;
else if (placementTarget != null)
targetSize = GetElementSize(placementTarget);
else
targetSize = Size.Empty;
// If we have a valid popup size and a valid target size, determine
// the offset needed to align the popup to the target rectangle.
if (!popupSize.IsEmpty && popupSize.Width > 0d && popupSize.Height > 0d
&& !targetSize.IsEmpty && targetSize.Width > 0d && targetSize.Height > 0d)
{
switch (placement)
{
// Horizontal alignment offset.
case PlacementMode.Top:
case PlacementMode.Bottom:
switch (GetHorizontalPlacementAlignment(popup))
{
case AlignmentX.Left:
offsetX = 0d;
break;
case AlignmentX.Center:
offsetX = -(popupSize.Width - targetSize.Width) / 2d;
break;
case AlignmentX.Right:
offsetX = -(popupSize.Width - targetSize.Width);
break;
default:
break;
}
break;
// Vertical alignment offset.
case PlacementMode.Left:
case PlacementMode.Right:
switch (GetVerticalPlacementAlignment(popup))
{
case AlignmentY.Top:
offsetY = 0d;
break;
case AlignmentY.Center:
offsetY = -(popupSize.Height - targetSize.Height) / 2d;
break;
case AlignmentY.Bottom:
offsetY = -(popupSize.Height - targetSize.Height);
break;
default:
break;
}
break;
default:
break;
}
}
}
// Add the developer specified offsets to the offsets we've calculated.
offsetX += GetHorizontalOffset(popup);
offsetY += GetVerticalOffset(popup);
// Apply the final computed offsets to the popup.
popup.SetCurrentValue(Popup.HorizontalOffsetProperty, offsetX);
popup.SetCurrentValue(Popup.VerticalOffsetProperty, offsetY);
}
private static Size GetElementSize(UIElement element)
{
if (element is null)
return new Size(0d, 0d);
else if (element is FrameworkElement frameworkElement)
return new Size(frameworkElement.ActualWidth, frameworkElement.ActualHeight);
else
return element.RenderSize;
}
#endregion Methods
#region Event handlers
private static void Popup_Unloaded(object sender, RoutedEventArgs e)
{
if (sender is Popup popup)
{
// Stop monitoring the popup state, because it was unloaded.
SetIsMonitoringState(popup, false);
}
}
private static void Popup_Opened(object sender, EventArgs e)
{
if (sender is Popup popup)
{
OnMonitorState(popup);
}
}
#endregion Event handlers
}
}
工作原理
上面的代码创建了一个静态类，它为Popup 控件公开了4 个附加的依赖属性。即HorizontalPlacementAlignment、VerticalPlacementAlignment、HorizontalOffset和VerticalOffset。
HorizontalPlacementAlignment 和 VerticalPlacementAlignment 附加的依赖属性允许您将弹出窗口与其PlacementTarget 或PlacementRectangle 对齐。为此，该机制使用Popup.HorizontalOffset 和Popup.VerticalOffset 属性来定位Popup。
因为该机制使用Popup.HorizontalOffset 和Popup.VerticalOffset 属性来工作，所以这个类提供了自己的HorizontalOffset 和VerticalOffset 属性（附加的依赖属性）。除了对齐之外，您还可以使用它们来调整 Popup 的位置。
每次打开Popup 时，该机制都会自动更新Popup 的位置。但是，当弹出窗口大小发生变化或放置目标或放置矩形大小发生变化时，它的位置不会自动更新。尽管如此，只要投入更多的工作，该功能就可以轻松实现。
使用示例
您将使用Popup 上的附加属性，如下例所示。在这个例子中，我们有一个简单的Button 和一个Popup。 Popup 与Button 的底部对齐显示，并以Button 的中心为中心水平居中。
<Button x:Name="MyTargetElement">My Button</Button>
<Popup xmlns:ui="clr-namespace:MyProjectName.Ui"
PlacementTarget="{Binding ElementName=MyTargetElement}"
Placement="Bottom"
ui:PopupProperties.HorizontalPlacementAlignment="Center"
ui:PopupProperties.VerticalOffset="2">
</Popup>
通过将ui:PopupProperties.HorizontalPlacementAlignment="Center" 和ui:PopupProperties.VerticalOffset="2" 添加到Popup，它将与其放置目标的水平中心对齐，并具有2 个WPF 单位的垂直偏移量。
注意在Popup 上使用xmlns:ui="clr-namespace:MyProjectName.Ui"。此属性仅从项目上的 MyProjectName.Ui 命名空间导入类型，并通过在 XAML 属性上使用 ui: 前缀使它们可用。在示例中，为简单起见，此属性设置在 Popup 上，但您通常会在使用这些自定义附加依赖项属性的 Window 或 ResourceDictionary 上设置它。
结论
使用附加的依赖属性来实现此功能的想法是使其在 XAML 中的使用尽可能简单。对于简单的一次性需求，使用转换器可能更易于实现。但是，在这种情况下，使用附加的依赖属性可能会提供一种更加动态和使用友好的方法。