WPF中Popup不向上传递IsKeyboardFocusWithin属性值如何解决

问题根本原因 #

WPF 中 Popup 的内容会被渲染在独立的 PopupRoot 视觉树中，和其父宿主元素（示例里的 Grid、Window，以及实际场景里的 ListViewItem）的视觉树完全分离，因此焦点相关属性、路由事件默认不会向上冒泡到宿主的元素链，这就是 IsKeyboardFocusWithin 属性不会向上传递的核心原因。

解决方案 #

原生的 IsKeyboardFocusWithin 是只读依赖属性，无法直接手动修改，你可以选择以下两种方案实现需求：

方案1：自定义附加属性同步焦点状态（适合通用场景）

通过自定义附加属性，同时监听宿主本身和其内部所有 Popup 的焦点状态，将合并后的焦点状态对外暴露，触发器直接绑定该附加属性即可。

1. 首先实现焦点同步辅助类：

using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls.Primitives;
using System.Windows.Media;
using System.ComponentModel;
using System.Linq;

public static class PopupFocusHelper
{
    // 对外暴露的合并焦点状态附加属性
    public static readonly DependencyProperty IsFocusWithinIncludePopupProperty = DependencyProperty.RegisterAttached(
        "IsFocusWithinIncludePopup", typeof(bool), typeof(PopupFocusHelper), new PropertyMetadata(false));

    public static bool GetIsFocusWithinIncludePopup(DependencyObject element)
    {
        return (bool)element.GetValue(IsFocusWithinIncludePopupProperty);
    }

    public static void SetIsFocusWithinIncludePopup(DependencyObject element, bool value)
    {
        element.SetValue(IsFocusWithinIncludePopupProperty, value);
    }

    // 启用同步的开关附加属性
    public static readonly DependencyProperty SynchronizePopupFocusProperty = DependencyProperty.RegisterAttached(
        "SynchronizePopupFocus", typeof(bool), typeof(PopupFocusHelper), new PropertyMetadata(false, OnSynchronizePopupFocusChanged));

    public static bool GetSynchronizePopupFocus(DependencyObject element)
    {
        return (bool)element.GetValue(SynchronizePopupFocusProperty);
    }

    public static void SetSynchronizePopupFocus(DependencyObject element, bool value)
    {
        element.SetValue(SynchronizePopupFocusProperty, value);
    }

    private static void OnSynchronizePopupFocusChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not FrameworkElement host) return;
        if ((bool)e.NewValue)
        {
            host.Loaded += HostOnLoaded;
            UpdateFocusState(host);
        }
        else
        {
            host.Loaded -= HostOnLoaded;
            // 此处可自行补充监听事件的清理逻辑，避免内存泄漏
        }
    }

    private static void HostOnLoaded(object sender, RoutedEventArgs e)
    {
        if (sender is not FrameworkElement host) return;
        // 监听宿主本身焦点变化
        var hostFocusDesc = DependencyPropertyDescriptor.FromProperty(UIElement.IsKeyboardFocusWithinProperty, typeof(FrameworkElement));
        hostFocusDesc.AddValueChanged(host, (_, _) => UpdateFocusState(host));
        
        // 遍历监听所有子Popup的焦点变化
        foreach (var popup in FindVisualChildren<Popup>(host))
        {
            var popupFocusDesc = DependencyPropertyDescriptor.FromProperty(UIElement.IsKeyboardFocusWithinProperty, typeof(Popup));
            popupFocusDesc.AddValueChanged(popup, (_, _) => UpdateFocusState(host));
        }
    }

    private static void UpdateFocusState(FrameworkElement host)
    {
        bool hasHostFocus = host.IsKeyboardFocusWithin;
        bool hasPopupFocus = FindVisualChildren<Popup>(host).Any(p => p.IsKeyboardFocusWithin);
        SetIsFocusWithinIncludePopup(host, hasHostFocus || hasPopupFocus);
    }

    private static IEnumerable<T> FindVisualChildren<T>(DependencyObject root) where T : DependencyObject
    {
        if (root == null) yield break;
        for (int i = 0; i < VisualTreeHelper.GetChildrenCount(root); i++)
        {
            var child = VisualTreeHelper.GetChild(root, i);
            if (child is T target) yield return target;
            foreach (var subChild in FindVisualChildren<T>(child)) yield return subChild;
        }
    }
}

2. 修改XAML代码，绑定新的附加属性即可：

<Window
    x:Class="PopupDemo.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:local="clr-namespace:PopupDemo"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Title="MainWindow"
    Width="800"
    Height="450"
    mc:Ignorable="d"
    local:PopupFocusHelper.SynchronizePopupFocus="True">
    <Grid>
        <Popup IsOpen="True" StaysOpen="True">
            <Button Content="Test" />
        </Popup>
    </Grid>

    <Window.Style>
        <Style TargetType="{x:Type Window}">
            <Style.Triggers>
                <Trigger Property="local:PopupFocusHelper.IsFocusWithinIncludePopup" Value="True">
                    <Setter Property="Background" Value="Red" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Style>
</Window>

方案2：监听Popup焦点事件直接处理业务逻辑（适合仅需选中ListViewItem的场景）

如果你的需求只是选中Popup所属的ListViewItem，不需要通用的焦点属性，可以直接监听Popup内元素的GotKeyboardFocus事件，在事件处理方法中通过Popup的PlacementTarget找到对应的ListViewItem，手动设置IsSelected = true即可，实现更轻量化。

内容的提问来源于stack exchange，提问作者L1oN