WPF自定义样式TreeView实现单选展开及SelectedItemChanged事件不触发问题

问题根源 #

你自定义的TreeViewItem模板存在嵌套ToggleButton的问题，外层整个Header区域被做成了绑定IsExpanded的ToggleButton，所有点击都被这个ToggleButton拦截处理，不会触发TreeViewItem的选中逻辑，所以IsSelected不会更新，SelectedItemChanged事件自然无法触发。

修复步骤 #

1. 修复选中事件不触发的问题

调整TreeViewItem模板，仅保留右侧箭头作为展开控制按钮，头部主区域点击触发选中逻辑：

<!-- 替换原来TreeViewItem模板中的DockPanel部分 -->
<DockPanel Background="{TemplateBinding Background}">
    <!-- 头部区域改为Border，点击触发选中 -->
    <Border Name="HeaderSite"
            DockPanel.Dock="Top"
            BorderThickness="0" 
            Cursor="Hand"
            Opacity=".87"
            Foreground="{TemplateBinding Foreground}"
            MouseLeftButtonDown="HeaderSite_MouseLeftButtonDown">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="Auto"/>
            </Grid.ColumnDefinitions>
            <ContentPresenter Grid.Column="0" 
                              Content="{TemplateBinding Header}"
                              ContentTemplate="{TemplateBinding HeaderTemplate}"
                              ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}"
                              ContentStringFormat="{TemplateBinding HeaderStringFormat}"
                              VerticalAlignment="Center"
                              Margin="0,0,16,0" />
            <!-- 仅右侧箭头为展开控制按钮 -->
            <ToggleButton Grid.Column="1"
                          VerticalAlignment="Center"
                          Foreground="{TemplateBinding Foreground}" 
                          x:Name="Expander" 
                          Visibility="{Binding HasItems, Converter={StaticResource BoolToVisibilityConverter}, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type TreeViewItem}}}"
                          IsChecked="{Binding Path=IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}">
                <ToggleButton.Template>
                    <ControlTemplate TargetType="{x:Type ToggleButton}">
                        <Border Background="Transparent">
                            <Path Data="M7.41,8.58L12,13.17L16.59,8.58L18,10L12,16L6,10L7.41,8.58Z" 
                                  Opacity=".38" 
                                  x:Name="ExpandPath" 
                                  RenderTransformOrigin="0.5,0.5" 
                                  Height="24" 
                                  Width="24" 
                                  Fill="{TemplateBinding Foreground}"/>
                        </Border>
                    </ControlTemplate>
                </ToggleButton.Template>
            </ToggleButton>
        </Grid>
    </Border>
    <!-- 下方内容区域保持原来的代码不变 -->
    <Border Name="ContentSite" DockPanel.Dock="Bottom">
        <StackPanel x:Name="ItemsPanel" Margin="10 0 0 0">
            <StackPanel.Height>
                <MultiBinding Converter="{StaticResource MathMlpMultipleConverter}">
                    <Binding ElementName="ItemsHost" Path="ActualHeight"/>
                    <Binding ElementName="ItemsHost" Path="Opacity"/>
                </MultiBinding>
            </StackPanel.Height>
            <ItemsPresenter x:Name="ItemsHost" VerticalAlignment="Top" Opacity="0" Visibility="Collapsed"/>
        </StackPanel>
    </Border>
</DockPanel>

后台添加头部点击事件和辅助方法：

// 头部点击触发选中
private void HeaderSite_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    var treeViewItem = FindVisualParent<TreeViewItem>(sender as DependencyObject);
    if (treeViewItem == null) return;
    treeViewItem.IsSelected = true;
    e.Handled = true;
}

// 视觉树查找父节点辅助方法
private T FindVisualParent<T>(DependencyObject child) where T : DependencyObject
{
    while (child != null)
    {
        if (child is T parent)
            return parent;
        child = VisualTreeHelper.GetParent(child);
    }
    return null;
}

2. 实现自动折叠非关联节点逻辑

现在SelectedItemChanged已经可以正常触发，添加如下逻辑即可实现需求：

private void TreeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
    var treeView = sender as TreeView;
    if (treeView == null || e.NewValue == null) return;
    
    var selectedItem = e.NewValue as TreeViewItem;
    // 先折叠所有节点
    foreach (TreeViewItem rootItem in treeView.Items)
    {
        CollapseAll(rootItem);
    }
    
    // 展开选中节点的所有祖先节点
    var parent = VisualTreeHelper.GetParent(selectedItem);
    while (parent != null)
    {
        if (parent is TreeViewItem item)
        {
            item.IsExpanded = true;
        }
        if (parent is TreeView)
            break;
        parent = VisualTreeHelper.GetParent(parent);
    }
}

// 递归折叠节点及其所有子节点
private void CollapseAll(TreeViewItem item)
{
    item.IsExpanded = false;
    foreach (TreeViewItem subItem in item.Items)
    {
        CollapseAll(subItem);
    }
}

如果你的TreeView是数据绑定模式而非直接硬编码TreeViewItem，只需要在获取选中节点时通过ItemContainerGenerator.ContainerFromItem方法拿到对应的TreeViewItem即可，逻辑和上面一致。

内容的提问来源于stack exchange，提问作者Roberto Vargas