如何在XAML TreeView HierarchicalDataTemplate中垂直对齐项目符号？

Question

When a node in my TreeView has multiple lines, the TreeView bullet gets vertically centered.

How can I top-align the TreeView bullet?

alt text http://www.deviantsart.com/upload/1uh2k8p.png

<pages:BasePage.Resources>
    <data:HierarchicalDataTemplate x:Key="OutlineTemplate"
        ItemsSource="{Binding OutlineDocumentObjects}">
        <TextBlock Text="{Binding Line}" 
            TextWrapping="Wrap" 
            VerticalAlignment="Top"
            Width="600"/>
    </data:HierarchicalDataTemplate>
</pages:BasePage.Resources>

<StackPanel Style="{StaticResource StackPanelPageWrapperStyle}">
    <tk:TreeView x:Name="TheTreeView" 
        ItemsSource="{Binding TheOutline.OutlineDocumentObjects}"
        ItemTemplate="{StaticResource OutlineTemplate}">
    </tk:TreeView>
</StackPanel>

Answer 1

Good question... of course, it could be done by redefining the template, but it's a pain... (if you want to go that way, extract the template with StyleSnooper or ShowMeTheTemplate and change the VerticalAlignment of the ToggleButton )

Another way is to inherit TreeViewItem and override the OnApply method. Since the ToggleButton has a name ("Expander") in the default template, you can find it and apply the VerticalAlignment you want :

public class TopAlignedTreeViewItem : TreeViewItem
{
    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        if (Template != null)
        {
            ToggleButton btn = Template.FindName("Expander", this) as ToggleButton;
            if (btn != null)
            {
                btn.VerticalAlignment = VerticalAlignment.Top;
            }
        }
    }
}

For the TreeView to generate TopAlignedTreeViewItem s instead of TreeViewItem s, you also need to make your own TreeView :

public class TopAlignedTreeView : TreeView
{
    protected override bool IsItemItsOwnContainerOverride(object item)
    {
        return (item is TopAlignedTreeViewItem);
    }

    protected override System.Windows.DependencyObject GetContainerForItemOverride()
    {
        return new TopAlignedTreeViewItem();
    }
}