This article discusses how to customize the item layout in a WPF TreeView. The layout we will examine is quite similar to an "org chart", where each level of items is displayed in a horizontal row directly beneath their respective parent. Along the way we will see how the power of templates and styles in WPF can provide incredible flexibility for customizing an application's user interface.
This article is not for WPF beginners. It assumes that you already have knowledge of XAML, control templates, styles, triggers, hierarchical data templates, data binding, and other fundamentals of WPF.
I also posted another article regarding layout customization for the TreeView control. If you are interested in seeing another way that the TreeView can be customized, you might want to read Advanced Custom TreeView Layout in WPF.
Before diving into the XAML which makes the magic happen, let's first take a look at what we are aiming to achieve. If I populate a TreeView with some simple data and view it, by default it looks pretty plain. Here is the "before" picture:
What you see above is certainly not a breathtaking representation of the data. However, after we customize the way that TreeViewItems are rendered and how the TreeView positions its items, the same TreeView control can look like this:
The first step is to create a custom ControlTemplate for the TreeViewItem class. If you wrap that template in a typed Style (i.e. a Style with no Key) then it will automatically be applied to every TreeViewItem instance by default. The TreeViewItem control template should have two things: a ContentPresenter whose Name is 'PART_Header' and an ItemsPresenter. The ContentPresenter is used to display the content of the item. The ItemsPresenter is used to display it's child items.
In addition to customizing the TreeViewItem control template, you also must modify the ItemsPanel of TreeViewItem. In order for the child items to be displayed in a horizontal row, I set the TreeViewItem.ItemsPanel property to a StackPanel with a horizontal orientation. That setting was also applied in the typed Style mentioned previously.
Let's take a look at an abridged version of the typed Style:
<Style TargetType="TreeViewItem">
<Style.Resources>
<!-- Resources omitted for clarity… -->
</Style.Resources>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="TreeViewItem">
<Grid Margin="2">
<Grid.RowDefinitions>
<!--The top row contains the item's content.-->
<RowDefinition Height="Auto" />
<!--The bottom row contains the item's children.-->
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<!-- This Border and ContentPresenter displays the
content of the TreeViewItem. -->
<Border Name="Bd"
Background="{StaticResource ItemAreaBrush}"
BorderBrush="{StaticResource ItemBorderBrush}"
BorderThickness="0.6"
CornerRadius="8"
Padding="6"
>
<ContentPresenter Name="PART_Header"
ContentSource="Header"
HorizontalAlignment="Center"
VerticalAlignment="Center" />
</Border>
<!-- The ItemsPresenter displays the item's children. -->
<ItemsPresenter Grid.Row="1"/>
</Grid>
<ControlTemplate.Triggers>
<!--When the item is selected in the TreeView, use the
"selected" colors and give it a drop shadow. -->
<Trigger Property="IsSelected" Value="True">
<Setter
TargetName="Bd"
Property="Panel.Background"
Value="{StaticResource SelectedItemAreaBrush}" />
<Setter
TargetName="Bd"
Property="Border.BorderBrush"
Value="{StaticResource SelectedItemBorderBrush}" />
<Setter
TargetName="Bd"
Property="TextElement.Foreground"
Value="{DynamicResource
{x:Static SystemColors.HighlightTextBrushKey}}" />
<Setter
TargetName="Bd"
Property="Border.BitmapEffect"
Value="{StaticResource DropShadowEffect}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<!-- Make each TreeViewItem show it's children
in a horizontal StackPanel. -->
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<StackPanel
HorizontalAlignment="Center"
IsItemsHost="True"
Margin="4,6"
Orientation="Horizontal" />
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
</Style>
The final step is to make the TreeView center the root item(s) horizontally. Doing so will provide symmetry between the items, as seen in the screenshot above. This step is a simple matter of setting the TreeView's ItemsPanel property to a Grid whose HorizontalAlignment is set to 'Center'. Let's take a look at the XAML for a Window which contains our customized TreeView:
<Window x:Class="CustomTreeViewLayout.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:CustomTreeViewLayout"
Title="Custom TreeView" Height="350" Width="780"
Loaded="OnLoaded"
WindowStartupLocation="CenterScreen"
FontSize="11"
>
<TreeView Name="tree">
<TreeView.Resources>
<ResourceDictionary>
<!-- Import the resource dictionary file which
contains the Style that makes TreeViewItems
display their child items in an organization
chart layout. -->
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="OrgChartTreeViewItemStyle.xaml" />
</ResourceDictionary.MergedDictionaries>
<!-- This template explains how to render
a Node object and its child nodes. -->
<HierarchicalDataTemplate
DataType="{x:Type local:Node}"
ItemsSource="{Binding ChildNodes}"
>
<TextBlock Text="{Binding Text}" />
</HierarchicalDataTemplate>
</ResourceDictionary>
</TreeView.Resources>
<!-- Put the root item(s) in a centered Grid so that
they will be centered and retain their width. -->
<TreeView.ItemsPanel>
<ItemsPanelTemplate>
<Grid
HorizontalAlignment="Center"
IsItemsHost="True" />
</ItemsPanelTemplate>
</TreeView.ItemsPanel>
</TreeView>
</Window>
I am not going to discuss the code which populates the TreeView with dummy data. Feel free to peruse that code (and all the rest of it) in the source code download, which is available at the top of this article.
Customizing the ControlTemplate for the TreeViewItem class was easy once I discovered a little trick. I serialized the default TreeViewItem control template to XAML and then modified that until I got the result I was looking for.
Unfortunately there is no supported way to programmatically set the selected item in a TreeView. The TreeView's SelectedItem property does not have a setter. As a result, I could not customize the keyboard navigation for the TreeView. The demo project prevents the TreeView from responding to keyboard input altogether. If you enable keyboard navigation in the demo you will find that it is very unintuitive to navigate the items. Hopefully one day there will be a way to customize the keyboard navigation of a TreeView, but until then...
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