Question

Okay what I want to do is something like this.

                      |-->  Child 1 (Parent=Root 2)         
                      |
                      |                                  Child 2.0 (Parent=Child 2)
                      |                                      Child 2.1 (Parent=Child 2)
                      |-->  Child 2 (Parent=Root 2) -------> Child 2.2 (Parent=Child 2)
                      |                                      Child 2.3 (Parent=Child 2)
                      |                                      Child 2.4 (Parent=Child 2)
                      |                                      Child 2.5 (Parent=Child 2)
                      |
                      |-->  Child 3 (Parent=Root 2) 
Root ------- Root 2 ----->  Child 4 (Parent=Root 2) -----> Root 3 ------> Root 4    
                      |-->  Child 5 (Parent=Root 2) 
                      |
                      |                                 Child 2.0 (Parent=Child 6)
                      |                                     Child 2.1 (Parent=Child 6)
                      |-->  Child 6 (Parent=Root 2) ------> Child 2.2 (Parent=Child 6)
                      |                                     Child 2.3 (Parent=Child 2)
                      |                                     Child 2.4 (Parent=Child 2)
                      |
                      |-->  Child 7 (Parent=Root 2)

Basically each Root element is Horizontal so I am using a VirtualizingStackPanel with Orientation Horizontal. Which is exactly what I want right now. Now the problem I have is the children. I basically want the Children to operate to the right of the Root 2 Element who is the parent node of the children.

I am simulating a Tree that is actually Horizontal instead of a Vertical tree.

Answer 1

If I understood your question correctly then you want something like this?

I added a 1px black border around each TreeViewItem to see what went where so that part can be removed.

I can’t think of any other solution then to re-template TreeViewItem for this. First I used a Horizontal VirtualizingStackPanel like you did. Then I removed the RowDefinitions and added another ColumnDefinition in the TreeViewItem template. To get matching Widths for the Columns I used IsSharedSizeScope for the Middle Column.

Here’s the Xaml

<TreeView Grid.IsSharedSizeScope="True">
    <TreeView.ItemsPanel>
        <ItemsPanelTemplate>
            <VirtualizingStackPanel Orientation="Horizontal"/>
        </ItemsPanelTemplate>
    </TreeView.ItemsPanel>
    <TreeView.ItemContainerStyle>
        <Style TargetType="{x:Type TreeViewItem}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type TreeViewItem}">
                        <Border BorderBrush="Black" BorderThickness="1">
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition MinWidth="19" Width="Auto"/>
                                    <ColumnDefinition Width="Auto" SharedSizeGroup="Column1"/>
                                    <ColumnDefinition Width="*" />
                                </Grid.ColumnDefinitions>
                                <ToggleButton x:Name="Expander" VerticalAlignment="Center" ClickMode="Press" IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ExpandCollapseToggleStyle}"/>
                                <Border x:Name="Bd" VerticalAlignment="Center" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.Column="1" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="true">
                                    <ContentPresenter x:Name="PART_Header" ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                                </Border>
                                <ItemsPresenter x:Name="ItemsHost" Grid.Column="2" VerticalAlignment="Center" />
                            </Grid>
                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsExpanded" Value="false">
                                <Setter Property="Visibility" TargetName="ItemsHost" Value="Collapsed"/>
                            </Trigger>
                            <Trigger Property="HasItems" Value="false">
                                <Setter Property="Visibility" TargetName="Expander" Value="Hidden"/>
                            </Trigger>
                            <Trigger Property="IsSelected" Value="true">
                                <Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
                                <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
                            </Trigger>
                            <MultiTrigger>
                                <MultiTrigger.Conditions>
                                    <Condition Property="IsSelected" Value="true"/>
                                    <Condition Property="IsSelectionActive" Value="false"/>
                                </MultiTrigger.Conditions>
                                <Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
                                <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
                            </MultiTrigger>
                            <Trigger Property="IsEnabled" Value="false">
                                <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </TreeView.ItemContainerStyle>
    <TreeView.Resources>
        <PathGeometry x:Key="TreeArrow" Figures="M0,0 L0,6 L6,0 z"/>
        <Style x:Key="ExpandCollapseToggleStyle" TargetType="{x:Type ToggleButton}">
            <Setter Property="Focusable" Value="False"/>
            <Setter Property="Width" Value="16"/>
            <Setter Property="Height" Value="16"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ToggleButton}">
                        <Border Background="Transparent" Height="16" Padding="5,5,5,5" Width="16">
                            <Path x:Name="ExpandPath" Data="{StaticResource TreeArrow}" Fill="Transparent" Stroke="#FF989898">
                                <Path.RenderTransform>
                                    <RotateTransform Angle="135" CenterY="3" CenterX="3"/>
                                </Path.RenderTransform>
                            </Path>
                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsMouseOver" Value="True">
                                <Setter Property="Stroke" TargetName="ExpandPath" Value="#FF1BBBFA"/>
                                <Setter Property="Fill" TargetName="ExpandPath" Value="Transparent"/>
                            </Trigger>
                            <Trigger Property="IsChecked" Value="True">
                                <Setter Property="RenderTransform" TargetName="ExpandPath">
                                    <Setter.Value>
                                        <RotateTransform Angle="180" CenterY="3" CenterX="3"/>
                                    </Setter.Value>
                                </Setter>
                                <Setter Property="Fill" TargetName="ExpandPath" Value="#FF595959"/>
                                <Setter Property="Stroke" TargetName="ExpandPath" Value="#FF262626"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </TreeView.Resources>
</TreeView>