c# - Replace part of default template in WPF

Question

is there any "best practice" way to replace a part of the default template. The current use case is a treeview. As default, the treeview has this small triangle shapes to expand and collapse.

I know how to replace these if I replace the whole control template, as shown in the code below. I am not sure if there is a way to "keep all default, just change XY". Its not a style, I basically need to replace a part of an existing control template.

To illustrate, take a look at the following XAML. The first smaller block is the relevant XAML I want to be able to adapt.

The bigger second and third part are basically a copy of the default templates, only to administer the "changed" part from the beginning.

Is there a better way to do this, saving the long and confusing XAML in the second half?

        <ResourceDictionary 
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >





  <Style x:Key="ExpandCollapseToggleStyle" TargetType="ToggleButton">
    <Setter Property="Focusable" Value="False"/>
     <Setter Property="Template">
      <Setter.Value>
        <ControlTemplate TargetType="ToggleButton">
          <Grid
            Width="15"
            Height="13"
            Background="Transparent">
            <Path x:Name="ExpandPath"
              HorizontalAlignment="Left" 
              VerticalAlignment="Center" 
              Margin="1,1,1,1"
              Fill="Black"
              Data="M 4 0 L 8 4 L 4 8 Z"/>
          </Grid>
          <ControlTemplate.Triggers>
            <Trigger Property="IsChecked"
                 Value="True">
              <Setter Property="Data"
                  TargetName="ExpandPath"
                  Value="M 0 4 L 8 4 L 4 8 Z"/>
            </Trigger>
          </ControlTemplate.Triggers>
        </ControlTemplate>
      </Setter.Value>
    </Setter>
  </Style>

<Setter Property="Template">
  <Setter.Value>
    <ControlTemplate TargetType="{x:Type TreeViewItem}">
      <Grid>
        <Grid.ColumnDefinitions>
          <ColumnDefinition MinWidth="19"
                    Width="Auto"/>
          <ColumnDefinition Width="Auto"/>
          <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
          <RowDefinition Height="Auto"/>
          <RowDefinition/>
        </Grid.RowDefinitions>
        <ToggleButton x:Name="Expander"
                Style="{StaticResource ExpandCollapseToggleStyle}"
                IsChecked="{Binding Path=IsExpanded,
                            RelativeSource={RelativeSource TemplatedParent}}"
                ClickMode="Press"/>
        <Border Name="Bd"
            Grid.Column="1"
            Background="{TemplateBinding Background}"
            BorderBrush="{TemplateBinding BorderBrush}"
            BorderThickness="{TemplateBinding BorderThickness}"
            Padding="{TemplateBinding Padding}">
          <ContentPresenter x:Name="PART_Header"
                    ContentSource="Header"
                    HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"/>
        </Border>
        <ItemsPresenter x:Name="ItemsHost"
                Grid.Row="1"
                Grid.Column="1"
                Grid.ColumnSpan="2"/>
      </Grid>
      <ControlTemplate.Triggers>
        <Trigger Property="IsExpanded"
             Value="false">
          <Setter TargetName="ItemsHost"
              Property="Visibility"
              Value="Collapsed"/>
        </Trigger>
        <Trigger Property="HasItems"
             Value="false">
          <Setter TargetName="Expander"
              Property="Visibility"
              Value="Hidden"/>
        </Trigger>
        <MultiTrigger>
          <MultiTrigger.Conditions>
            <Condition Property="HasHeader"
                   Value="false"/>
            <Condition Property="Width"
                   Value="Auto"/>
          </MultiTrigger.Conditions>
          <Setter TargetName="PART_Header"
              Property="MinWidth"
              Value="75"/>
        </MultiTrigger>
        <MultiTrigger>
          <MultiTrigger.Conditions>
            <Condition Property="HasHeader"
                   Value="false"/>
            <Condition Property="Height"
                   Value="Auto"/>
          </MultiTrigger.Conditions>
          <Setter TargetName="PART_Header"
              Property="MinHeight"
              Value="19"/>
        </MultiTrigger>
        <Trigger Property="IsSelected"
             Value="true">
          <Setter TargetName="Bd"
              Property="Background"
              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 TargetName="Bd"
              Property="Background"
              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>

Answer 1

Unfortunately, I think you have to replace the entire template:

From MSDN: http://msdn.microsoft.com/en-us/library/aa970773.aspx

Controls in Windows Presentation Foundation (WPF) have a ControlTemplate that contains the visual tree of that control. You can change the structure and appearance of a control by modifying the ControlTemplate of that control. There is no way to replace only part of the visual tree of a control; to change the visual tree of a control you must set the Template property of the control to its new and complete ControlTemplate.