WPF Expander Button Styled so it is inside Expander Header

Question

I am using the Expander control and have styled the header as shown in the picture below:

http://www.hughgrice.com/Expander.jpg

The problem I have is that I want the expander button to be contained within the header so that the line for the end of the header template aligns with the Expander content i.e. I ultimatly want to end up with something similar to the image below:

http://www.hughgrice.com/Expander.gif

Thanks in advance.

Answer 1

I see that you want to actually move the expander button into your HeaderTemplate, not just restyle it. This is easily done with FindAncestor:

First add a ToggleButton and bind its IsChecked property using FindAncestor, along these lines:

<DataTemplate x:Key="MyHeaderTemplate">
  <Border ...>
    <DockPanel>
      <!-- Expander button -->
      <ToggleButton
         IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource FindAncestor,Header,1}}"
         Content=... />

      <!-- Other content here -->
      ...
    </DockPanel>
  </Border>
</DataTemplate>           

This adds an expand button inside the header template but does not hide the original button provided by the Expander. To do this I recommend you replace the Expander's ControlTemplate.

Here is a complete copy of Expander's ControlTemplate with the ToggleButton replaced with a simple ContentPresenter:

<ControlTemplate x:Key="ExpanderWithoutButton" TargetType="{x:Type Expander}">
  <Border BorderBrush="{TemplateBinding BorderBrush}"
          BorderThickness="{TemplateBinding BorderThickness}"
          Background="{TemplateBinding Background}"
          CornerRadius="3"
          SnapsToDevicePixels="true">
    <DockPanel>
      <ContentPresenter
        Content="{TemplateBinding Header}"
        ContentTemplate="{TemplateBinding HeaderTemplate}"
        ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}"
        DockPanel.Dock="Top"
        Margin="1"
        Focusable="false" />
      <ContentPresenter
        x:Name="ExpandSite"
        Visibility="Collapsed"
        HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
        VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
        Margin="{TemplateBinding Padding}"
        Focusable="false" />
    </DockPanel>
  </Border>
  <ControlTemplate.Triggers>
    <Trigger Property="IsExpanded" Value="true">
      <Setter Property="Visibility" Value="Visible" TargetName="ExpandSite"/>
    </Trigger>
    <Trigger Property="IsEnabled" Value="false">
      <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
    </Trigger>
  </ControlTemplate.Triggers>
</ControlTemplate>

It might be used as follows:

<Expander Template="{StaticResource ExpanderWithoutButton}">

  <Expander.HeaderTemplate>
    <DataTemplate ...>
      <Border ...>
        <DockPanel>
          <ToggleButton ...
            IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource FindAncestor,Header,1}}" />
          ... other header template content here ...

A simpler alternative would be to just set a negative margin in yourHeaderTemplate to cover the expander button. Instead of the ControlTemplate shown above, your DataTemplat would just contain something like this:

<DataTemplate ...>
  <Border Margin="-20 0 0 0" ... />

Adjust the negative margin to get the look you want. This solution is simpler but inferior in that if you switch to a different system theme the required margin may change and your expander may no longer look good.