Can't get WPF TextBlock to bind to Property - wpf

Question

In my project I have a custom control which is an expander. The button's content that makes the control expand or collapse should change depending on the state. I got most of it to work but I fail at binding text to the content which I use for the button.
Here's my XAML-Code from Generic.xaml:
<ControlTemplate x:Key="PndExpanderControlVertical" TargetType="{x:Type local:PndExpanderControl}">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<dx:DXExpander x:Name="expander" Grid.Column="0" FlowDirection="LeftToRight" VerticalExpand="None" HorizontalExpand="FromLeftToRight" IsExpanded="True">
<dxlc:GroupBox x:Name="group_box" Padding="0" Header="Header"/>
</dx:DXExpander>
<Button Grid.Column="1" Padding="1" x:Name="expand_button">
<Button.Style>
<Style TargetType="Button">
<Style.Triggers>
<!-- Button-Style, expanded -->
<DataTrigger Binding="{Binding IsExpanded, ElementName=expander}" Value="True">
<Setter Property="Content" Value="↧ ↧"/>
<Setter Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="90"/>
</Setter.Value>
</Setter>
</DataTrigger>
<!-- Button-Style, collapsed -->
<DataTrigger Binding="{Binding IsExpanded, ElementName=expander}" Value="False">
<Setter Property="Content">
<Setter.Value>
<TextBlock>
<TextBlock Text="↥ "/>
<TextBlock Text="{Binding Header, ElementName=group_box}"/>
<TextBlock Text=" ↥"/>
</TextBlock>
</Setter.Value>
</Setter>
<Setter Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="90"/>
</Setter.Value>
</Setter>
</DataTrigger>
</Style.Triggers>
</Style>
</Button.Style>
</Button>
</Grid>
</Border>
</ControlTemplate>
As you can see I use a nested text block to combine the group_box's header with some arrows. However the binding of the middle text block does not work.
To be honest, I work with WPF for a while now but data binding (to the right source) is still a mystery to me. Most of the time I get it to work somehow but if it fails I've no Idea what to do. I googled a few hours and tried various thing but nothing worked for me.
Any help is appreciated.

Answer 1

You could bind to the Header of the Content of the Expander:
<TextBlock Text="↥ "/>
<TextBlock Text="{Binding Content.Header, ElementName=expander}"/>
<TextBlock Text=" ↥"/>
You cannot use an ElementName to bind directly to the GroupBox since it is not in the same name scope as the Button.
Edit:
Ok, it only works if the expander is collapsed by default.
But you could use an x:Reference to bind to the GroupBox:
<TextBlock Text="↥ "/>
<TextBlock Text="{Binding Header, Source={x:Reference group_box}}"/>
<TextBlock Text=" ↥"/>