具有多个按钮的WPF扩展器

Question

I have a requirement to include buttons that will trigger logic on an Expander in WPF. I have included an image below.

I'm unsure how to accomplish this in WPF, or if its possible. The buttons needs to wire to backend fucntionality to create objects. Each is an expander and should have those buttons on the right of the expander header regardless if they are expanded or not. The button should call _Click method on the code behind when clicked.

Can this be done with an expander and how can I accomplish this?

Answer 1

To make your Button s appear at the extreme right, you have to use a DockPanel as your Expander.HeaderTemplate . But Expander uses ToggleButton in its top part where expanding circle is shown. This ToggleButton is having its HorizontalAlignment set as Left. So, we need to change this setting in the Style with ExpanderDownHeaderStyle Key as

<ContentPresenter Grid.Column="1" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" ... />

Copy paste the below piece of XAML as it is.

<Window.Resources>
        <SolidColorBrush x:Key="Expander.MouseOver.Circle.Stroke" Color="#FF3C7FB1"/>
        <SolidColorBrush x:Key="Expander.MouseOver.Circle.Fill" Color="Transparent"/>
        <SolidColorBrush x:Key="Expander.MouseOver.Arrow.Stroke" Color="#222"/>
        <SolidColorBrush x:Key="Expander.Pressed.Circle.Stroke" Color="#FF526C7B"/>
        <SolidColorBrush x:Key="Expander.Pressed.Circle.Fill" Color="Transparent"/>
        <SolidColorBrush x:Key="Expander.Pressed.Arrow.Stroke" Color="#FF003366"/>
        <SolidColorBrush x:Key="Expander.Disabled.Circle.Stroke" Color="DarkGray"/>
        <SolidColorBrush x:Key="Expander.Disabled.Circle.Fill" Color="Transparent"/>
        <SolidColorBrush x:Key="Expander.Disabled.Arrow.Stroke" Color="#666"/>
        <SolidColorBrush x:Key="Expander.Static.Circle.Fill" Color="Transparent"/>
        <SolidColorBrush x:Key="Expander.Static.Circle.Stroke" Color="DarkGray"/>
        <SolidColorBrush x:Key="Expander.Static.Arrow.Stroke" Color="#666"/>
        <Style x:Key="ExpanderRightHeaderStyle" TargetType="{x:Type ToggleButton}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ToggleButton}">
                        <Border Padding="{TemplateBinding Padding}">
                            <Grid Background="Transparent" SnapsToDevicePixels="False">
                                <Grid.RowDefinitions>
                                    <RowDefinition Height="19"/>
                                    <RowDefinition Height="*"/>
                                </Grid.RowDefinitions>
                                <Grid>
                                    <Grid.LayoutTransform>
                                        <TransformGroup>
                                            <TransformGroup.Children>
                                                <TransformCollection>
                                                    <RotateTransform Angle="-90"/>
                                                </TransformCollection>
                                            </TransformGroup.Children>
                                        </TransformGroup>
                                    </Grid.LayoutTransform>
                                    <Ellipse x:Name="circle" Fill="{StaticResource Expander.Static.Circle.Fill}" HorizontalAlignment="Center" Height="19" Stroke="{StaticResource Expander.Static.Circle.Stroke}" VerticalAlignment="Center" Width="19"/>
                                    <Path x:Name="arrow" Data="M 1,1.5 L 4.5,5 L 8,1.5" HorizontalAlignment="Center" SnapsToDevicePixels="false" Stroke="{StaticResource Expander.Static.Arrow.Stroke}" StrokeThickness="2" VerticalAlignment="Center"/>
                                </Grid>
                                <ContentPresenter HorizontalAlignment="Center" Margin="0,4,0,0" Grid.Row="1" RecognizesAccessKey="True" SnapsToDevicePixels="True" VerticalAlignment="Top"/>
                            </Grid>
                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsChecked" Value="true">
                                <Setter Property="Data" TargetName="arrow" Value="M 1,4.5  L 4.5,1  L 8,4.5"/>
                            </Trigger>
                            <Trigger Property="IsMouseOver" Value="true">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.MouseOver.Circle.Stroke}"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.MouseOver.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.MouseOver.Arrow.Stroke}"/>
                            </Trigger>
                            <Trigger Property="IsPressed" Value="true">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.Pressed.Circle.Stroke}"/>
                                <Setter Property="StrokeThickness" TargetName="circle" Value="1.5"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.Pressed.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.Pressed.Arrow.Stroke}"/>
                            </Trigger>
                            <Trigger Property="IsEnabled" Value="false">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.Disabled.Circle.Stroke}"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.Disabled.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.Disabled.Arrow.Stroke}"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
        <Style x:Key="ExpanderUpHeaderStyle" TargetType="{x:Type ToggleButton}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ToggleButton}">
                        <Border Padding="{TemplateBinding Padding}">
                            <Grid Background="Transparent" SnapsToDevicePixels="False">
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="19"/>
                                    <ColumnDefinition Width="*"/>
                                </Grid.ColumnDefinitions>
                                <Grid>
                                    <Grid.LayoutTransform>
                                        <TransformGroup>
                                            <TransformGroup.Children>
                                                <TransformCollection>
                                                    <RotateTransform Angle="180"/>
                                                </TransformCollection>
                                            </TransformGroup.Children>
                                        </TransformGroup>
                                    </Grid.LayoutTransform>
                                    <Ellipse x:Name="circle" Fill="{StaticResource Expander.Static.Circle.Fill}" HorizontalAlignment="Center" Height="19" Stroke="{StaticResource Expander.Static.Circle.Stroke}" VerticalAlignment="Center" Width="19"/>
                                    <Path x:Name="arrow" Data="M 1,1.5 L 4.5,5 L 8,1.5" HorizontalAlignment="Center" SnapsToDevicePixels="false" Stroke="{StaticResource Expander.Static.Arrow.Stroke}" StrokeThickness="2" VerticalAlignment="Center"/>
                                </Grid>
                                <ContentPresenter Grid.Column="1" HorizontalAlignment="Left" Margin="4,0,0,0" RecognizesAccessKey="True" SnapsToDevicePixels="True" VerticalAlignment="Center"/>
                            </Grid>
                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsChecked" Value="true">
                                <Setter Property="Data" TargetName="arrow" Value="M 1,4.5  L 4.5,1  L 8,4.5"/>
                            </Trigger>
                            <Trigger Property="IsMouseOver" Value="true">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.MouseOver.Circle.Stroke}"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.MouseOver.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.MouseOver.Arrow.Stroke}"/>
                            </Trigger>
                            <Trigger Property="IsPressed" Value="true">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.Pressed.Circle.Stroke}"/>
                                <Setter Property="StrokeThickness" TargetName="circle" Value="1.5"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.Pressed.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.Pressed.Arrow.Stroke}"/>
                            </Trigger>
                            <Trigger Property="IsEnabled" Value="false">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.Disabled.Circle.Stroke}"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.Disabled.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.Disabled.Arrow.Stroke}"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
        <Style x:Key="ExpanderLeftHeaderStyle" TargetType="{x:Type ToggleButton}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ToggleButton}">
                        <Border Padding="{TemplateBinding Padding}">
                            <Grid Background="Transparent" SnapsToDevicePixels="False">
                                <Grid.RowDefinitions>
                                    <RowDefinition Height="19"/>
                                    <RowDefinition Height="*"/>
                                </Grid.RowDefinitions>
                                <Grid>
                                    <Grid.LayoutTransform>
                                        <TransformGroup>
                                            <TransformGroup.Children>
                                                <TransformCollection>
                                                    <RotateTransform Angle="90"/>
                                                </TransformCollection>
                                            </TransformGroup.Children>
                                        </TransformGroup>
                                    </Grid.LayoutTransform>
                                    <Ellipse x:Name="circle" Fill="{StaticResource Expander.Static.Circle.Fill}" HorizontalAlignment="Center" Height="19" Stroke="{StaticResource Expander.Static.Circle.Stroke}" VerticalAlignment="Center" Width="19"/>
                                    <Path x:Name="arrow" Data="M 1,1.5 L 4.5,5 L 8,1.5" HorizontalAlignment="Center" SnapsToDevicePixels="false" Stroke="{StaticResource Expander.Static.Arrow.Stroke}" StrokeThickness="2" VerticalAlignment="Center"/>
                                </Grid>
                                <ContentPresenter HorizontalAlignment="Center" Margin="0,4,0,0" Grid.Row="1" RecognizesAccessKey="True" SnapsToDevicePixels="True" VerticalAlignment="Top"/>
                            </Grid>
                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsChecked" Value="true">
                                <Setter Property="Data" TargetName="arrow" Value="M 1,4.5  L 4.5,1  L 8,4.5"/>
                            </Trigger>
                            <Trigger Property="IsMouseOver" Value="true">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.MouseOver.Circle.Stroke}"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.MouseOver.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.MouseOver.Arrow.Stroke}"/>
                            </Trigger>
                            <Trigger Property="IsPressed" Value="true">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.Pressed.Circle.Stroke}"/>
                                <Setter Property="StrokeThickness" TargetName="circle" Value="1.5"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.Pressed.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.Pressed.Arrow.Stroke}"/>
                            </Trigger>
                            <Trigger Property="IsEnabled" Value="false">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.Disabled.Circle.Stroke}"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.Disabled.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.Disabled.Arrow.Stroke}"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
        <Style x:Key="ExpanderHeaderFocusVisual">
            <Setter Property="Control.Template">
                <Setter.Value>
                    <ControlTemplate>
                        <Border>
                            <Rectangle Margin="0" SnapsToDevicePixels="true" Stroke="Black" StrokeThickness="1" StrokeDashArray="1 2"/>
                        </Border>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
        <Style x:Key="ExpanderDownHeaderStyle" TargetType="{x:Type ToggleButton}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ToggleButton}">
                        <Border Padding="{TemplateBinding Padding}">
                            <Grid Background="Transparent" SnapsToDevicePixels="False">
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="19"/>
                                    <ColumnDefinition Width="*"/>
                                </Grid.ColumnDefinitions>
                                <Ellipse x:Name="circle" Fill="{StaticResource Expander.Static.Circle.Fill}" HorizontalAlignment="Center" Height="19" Stroke="{StaticResource Expander.Static.Circle.Stroke}" VerticalAlignment="Center" Width="19"/>
                                <Path x:Name="arrow" Data="M 1,1.5 L 4.5,5 L 8,1.5" HorizontalAlignment="Center" SnapsToDevicePixels="false" Stroke="{StaticResource Expander.Static.Arrow.Stroke}" StrokeThickness="2" VerticalAlignment="Center"/>
                                <ContentPresenter Grid.Column="1" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" Margin="4,0,0,0" RecognizesAccessKey="True" SnapsToDevicePixels="True" VerticalAlignment="Center"/>
                            </Grid>
                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsChecked" Value="true">
                                <Setter Property="Data" TargetName="arrow" Value="M 1,4.5  L 4.5,1  L 8,4.5"/>
                            </Trigger>
                            <Trigger Property="IsMouseOver" Value="true">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.MouseOver.Circle.Stroke}"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.MouseOver.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.MouseOver.Arrow.Stroke}"/>
                            </Trigger>
                            <Trigger Property="IsPressed" Value="true">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.Pressed.Circle.Stroke}"/>
                                <Setter Property="StrokeThickness" TargetName="circle" Value="1.5"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.Pressed.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.Pressed.Arrow.Stroke}"/>
                            </Trigger>
                            <Trigger Property="IsEnabled" Value="false">
                                <Setter Property="Stroke" TargetName="circle" Value="{StaticResource Expander.Disabled.Circle.Stroke}"/>
                                <Setter Property="Fill" TargetName="circle" Value="{StaticResource Expander.Disabled.Circle.Fill}"/>
                                <Setter Property="Stroke" TargetName="arrow" Value="{StaticResource Expander.Disabled.Arrow.Stroke}"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
        <Style x:Key="ExpanderStyle1" TargetType="{x:Type Expander}">
            <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
            <Setter Property="Background" Value="Transparent"/>
            <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
            <Setter Property="VerticalContentAlignment" Value="Stretch"/>
            <Setter Property="BorderBrush" Value="Transparent"/>
            <Setter Property="BorderThickness" Value="1"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type Expander}">
                        <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="3" SnapsToDevicePixels="true">
                            <DockPanel>
                                <ToggleButton x:Name="HeaderSite" ContentTemplate="{TemplateBinding HeaderTemplate}" ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}" Content="{TemplateBinding Header}" DockPanel.Dock="Top" Foreground="{TemplateBinding Foreground}" FontWeight="{TemplateBinding FontWeight}" FocusVisualStyle="{StaticResource ExpanderHeaderFocusVisual}" FontStyle="{TemplateBinding FontStyle}" FontStretch="{TemplateBinding FontStretch}" FontSize="{TemplateBinding FontSize}" FontFamily="{TemplateBinding FontFamily}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Margin="1" MinWidth="0" MinHeight="0" Padding="{TemplateBinding Padding}" Style="{StaticResource ExpanderDownHeaderStyle}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
                                <ContentPresenter x:Name="ExpandSite" DockPanel.Dock="Bottom" Focusable="false" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" Visibility="Collapsed" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                            </DockPanel>
                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsExpanded" Value="true">
                                <Setter Property="Visibility" TargetName="ExpandSite" Value="Visible"/>
                            </Trigger>
                            <Trigger Property="ExpandDirection" Value="Right">
                                <Setter Property="DockPanel.Dock" TargetName="ExpandSite" Value="Right"/>
                                <Setter Property="DockPanel.Dock" TargetName="HeaderSite" Value="Left"/>
                                <Setter Property="Style" TargetName="HeaderSite" Value="{StaticResource ExpanderRightHeaderStyle}"/>
                            </Trigger>
                            <Trigger Property="ExpandDirection" Value="Up">
                                <Setter Property="DockPanel.Dock" TargetName="ExpandSite" Value="Top"/>
                                <Setter Property="DockPanel.Dock" TargetName="HeaderSite" Value="Bottom"/>
                                <Setter Property="Style" TargetName="HeaderSite" Value="{StaticResource ExpanderUpHeaderStyle}"/>
                            </Trigger>
                            <Trigger Property="ExpandDirection" Value="Left">
                                <Setter Property="DockPanel.Dock" TargetName="ExpandSite" Value="Left"/>
                                <Setter Property="DockPanel.Dock" TargetName="HeaderSite" Value="Right"/>
                                <Setter Property="Style" TargetName="HeaderSite" Value="{StaticResource ExpanderLeftHeaderStyle}"/>
                            </Trigger>
                            <Trigger Property="IsEnabled" Value="false">
                                <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>    

Expander usage

<Expander Grid.Row="1" HorizontalContentAlignment="Stretch" HorizontalAlignment="Stretch" Style="{DynamicResource ExpanderStyle1}">            
    <Expander.HeaderTemplate>
        <DataTemplate>
            <DockPanel LastChildFill="False" Background="LimeGreen">
                <TextBlock Text="Expander1"/>
                <Button Content="Press" DockPanel.Dock="Right"/>
                <Button Content="Press" DockPanel.Dock="Right"/>
            </DockPanel>
        </DataTemplate>
    </Expander.HeaderTemplate>
    <StackPanel Background="LightSteelBlue"></StackPanel>
</Expander>

Output :

Answer 2

I'd suggest that you craft a specific class that does the job for you, here is a primitive skeleton along explanations.

On the top there's a sort of bread-crumb bar , which are the actual buttons that when pressed will expand the targeted section.

ExpanderGroup class, responsible of rendering and some UI logic

There are 2 collections, one for the headers of the bread-crumb bar, the other are for the Expander s themselves.

public partial class ExpanderGroup : UserControl
{
    public static readonly DependencyProperty ChildrenProperty = DependencyProperty.Register(
        "Children", typeof (ObservableCollection<Expander>), typeof (ExpanderGroup),
        new PropertyMetadata(default(ObservableCollection<Expander>)));

    public static readonly DependencyProperty ChildrenHeadersProperty = DependencyProperty.Register(
        "ChildrenHeaders", typeof (ObservableCollection<string>), typeof (ExpanderGroup),
        new PropertyMetadata(default(ObservableCollection<string>)));

    public ExpanderGroup()
    {
        InitializeComponent();
        Children = new ObservableCollection<Expander>();
        ChildrenHeaders = new ObservableCollection<string>();
    }

    public ObservableCollection<Expander> Children
    {
        get { return (ObservableCollection<Expander>) GetValue(ChildrenProperty); }
        set { SetValue(ChildrenProperty, value); }
    }

    public ObservableCollection<string> ChildrenHeaders
    {
        get { return (ObservableCollection<string>) GetValue(ChildrenHeadersProperty); }
        set { SetValue(ChildrenHeadersProperty, value); }
    }

    private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
    {
        // find the corresponding expander to toggle button clicked
        // it is very primitive as it expects an exact mapping by name
        var button = (ToggleButton) sender;
        var text = (string) button.Content;
        var expander = Children.Single(s => (string) s.Header == text);

        // toggle expansion status
        expander.IsExpanded = button.IsChecked != null && (bool) button.IsChecked;

        // little helper that expands the root expander
        // (though its template should be changed for better UX)
        RootExpander.IsExpanded = Children.Any(s => s.IsExpanded);
    }
}

Notes:

• I used a really simple logic, ie I retrieve the corresponding Expander according its header, you could enhance that further, either by referring to its index or any other means.
• There is a 2nd collection ChildrenHeaders , because if you'd try to fetch Expanders from 2 sources, they would be appear only on one of the containers since an UIElement can have only one parent.
• You might also want to craft a mechanism so that when an Expander is directly manipulated, the according button in the bread crumb is toggled or not. Hint : a good place for this would be the PropertyChangedCallback of Children where you'd build a little more inspired relationship between a toggle button and the targeted Expander

ExpanderGroup XAML part

<UserControl x:Class="delme.ExpanderGroup"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:local="clr-namespace:delme"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:system="clr-namespace:System;assembly=mscorlib"
             x:Name="Root"
             d:DesignHeight="300"
             d:DesignWidth="300"
             mc:Ignorable="d">
    <Grid>
        <StackPanel>

            <ItemsControl Background="LightBlue" ItemsSource="{Binding ElementName=Root, Path=ChildrenHeaders}">
                <ItemsControl.ItemsPanel>
                    <ItemsPanelTemplate>
                        <StackPanel IsItemsHost="True" Orientation="Horizontal" />
                    </ItemsPanelTemplate>
                </ItemsControl.ItemsPanel>
                <ItemsControl.ItemTemplate>
                    <DataTemplate DataType="system:String">
                        <ToggleButton Width="75"
                                      Height="22"
                                      Margin="2"
                                      Click="ButtonBase_OnClick"
                                      Content="{Binding}" />
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>

            <Expander x:Name="RootExpander">
                <ItemsControl Background="LightCyan" ItemsSource="{Binding ElementName=Root, Path=Children}">
                    <ItemsControl.ItemsPanel>
                        <ItemsPanelTemplate>
                            <StackPanel IsItemsHost="True" />
                        </ItemsPanelTemplate>
                    </ItemsControl.ItemsPanel>
                </ItemsControl>
            </Expander>

        </StackPanel>
    </Grid>
</UserControl>

Nothing extraordinary here, just kept it to the bare minimum for the sake of the example. You might tweak it a little more, especially re-defining the template of RootExpander as IMO that little arrow should be simply hidden for a better UX. You can/should use Blend to very easily tweak its template.

Usage example

<Window x:Class="delme.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:local="clr-namespace:delme"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:system="clr-namespace:System;assembly=mscorlib"
        Title="MainWindow"
        Width="525"
        Height="350"
        mc:Ignorable="d">
    <Grid>

        <local:ExpanderGroup>
            <local:ExpanderGroup.ChildrenHeaders>
                <system:String>Expander1</system:String>
                <system:String>Expander2</system:String>
            </local:ExpanderGroup.ChildrenHeaders>
            <local:ExpanderGroup.Children>
                <Expander Header="Expander1">
                    <TextBlock Text="abcd" />
                </Expander>
                <Expander Header="Expander2">
                    <TextBlock Text="efgh" />
                </Expander>
            </local:ExpanderGroup.Children>
        </local:ExpanderGroup>

    </Grid>
</Window>

Go on and improve upon that design !