WPF GroupStyle only for Groups with more than 1 item

Question

I am trying to show different Groups in a Listview which contain an expander and a header. However I don't want my SecondLevel Group to show a header if the Group only contains 1 item. Since this would be quite inconvenient. My Code:

<Window x:Class="ListViewGrouping.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:listViewGrouping="clr-namespace:ListViewGrouping"
    Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
    <listViewGrouping:GroupItemStyleSelector x:Key="groupItemStyleSelector"/>
    <!-- Style for the first level GroupItem -->
        <Style x:Key="FirstLevel" TargetType="{x:Type GroupItem}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type GroupItem}">
                        <Expander IsExpanded="True">
                            <Expander.Header>
                                <Grid>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="Auto"/>
                                        <ColumnDefinition Width="Auto"/>
                                        <ColumnDefinition Width="Auto"/>
                                    </Grid.ColumnDefinitions>
                                    <TextBlock Grid.Column="0" Margin="3.5,0" Text="{Binding Name}" TextBlock.FontWeight="Bold"/>
                                    <TextBlock Grid.Column="1" Margin="3.5,0" Text="Elements:"/>
                                    <TextBlock Grid.Column="2" Margin="3.5,0" Text="{Binding ItemCount}"/>
                                </Grid>
                            </Expander.Header>
                            <ItemsPresenter />
                        </Expander>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
        <!-- Style for the second level GroupItem -->
        <Style x:Key="SecondLevel" TargetType="{x:Type GroupItem}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type GroupItem}">
                        <Expander IsExpanded="True" Margin="15,0,0,0">
                            <Expander.Header>
                                <TextBlock Text="{Binding Name}" TextBlock.FontWeight="Bold"/>
                            </Expander.Header>
                            <ItemsPresenter />
                        </Expander>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Grid>
        <ListView Name="mailView" ItemsSource="{Binding}">
            <ListView.GroupStyle>
                <GroupStyle ContainerStyleSelector="{StaticResource groupItemStyleSelector}" />
            </ListView.GroupStyle>
            <ListView.View>
                <GridView>
                    <GridViewColumn Header="ID" DisplayMemberBinding="{Binding ID}"/>
                    <GridViewColumn Header="Subject" DisplayMemberBinding="{Binding Subject}"/>
                    <GridViewColumn Header="Sender" DisplayMemberBinding="{Binding Sender}"/>
                    <GridViewColumn Header="Support-ID" DisplayMemberBinding="{Binding Support_ID}"/>
                    <GridViewColumn Header="Supporter" DisplayMemberBinding="{Binding Supporter}"/>
                    <GridViewColumn Header="Received" DisplayMemberBinding="{Binding ReceivedDate}"/>
                </GridView>
            </ListView.View>
        </ListView>
    </Grid>

Code behind:

    private void createMailList()
    {
        _mails.Add(new Mail("LIC", "Lizenz geht nicht", "Xeun", "LIC.2013.01.10.002", "Xeun", "25.09.2013"));
        _mails.Add(new Mail("CD", "Alles doof", "Xeun", "CD.2013.01.10.002", "Xeun", "25.09.2013"));
        _mails.Add(new Mail("CD", "Re:Alles doof", "Xeun", "CD.2013.01.10.002", "Xeun", "25.09.2013"));
        _mails.Add(new Mail("CD", "CD kaputt", "Xeun", "CD.2013.01.10.003", "Xeun", "25.09.2013"));
        _mails.Add(new Mail("CD", "Geht nicht", "Xeun", "CD.2013.01.10.001", "Xeun", "25.09.2013"));
        _mails.Add(new Mail("LIC", "Kaputt", "Xeun", "LIC.2013.01.10.001", "Xeun", "25.09.2013"));
    }
    public MainWindow()
    {

        InitializeComponent();

        createMailList();
        DataContext = _mails;
        ICollectionView view = CollectionViewSource.GetDefaultView(_mails);
        PropertyGroupDescription groupDescription = new PropertyGroupDescription("ID");
        view.GroupDescriptions.Add(groupDescription);
        view.GroupDescriptions.Add(new PropertyGroupDescription("Support_ID"));
    }

}
public class GroupItemStyleSelector : StyleSelector
{
    public override Style SelectStyle(object item, DependencyObject container)
    {
        Style s;

        CollectionViewGroup group = item as CollectionViewGroup;
        Window window = Application.Current.MainWindow;

        if (!group.IsBottomLevel)
        {
            s = window.FindResource("FirstLevel") as Style;
        }
        else
        {
            s = window.FindResource("SecondLevel") as Style;
        }

        return s;
    }
}

I hope I explained my problem well enough - I have attached a screenshot of the small app - the Groups marked as red only contain one item and should not be shown as group.

Answer 1

1. For both styles split the ControlTemplate in 2, one with an expander and one with out.

2. Create a converter which checks the if the group size (your group is of type CollectionViewGroup)

    return yourGroup.Items.Count > 1 
   

3. Place a DataTrigger as seen below in each style which checks the groups size via the converter (your DataContext is your group so the binding is Binding="{Binding}"

xaml :

 <ControlTemplate TargetType="{x:Type GroupItem}" x:Key="withExpander">
     <Expander IsExpanded="True">
         <Expander.Header>
               .....
         </Expander.Header>
         <ItemsPresenter />
      </Expander>
  </ControlTemplate>


  <ControlTemplate TargetType="{x:Type GroupItem}" x:Key="withOutExpander">        
        <ItemsPresenter />          
  </ControlTemplate>

  <Style x:Key="FirstLevel" TargetType="{x:Type GroupItem}">
      <Setter Property="Template" Value="{StaticResource withExpander}" />

      <Style.Triggers>
          <DataTrigger Binding="{Binding , Converter={StaticResource GroupSizeToExpanderConverter}" Value="False">
                  <Setter Property="Template" Value="{StaticResource withOutExpander}"/>
          </DataTrigger>    
      </Style.Triggers>
  </Style>

Edit :

*the converter value will be the Group itself (of type CollectionViewGroup)

The Converter :

public class GroupSizeToExpanderConverter: IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        CollectionViewGroup grp = (CollectionViewGroup)value;
        return grp.Items.Count() > 1; // ALTERNATIVLY grp.ItemCount;             
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}