如何获取组中的第一个ListBoxItem

Question

I have a ListBox sorted and grouped by SortDescription and GroupDescription and now I want to know whether there is a way to know which item is the first one in each group. For example if I have a list of names I want to get the first Item in the list which starts with A, B, C, etc and change its template.

To be more clear I should say I want to change the DataTemplate of the first ListBoxItem in each group. How is it possible?

Edit

This is my sample CollectionView

<CollectionViewSource
x:Key="Myiew"
Source="{Binding Items}">
    <CollectionViewSource.GroupDescriptions>
        <PropertyGroupDescription PropertyName="FirstName" Converter="{StaticResource StringToFirstLetter}" />
    </CollectionViewSource.GroupDescriptions>
    <CollectionViewSource.SortDescriptions>
        <scm:SortDescription PropertyName="FirstName" />
    </CollectionViewSource.SortDescriptions>
</CollectionViewSource>

<Listbox ItemsSource={"StaticResources Myiew"} />

As I said everything works fine. the Items are sorted and if I change the GroupStyle I see that the items are grouped, but I don't want to set a group style. I just want to change the DataTemplate of the first item in each group.

Answer 1

Here is a sample for you

• create a converter class

this will just see if the value passed is null or not

class FirstItemDetector : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
       return value == null;
    }

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

then comes the declaration of converter where l: is the namespace to your converter class

<l:FirstItemDetector x:Key="FirstItemDetactor" />

then comes the listbox

• I added a group style (you can style however you like) solution is group independent
• added a data template as ItemTemplate of the listbox
• added a DataTrigger on PreviousData with converter as FirstItemDetactor declared as above
• in the setter of trigger I changed the foreground to red ( you have many choice including changing styles, show hide elements etc.

here is the listbox

    <ListBox ItemsSource="{Binding Source={StaticResource Myiew}}">
        <ListBox.GroupStyle>
            <GroupStyle>
                <GroupStyle.ContainerStyle>
                    <Style TargetType="{x:Type GroupItem}">
                        <Setter Property="Template">
                            <Setter.Value>
                                <ControlTemplate TargetType="{x:Type GroupItem}">
                                    <Grid>
                                        <Grid.RowDefinitions>
                                            <RowDefinition Height="auto" />
                                            <RowDefinition />
                                        </Grid.RowDefinitions>
                                        <Border BorderBrush="Black"
                                                BorderThickness=".5"
                                                Padding="4">
                                            <TextBlock Text="{Binding Name}"
                                                       HorizontalAlignment="Center" />
                                        </Border>
                                        <ItemsPresenter Grid.Row="1" />
                                    </Grid>
                                </ControlTemplate>
                            </Setter.Value>
                        </Setter>
                    </Style>
                </GroupStyle.ContainerStyle>
            </GroupStyle>
        </ListBox.GroupStyle>
        <ListBox.ItemTemplate>
            <DataTemplate>
                <TextBlock Text="{Binding}" x:Name="text" />
                <DataTemplate.Triggers>
                    <DataTrigger Binding="{Binding RelativeSource={RelativeSource PreviousData},Converter={StaticResource FirstItemDetactor}}"
                                 Value="True">
                        <Setter TargetName="text"
                                Property="Foreground"
                                Value="Red" />
                    </DataTrigger>
                </DataTemplate.Triggers>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

main role is played by the datatigger and the converter, it will work independently, with groups or even nested groups.