How to generate groups and items via ItemsSource

Question Comments

Hello,

This example has a binding error:
System.Windows.Data Error: 40 : BindingExpression path error: 'Content' property not found on 'object' ''Item'

To fix it I've changed the DataTemplate as follows:
                   <DataTemplate>
                       <TextBlock Text="{Binding Header}" Background="Black" Foreground="Yellow"/>
                   </DataTemplate>

Without it I see black bars instead of items.

Best regards,
Mikhail