Wsrtjtyk

Scenario: a datagrid to show custom events from custom sources.
The grid must group rows by source, ordering events by datetime descending (newer always on top). No manual sorting allowed, virtualization enabled, MVVM pattern.
Here's my issue: the group header itself must follow the datetime ordering, that is the group containg the latest event must scale to top.

ViewModel:

public class Event

{

   public Int32 Source_Id { get; set; }

   public String Source_Name { get; set; }

   public DateTime Event_DateTime { get; set; }

}

CollectionView is grouped by Source_Id and sorted by Event_DateTime:

<CollectionViewSource x:Key='alarms_src' Source="{Binding Alarms}">                

   <CollectionViewSource.GroupDescriptions>

      <PropertyGroupDescription PropertyName="Source_Id" />

   </CollectionViewSource.GroupDescriptions>

   <CollectionViewSource.SortDescriptions>

      <scm:SortDescription PropertyName="Event_DateTime" Direction="Descending" />

   </CollectionViewSource.SortDescriptions>

</CollectionViewSource>

GroupStyle is a simple expander with source name and latest event datetime into the header:

<DataGrid.GroupStyle>

    <GroupStyle>

        <GroupStyle.ContainerStyle>

            <Style TargetType="{x:Type GroupItem}">

                <Setter Property="Margin" Value="4"/>

                <Setter Property="Template">

                    <Setter.Value>

                        <ControlTemplate TargetType="{x:Type GroupItem}">

                            <Expander IsExpanded="True" Background="LightGray">

                                <Expander.Header>



                                    <StackPanel Orientation="Horizontal" Background="Transparent">



                                        <TextBlock Margin="8" Text="{Binding Items[0].Source_Name}"/>

                                        <TextBlock Margin="8" Text="{Binding Items[0].Event_DateTime}"/>



                                    </StackPanel>

                                </Expander.Header>



                                <Expander.Content>

                                    <ItemsPresenter/>

                                </Expander.Content>



                            </Expander>

                        </ControlTemplate>

                    </Setter.Value>

                </Setter>

            </Style>

        </GroupStyle.ContainerStyle>

    </GroupStyle>                

</DataGrid.GroupStyle>

Output:
simplified grid

The red arrows in the image points to the 4th row I've generated, with the latest timestamp.
The grid is correctly grouping and sorting INSIDE each group, but what I expect is that the Device_1 group would scale on top of the grid: how to obtain this?

Update: wetransfer link to sample project

CollectionViewSource.GetDefaultView(testdg.ItemsSource).Refresh(); 

Here testdg is my DataGrid Control name.

What i thought is to refresh the itemsource of the datagrid and the rest sorting will be taken care. Checkout the method.

    private void addEvent(Int32 source)

    {

        Event a = new Event();

        a.Source_Id = source;

        a.Source_Name = "Device_" + source.ToString();

        a.Event_DateTime = DateTime.Now;



        this.Alarms.Add(a);

        CollectionViewSource.GetDefaultView(testdg.ItemsSource).Refresh();

    }

Sorry, misunderstood your question previously.