title: Update Binding from DataGrid Group
body: <p>I have a WPF with a DataGrid and I group rows (by "Name" in the example). I would like to have summary row for each group ("Name" and the total "Amount" which I calculate in a Converter).
  The problem is that when I update a data value ("Amount" in the example), the summary row is not updated.
  So, I would like to force binding update of the summary row but I cannot access it as far as it is in a DataTemplate.</p>
  
  <pre><code>&lt;DataGrid ItemsSource="{Binding Source={StaticResource posCurrencyOpen}}"&gt;
      &lt;DataGrid.Columns&gt;
          &lt;DataGridTextColumn Header="Amount"  Binding="{Binding Amount, Converter={StaticResource Amount_Converter}, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"&gt;
      &lt;/DataGrid.Columns&gt;
      &lt;DataGrid.GroupStyle&gt;
          &lt;GroupStyle&gt;
              &lt;GroupStyle.HeaderTemplate&gt;
                  &lt;DataTemplate&gt;
                      &lt;StackPanel Orientation="Horizontal"&gt;
                          &lt;DataGridCell Content="{Binding Path=Name}"/&gt;
                          &lt;DataGridCell Content="{Binding Path=Items, Converter={StaticResource AmountGroup_Converter}, Mode=OneWay}"/&gt;
                      &lt;/StackPanel&gt;
                  &lt;/DataTemplate&gt;
              &lt;/GroupStyle.HeaderTemplate&gt;
          &lt;/GroupStyle&gt;
      &lt;/DataGrid.GroupStyle&gt;
  &lt;/DataGrid&gt;
  </code></pre>