How can I add a control to a GridView when an item is selected such that the control is on the row below the selected item and spans all the columns?

Question

I have a GridView with a horizontal orientation.
When the user selects an item in the GridView I would like to show a list (of child objects) between the row of the selected item and the row below the selected item. The list should span all the columns of the GridView.

<GridView ItemsSource="{Binding SrGroups}" ItemTemplate="{StaticResource ServiceReviewGroupTemplate}" ScrollViewer.HorizontalScrollMode="Disabled" ScrollViewer.VerticalScrollMode="Auto">
    <GridView.ItemsPanel>
        <ItemsPanelTemplate>
            <WrapGrid Orientation="Horizontal"></WrapGrid>
        </ItemsPanelTemplate>
    </GridView.ItemsPanel>
</GridView>

<DataTemplate x:Key="ServiceReviewGroupTemplate">
    <Grid Background="{StaticResource BlueMedium}" Height="150" Width="150">
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <TextBlock Grid.Row="0" Margin="10,0,10,10" Text="{Binding Name}" />
        <TextBlock Grid.Row="1" Margin="10,0,10,10">
            <Run Text="{Binding SrInstances.Count, Mode=OneWay}"/>
            <Run Text="Instances"  />
        </TextBlock>
    </Grid>
</DataTemplate>

Answer 1

Truth be told, I think the easiest way to do this would be to create a custom ItemsPanel which inherited from WrapGrid, but added a property to do this (upon item selected, show the assigned ContentTemplate in a row/column which spans the width/height of the grid closest to the center).

Answer 2

How many items will you be displaying in the sub list? I don't think your GridView will look uniformed. What you can do is bind a ListView to your GridViewItem Data Template. Check out Microsoft's WinRT Sample Code. It should have a multi-list GridView Sample or point you in the right direction.

Best of Luck