WPF对数据绑定的DataGrid 分组显示设置后，为什么分组下不能显示分组数据

[半途人]

<Window x:Class="WpfApplication1.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="Window1" Height="300" Width="300" Loaded="Window_Loaded">


    <Window.Resources>
        <local:People x:Key="data" ></local:People>
    
        <CollectionViewSource x:Key="peopleViewSource" d:DesignSource="{d:DesignInstance {x:Type local:People}}">
            <CollectionViewSource.GroupDescriptions>
                <PropertyGroupDescription PropertyName="City" />
            </CollectionViewSource.GroupDescriptions>
        </CollectionViewSource>
    </Window.Resources>
    <Grid DataContext="{StaticResource peopleViewSource}">
        <DataGrid x:Name="peopleDataGrid" AutoGenerateColumns="True" EnableRowVirtualization="True" ItemsSource="{Binding}"  RowDetailsVisibilityMode="Visible">
            <DataGrid.Columns>
                <DataGridTextColumn x:Name="cityColumn" Binding="{Binding City}" Header="City" Width="SizeToHeader"/>
                <DataGridTextColumn x:Name="cNameColumn" Binding="{Binding cName}" Header="c Name" Width="SizeToHeader"/>
                <DataGridTextColumn x:Name="fatherColumn" Binding="{Binding Father}" Header="Father" Width="SizeToHeader"/>
                <DataGridTextColumn x:Name="idColumn" Binding="{Binding Id}" Header="Id" Width="SizeToHeader"/>
                <DataGridTextColumn x:Name="phoneColumn" Binding="{Binding Phone}" Header="Phone" Width="SizeToHeader"/>
            </DataGrid.Columns>

            <DataGrid.GroupStyle >
                <GroupStyle>
                    <GroupStyle.ContainerStyle>
                        <Style TargetType="{x:Type GroupItem}">
                            <!--<Setter Property="Margin" Value="0,0,0,0"/>-->
                            <Setter Property="Template">
                                <Setter.Value>
                                    <ControlTemplate TargetType="{x:Type GroupItem}">
                                        <Expander IsExpanded="True" >
                                            <Expander.Header>
                                                <StackPanel>
                                                    <TextBlock FontWeight="Bold" Text="{Binding Path=Name}"/>
                                                    <TextBlock FontWeight="Bold" Text="{Binding Path=ItemCount,StringFormat=(共{0}条)}"/>
                                                </StackPanel>
                                            </Expander.Header>
                                        </Expander>
                                    </ControlTemplate>
                                </Setter.Value>
                            </Setter>
                        </Style>
                    </GroupStyle.ContainerStyle>
                </GroupStyle>
                <GroupStyle>
                    
                    <GroupStyle.HeaderTemplate>
                        
                        <DataTemplate>
                            
                            <DockPanel Background="LightBlue">
                                <TextBlock Text="{Binding Path=Name }" Foreground="Blue" Margin="30,0,0,0" Width="100"/>
                                <TextBlock Text="{Binding Path=ItemCount}" Foreground="Blue"/>


                            </DockPanel>
                        </DataTemplate>
                    </GroupStyle.HeaderTemplate>
                </GroupStyle>

            </DataGrid.GroupStyle>
        </DataGrid>
    </Grid>
</Window>