问 WPF MVVM -以编程方式触发数据标记排序方向的箭头

EN

默认情况下，当加载DataGrid时，ProductName将对数据进行排序。但是，网格视图的ProductName头将不会显示向上箭头图标。这对我来说是以编程方式触发图标吗？

XAML：

<DataGrid x:Name="GridProduct" 
          ItemsSource="{Binding Path=ProductResult}" 
          Style="{StaticResource defaultDataGridStyle}" 
          CellStyle="{StaticResource defaultCellStyle}"
          ColumnHeaderStyle="{StaticResource defaultCellHeaderStyle}"> 
  <DataGrid.Columns>
       <DataGridTextColumn Header="Product Name" Binding="{Binding ProductName}" />
       <DataGridTextColumn Header="Product Price" Binding="{Binding ProducPrice}"/> 
  </DataGrid.Columns>
</DataGrid>

风格：

<Style x:Key="defaultCellHeaderStyle" TargetType="DataGridColumnHeader" BasedOn="{StaticResource MetroDataGridColumnHeader}">
    <Setter Property="FontSize" Value="16"></Setter>
    <Setter Property="Command" Value="{Binding Path=DataContext.SortCommand, RelativeSource={RelativeSource AncestorType=DataGrid}}" />
    <Setter Property="CommandParameter" Value="{Binding Path=Content, RelativeSource={RelativeSource Self}}"></Setter>
</Style>
<Style x:Key="defaultCellStyle" TargetType="DataGridCell" BasedOn="{StaticResource MetroDataGridCell}">
    <Setter Property="FontSize" Value="16"></Setter>
    <Setter Property="HorizontalContentAlignment" Value="Center"></Setter>
    <Setter Property="HorizontalAlignment" Value="Stretch"></Setter>
</Style>

MVVM：

public List<Product> ProductResult
        _productResult = _productResult.OrderBy(x => x.Name).ToList();
        return _productResult;
}