WPF DataGrid MVVM当用户更改另一列时更改一列的值

Question

我正在使用DataGrid绑定到DataTable。

我想要做以下事情： 如果TargetUnit列的值为空，则将Combobox的SelectedIndex更改为1 否则将SelectedIndex更改为0.

我正在做以下事情：

                <DataGrid MaxHeight="400" Height="Auto" Width="Auto" IsEnabled="{Binding Enabled}" ItemsSource="{Binding Spread.DefaultView}" SelectedItem="{Binding SpreadSelectedItem, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" AutoGenerateColumns="False" CanUserAddRows="False">
                    <DataGrid.Columns>
                        <DataGridTextColumn IsReadOnly="True" Header="Value" Binding="{Binding Path=Value}" Width="70"/>
                        <DataGridTextColumn IsReadOnly="True" Header="No of Items"  Binding="{Binding Path=NoOfItems}" />
                        <DataGridTextColumn Header="Rename Unit"  Binding="{Binding Path=TargetUnit, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
                        <DataGridComboBoxColumn Header="Action Type" SelectedItemBinding="{Binding Path=ActionType, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource SpreadActionTypeConverter}}">
                            <DataGridComboBoxColumn.ElementStyle>
                                <Style TargetType="ComboBox">
                                    <Setter Property="ItemsSource" Value="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}}, Path=DataContext.UnitConversionActionTypes}"/>
                                    <Setter Property="SelectedIndex" Value="0"/>
                                    <Style.Triggers>
                                        <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGridRow}}, Path=TargetUnit}" Value="">
                                            <Setter Property="SelectedIndex" Value="1"/>
                                        </DataTrigger>
                                    </Style.Triggers>
                                </Style>
                            </DataGridComboBoxColumn.ElementStyle>
                            <DataGridComboBoxColumn.EditingElementStyle>
                                <Style TargetType="ComboBox">
                                    <Setter Property="ItemsSource" Value="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}}, Path=DataContext.UnitConversionActionTypes}"/>
                                    <Setter Property="SelectedIndex" Value="0"/>
                                    <Style.Triggers>
                                        <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGridRow}}, Path=TargetUnit}" Value="">
                                            <Setter Property="SelectedIndex" Value="1"/>
                                        </DataTrigger>
                                    </Style.Triggers>
                                </Style>
                            </DataGridComboBoxColumn.EditingElementStyle>
                        </DataGridComboBoxColumn>
                    </DataGrid.Columns>
                </DataGrid>

知道如何实现这一目标吗？