WPF DataGrid控件编辑时CellEditingTemplate，获取不到编辑的元素

该自定义模板列编辑时PreparingCellForEdit 获取不到编辑的元素
private void editDataGrid_PreparingCellForEdit(object sender, DataGridPreparingCellForEditEventArgs e)
{
e.EditingElement
}
e.EditingElement是null的，请教下怎么样下可以获取编辑后的元素或者值

<DataGrid x:Name="editDataGrid" IsReadOnly="False"
                      BeginningEdit="editDataGrid_BeginningEdit" 
                      CellEditEnding="editDataGrid_CellEditEnding"
                      PreparingCellForEdit="editDataGrid_PreparingCellForEdit"
                      >
                <DataGrid.Columns>
                    <DataGridTemplateColumn Width="320">
                        <DataGridTemplateColumn.CellTemplate>
                            <DataTemplate>
                                <Grid Width="320">
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition/>
                                        <ColumnDefinition/>
                                    </Grid.ColumnDefinitions>
                                    <StackPanel Orientation="Horizontal">
                                        <TextBlock TextTrimming="CharacterEllipsis" MaxWidth="60" Text="{Binding Areas[0].DutyTimeBuckets[0].DutyMemberNames1,Converter={StaticResource DutyMemberConverteer}}" ToolTipService.IsEnabled="False"/>
                                        <TextBlock Text="，" ToolTipService.IsEnabled="False"/>
                                        <TextBlock TextTrimming="CharacterEllipsis" MaxWidth="60" Text="{Binding Areas[0].DutyTimeBuckets[0].DutyMemberNames2,Converter={StaticResource DutyMemberConverteer}}" ToolTipService.IsEnabled="False"/>
                                    </StackPanel>

                                    <StackPanel Grid.Column="1" Orientation="Horizontal">
                                        <TextBlock TextTrimming="CharacterEllipsis" MaxWidth="60" Text="{Binding Areas[0].DutyTimeBuckets[1].DutyMemberNames1,Converter={StaticResource DutyMemberConverteer}}" ToolTipService.IsEnabled="False"/>
                                        <TextBlock Text="，" ToolTipService.IsEnabled="False"/>
                                        <TextBlock TextTrimming="CharacterEllipsis" MaxWidth="60" Text="{Binding Areas[0].DutyTimeBuckets[1].DutyMemberNames2,Converter={StaticResource DutyMemberConverteer}}" ToolTipService.IsEnabled="False"/>
                                    </StackPanel>
                                </Grid>
                            </DataTemplate>
                        </DataGridTemplateColumn.CellTemplate>
                        <DataGridTemplateColumn.CellEditingTemplate>
                            <DataTemplate>
                                <Grid Width="320">
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition/>
                                        <ColumnDefinition/>
                                    </Grid.ColumnDefinitions>
                                    <StackPanel Orientation="Horizontal">
                                        <TextBox MaxWidth="60" Text="{Binding Areas[0].DutyTimeBuckets[0].DutyMemberNames1,Converter={StaticResource DutyMemberConverteer}}" />
                                        <TextBlock Text="，" ToolTipService.IsEnabled="False"/>
                                        <TextBox MaxWidth="60" Text="{Binding Areas[0].DutyTimeBuckets[0].DutyMemberNames2,Converter={StaticResource DutyMemberConverteer}}" />
                                    </StackPanel>

                                    <StackPanel Grid.Column="1" Orientation="Horizontal">
                                        <TextBox MaxWidth="60" Text="{Binding Areas[0].DutyTimeBuckets[1].DutyMemberNames1,Converter={StaticResource DutyMemberConverteer}}" />
                                        <TextBlock Text="，" ToolTipService.IsEnabled="False"/>
                                        <TextBox MaxWidth="60" Text="{Binding Areas[0].DutyTimeBuckets[1].DutyMemberNames2,Converter={StaticResource DutyMemberConverteer}}" />
                                    </StackPanel>
                                </Grid>
                            </DataTemplate>
                        </DataGridTemplateColumn.CellEditingTemplate>
                    </DataGridTemplateColumn>
</DataGrid.Columns>
</DataGrid>

```