wpf datagrid with customedata source, get cell value after editing

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WPF

DataGrid

I am using wpf DataGrid with customedata source . the data source like blow:

public class SentencesField
   {
       public int No{get;set;}
       public string TargetSen { get; set; }
       public string SourceSen { get; set; }
       public static List<SentencesField> FillSentencesGrid(List<myXml.tu> m_list)
       {
           List<SentencesField> m_result = new List<SentencesField>();
           for (int i = 0; i < m_list.Count; i++)
           {
               SentencesField m_Field = new SentencesField();
               m_Field.No = i;
               m_Field.SourceSen = m_list[i].sourcelang;
               m_Field.TargetSen = m_list[i].targetlang;
               m_result.Add(m_Field);
           }
           return m_result;
       }
}

in xaml code:

<pre lang="xml"><DataGrid Name="m_SentencesFiled"
                       IsReadOnly="False"
                       AutoGenerateColumns="False" >
           <DataGrid.CellStyle>
               <Style TargetType="DataGridCell" >
                   <Style.Triggers>
                       <Trigger Property="IsKeyboardFocusWithin" Value="True">
                           <Setter Property="Background" Value="SeaGreen"/>
                           <Setter Property="BorderBrush" Value="Red"/>
                       </Trigger>
                   </Style.Triggers>
               </Style>
           </DataGrid.CellStyle>
           <!-- index column-->
           <DataGrid.Columns>
               <DataGridTextColumn Width="0.06*"  Binding="{Binding No}" IsReadOnly="True"  >
               </DataGridTextColumn>
               <!-- source sentences column-->
               <DataGridTemplateColumn Width="0.5*" >
                   <DataGridTemplateColumn.CellTemplate >
                       <DataTemplate>
                           <StackPanel>
                               <TextBox Text ="{Binding SourceSen,NotifyOnTargetUpdated=True}"
                                               TextWrapping="Wrap"
                                               LostFocus="SourceTextBox_LostFocus"
                                               />
                           </StackPanel>
                       </DataTemplate>
                   </DataGridTemplateColumn.CellTemplate>
               </DataGridTemplateColumn>
               <!-- target sentences column column-->
               <DataGridTemplateColumn Width="0.5*">
                   <DataGridTemplateColumn.CellTemplate >
                       <DataTemplate>
                           <StackPanel>
                               <TextBox Text ="{Binding TargetSen,NotifyOnTargetUpdated=True}"
                                               TextWrapping="Wrap"
                                               LostFocus="TargetTextBox_LostFocus"
                                             />
                           </StackPanel>
                       </DataTemplate>
                   </DataGridTemplateColumn.CellTemplate>
               </DataGridTemplateColumn>
           </DataGrid.Columns>
       </DataGrid>

when populating data grid

this.m_SentencesFiled.ItemsSource = SentencesField.FillSentencesGrid(m_SenList);

I want to get cell text value after editing, but inside the LostFocus event of textbox(which I put inside cell) I got same value before editing.

how could I know the cell value after editing a cell (there is a textbox inside cell)?
I look up many articles , but their saving changes in only consider when item source is database. but in my situation, it is a costume class.

could anyone tell me what to do in order to get the cell value after editing?

thanks in advance!