Binding not triggering Action<> - wpf

Question

I have some code like this
this is a custom datagrid which displays hierarchical data which should close and open.
<UserControl.DataContext>
<loc:Def1 x:Name="initdef1"/>
</UserControl.DataContext>
<Grid x:Name="LayoutRoot" HorizontalAlignment="Center" VerticalAlignment="Center" >
<data:DataGrid x:Name="_dataGrid" AutoGenerateColumns="False"
ItemsSource="{Binding Display, Mode=OneWay}"
SelectionMode="Extended" >
<data:DataGrid.Columns>
<data:DataGridTemplateColumn Header="Col1">
<data:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal" >
<CheckBox IsChecked="{Binding IsExpanded, Mode=TwoWay}" Margin="{Binding Path=Level, Converter={StaticResource ConvertToThickness}}"/>
<TextBlock Text="{Binding Cells[0]}" Margin="4" />
</StackPanel>
</DataTemplate>
</data:DataGridTemplateColumn.CellTemplate>
</data:DataGridTemplateColumn>
<data:DataGridTextColumn Header="Col2" Binding="{Binding Cells[1]}" />
</data:DataGrid.Columns>
</data:DataGrid>
with this is def1
this class has the actual logic for all the processing and loading and mapping the data to the grid.
public Def1()
{
_columns = new List<ColumnDef>();
_source = new ObservableCollection<Def2>();
Def2.RowExpanding += new Action<Def2>(RowDef_RowExpanding);
Def2.RowCollapsing += new Action<Def2>(RowDef_RowCollapsing);
}
void RowDef_RowExpanding(Def2 row)
{
foreach (RowDef child in row.Children)
child.IsVisible = true;
OnPropertyChanged("Display");
}
void RowDef_RowCollapsing(Def2 row)
{
foreach (Def2 child in row.Children)
{
if (row.IsExpanded.HasValue && row.IsExpanded.Value)
RowDef_RowCollapsing(child);
child.IsVisible = false;
}
OnPropertyChanged("Display");
}
and this in def2
this class has the logic on how should the rows behave.
public bool? IsExpanded
{
get { return _isExpanded; }
set
{
if (_isExpanded != value)
{
_isExpanded = value;
if (_isExpanded.Value)
{
if (RowDef.RowExpanding != null)
RowDef.RowExpanding(this);
}
else
{
if (RowDef.RowCollapsing != null)
RowDef.RowCollapsing(this);
}
}
}
}
The thing is when the checkbox is checked or unchecked nothing happens.

Answer 1

Ok I found the answer from a similar post wpf 4.0 datagrid template column two-way binding problem
So I changed the code to
<CheckBox IsChecked="{Binding Mode=TwoWay, Path=IsExpanded, UpdateSourceTrigger=PropertyChanged}" Margin="{Binding Path=Level, Converter={StaticResource ConvertToThickness}}" Checked="CheckBox_Checked" Unchecked="CheckBox_Unchecked" x:Name="checkbox1"/>
Now it works.
But can anybody explain why i needed to set UpdateSourceTrigger=PropertyChanged ?
It's not always required.
Thanks