Xamdatagrid is bind to EmployeeColl but when collection changes from combobox datasource property change is not firing.

How to get the datasource collection change on view model. So that I can do validation.

 Below is my viewmodel

ViewModel.cs

public class ViewModel : INotifyPropertyChanged
{
private ObservableCollection<string> _coll1;

public ICommand ClickCommand { get; set; }

public ObservableCollection<string> Coll1
{
get
{
return _coll1;
}
set
{
_coll1 = value;
NotifyPropertyChanged("Coll1");
}
}

private ObservableCollection<string> _coll2;

public ObservableCollection<string> Coll2
{
get
{
return _coll2;
}
set
{
_coll2 = value;
NotifyPropertyChanged("Coll2");
}
}

private ObservableCollection<Employee> _employeeColl;

public ObservableCollection<Employee> EmployeeColl
{
get
{
return _employeeColl;
}
set
{
_employeeColl = value;
NotifyPropertyChanged("EmployeeColl");
}
}

public ViewModel()
{
ClickCommand = new DelegateCommand(ClickExecute, ClickCanExecute);

Coll1 = new ObservableCollection<string> () { "AAA", "AASSSA", "AAAAXX"};
Coll2 = new ObservableCollection<string> () { "XX", "DD", "DDD" };

EmployeeColl = new ObservableCollection<Employee>() { new Employee("Emp1", "Dep1"), new Employee("Emp2", "Dep2") };

}

public void ClickExecute(object sender)
{

}

public bool ClickCanExecute(object sender)
{
return true;
}

public event PropertyChangedEventHandler PropertyChanged;

private void NotifyPropertyChanged(string propertyName = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}

Below is my xaml

<StackPanel>
<igDP:XamDataGrid DataSource="{Binding Path=EmployeeColl, UpdateSourceTrigger=PropertyChanged}">
<igDP:XamDataGrid.FieldSettings>
<igDP:FieldSettings/>
</igDP:XamDataGrid.FieldSettings>

<igDP:XamDataGrid.FieldLayoutSettings>
<igDP:FieldLayoutSettings AddNewRecordLocation="OnTopFixed" AllowAddNew="True" AllowDelete="True"
SupportDataErrorInfo="RecordsAndCells" DataErrorDisplayMode="ErrorIconAndHighlight"/>
</igDP:XamDataGrid.FieldLayoutSettings>

<igDP:XamDataGrid.FieldLayouts>
<igDP:FieldLayout>
<igDP:ComboBoxField Name="Name" Width="*">
<igDP:ComboBoxField.EditorStyle>
<Style TargetType="igEditors:XamComboEditor">
<Setter Property="ItemsSource"
Value="{Binding Path=DataContext.Coll1,NotifyOnSourceUpdated=True, RelativeSource={RelativeSource AncestorType={x:Type igDP:XamDataGrid}}}" />
</Style>
</igDP:ComboBoxField.EditorStyle>
</igDP:ComboBoxField>
<igDP:ComboBoxField Name="Department" Width="*">
<igDP:ComboBoxField.EditorStyle>
<Style TargetType="igEditors:XamComboEditor">
<Setter Property="ItemsSource"
Value="{Binding Path=DataContext.Coll2, RelativeSource={RelativeSource AncestorType={x:Type igDP:XamDataGrid}}}" />
</Style>
</igDP:ComboBoxField.EditorStyle>
</igDP:ComboBoxField>
</igDP:FieldLayout>
</igDP:XamDataGrid.FieldLayouts>
</igDP:XamDataGrid>
<Button Height="20" Command="{Binding ClickCommand}"/>
</StackPanel>