title: updating tabitem datatemplate when changing selected tab
body: <p>I have an object(MyCals) that is type of Cal Collection. (Cal is an object).</p>
  
  <p>The MyCals consists of 2 Cal, where each Cal has List of Events (Events does not has the INotifyPropertyChanged interface implemented, it has many properties like Summary, Id,...)</p>
  
  <pre><code>&lt;TabControl ItemsSource="{Binding Path=MyCals, UpdateSourceTrigger=PropertyChanged}"&gt;
                  &lt;TabControl.ItemTemplate&gt;
                      &lt;DataTemplate&gt;
                          &lt;!--  Tab Header --&gt;
                          &lt;TextBlock Text="{Binding Path=ProductID}" /&gt;
                      &lt;/DataTemplate&gt;
                  &lt;/TabControl.ItemTemplate&gt;
  
                  &lt;!--  Content  --&gt;
                  &lt;TabControl.ContentTemplate&gt;
                      &lt;DataTemplate&gt;
                          &lt;Grid&gt;
                         &lt;DataGrid AutoGenerateColumns="False"
                                    HorizontalGridLinesBrush="LightGray"
                                    ItemsSource="{Binding Path=Events, UpdateSourceTrigger=PropertyChanged}"
                                    VerticalGridLinesBrush="LightGray"&gt;
                              &lt;DataGrid.Columns&gt;
                                  &lt;DataGridTextColumn Binding="{Binding Summary}" Header="Summary" /&gt;
                                                              &lt;/DataGrid.Columns&gt;
                          &lt;/DataGrid&gt;
                          &lt;/Grid&gt;
                      &lt;/DataTemplate&gt;
                  &lt;/TabControl.ContentTemplate&gt;
              &lt;/TabControl&gt;
  </code></pre>
  
  <p>Now the TabControl will show 2 tabs with the right header, but the Datagrid wont update when I change select the other tab. So if the First Tab is selected, that datagrid will be showed with the Data from first Cal, but when I select the second tab, data in the datagrid are not getting updated 
  (Same Events in the MyCals[1])</p>
  
  <p>any help is really appreciated</p>