title: Mvvm wpf: update value binding before leaving focus of input element
body: <p>I have this element in my xaml (DevExpress item):</p>
  
  <pre><code>&lt;dxe:ComboBoxEdit IsTextEditable="False" EditValue="{Binding IDTIPOCONN}" 
       ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=Window},Path=DataContext.ttc}"  /&gt;
  </code></pre>
  
  <p>All the binding are correct, and when I change the value, I trigger some event from my ViewModel.</p>
  
  <p>My problem is the update of the value is executed only when I leave the focus of my comboBox. I need, instead, to execute my action when the value is changed, before leaving its foucs.</p>
  
  <p>How can I do it? I need this because selecting one or another from the list, I show to the user some hidden elements.</p>