title: Textbox Binding to both LostFocus and Property Update
body: <p>Currently I bind to my <code>TextBox</code>es as:</p>
  
  <pre><code>Text="{Binding DocValue,
           Mode=TwoWay,
           ValidatesOnDataErrors=True,
           UpdateSourceTrigger=PropertyChanged}"
  </code></pre>
  
  <p>This works great in getting every keystroke to do button status checking (which I want). </p>
  
  <p>In addition, I  would like to track the <code>LostFocus</code> event on the <code>TextBox</code> (through binding) and do some additional calculations that might be too intensive for each keystroke.</p>
  
  <p>Anyone have thoughts on how to accomplish both?</p>