title: Listen to DependencyProperty changed event and get the old value
body: <p>I have the following code to subscribe to property changed event for <code>VisiblePosition</code> property of <code>Column</code> class: </p>
  
  <pre><code>DependencyPropertyDescriptor dpd = DependencyPropertyDescriptor.FromProperty(ColumnBase.VisiblePositionProperty, typeof(Column));
  
  if (dpd != null)
  {
     dpd.AddValueChanged(col, ColumnVisiblePositionChangedHandler);
  }
  </code></pre>
  
  <p>Here is the definition of the <code>ColumnVisiblePositionChangedHandler</code> method:</p>
  
  <pre><code>static internal void ColumnVisiblePositionChangedHandler(object sender, EventArgs e)
  </code></pre>
  
  <p>The problem is I need to get the old value of the property. How do I do that?</p>
  
  <p>Thanks, </p>