protected virtual void OnActiveContentChanged(EventArgs e, IDockContent prevContent)
 {
     var activeContent = ActiveContent;
     EventHandler handler;
     if (null != prevContent)
     {
         prevContent.OnDeactivate(e);
     }
     handler = (EventHandler)Events[ActiveContentChangedEvent];
     if (handler != null)
         handler(this, e);
     if (null != activeContent)
     {
         activeContent.OnActivated(e);
     }
 }