title: wpf TextBox TexPreviewTextInput not working with binding
body: <p>I'm trying to catch TexPreviewTextInput event from TextBox on binding change and it doesn't work.
  I want to be able to get the current value and the new value, I'm not changing some of the charterers, but the whole string with binding.</p>
  
  <pre><code>&lt;TextBox x:Name="TB" Text="{Binding TBText}" PreviewTextInput="Sender_TexPreviewTextInput"/&gt;
  </code></pre>
  
  <p>and the code behind is:</p>
  
  <pre><code>    public MainWindow()
      {
          InitializeComponent();
          TB.Text = "First Value";
          Loaded+=MainWindow_Loaded;
      }
  
      public string  TBText { get; set; }
  
      void MainWindow_Loaded(object sender, RoutedEventArgs e)
      {
          TB.Text = "Changed Value";
      }
  
      void Sender_TexPreviewTextInput(object sender, TextCompositionEventArgs e)
      {
          //Do Somthing
      }
  </code></pre>