title: WPF: Textbox not firing onTextInput event
body: <p>So basically, I have a bunch of TextBoxes that the user gets to fill out. I've got a button that I want to keep disabled until all the TextBoxes have had text entered in them. Here is a sample XAML TextBox that I'm using:</p>
  
  <pre><code>&lt;TextBox Name="DelayedRecallScore" TextInput="CheckTextBoxFilled" Width="24" /&gt;
  </code></pre>
  
  <p>And here is the function that I'm trying to trigger:</p>
  
  <pre><code>  //Disables the OK button until all score textboxes have content
      private void CheckTextBoxFilled(object sender, RoutedEventArgs e)
      {
          /*
          foreach (TextBox scorebox in TextBoxList)
          {
              if (string.IsNullOrEmpty(scorebox.Text))
              {
                  Ok_Button.IsEnabled = false;
                  return;
              }
          }
          Ok_Button.IsEnabled = true;
           */
          MessageBox.Show("THIS MAKES NO SENSE");
      }
  </code></pre>
  
  <p>The MessageBox is not showing up when TextInput should be getting triggered. As an experiment I tried triggering CheckTextBoxFilled() on PreviewTextInput, and it worked fine then, meaning that for whatever reason, the function just isn't getting called. I also have a validation function that is triggered by PreviewTextInput, which works as it should. At first I thought PreviewTextInput might somehow be interfering with TextInput, so I took PreviewTextInput off the TextBox, but that hasn't managed to fix anything.  I'm completely befuddled by why this might happen, so any help would be appreciated. </p>