UpdateSourceTrigger=PropertyChanged and Converter

Question

I have a simple Converter that adds a "+" symbol to a positive number that is entered in a TextBox. When the number is entered I want to initiate some action, but I don't want to wait until the TextBox loses focus: I want to update the binding immediately as the user enters the text.

The default behaviour of a TextBox is that when a user navigates away from the box, the binding source is updated (UpdateSourceTrigger=LostFocus). In this scenario, my converter works as expected, and the + is added. However, when I change it to the following, the + is never added.

<TextBox Text="{Binding Value, Converter={StaticResource PlusConverter}, UpdateSourceTrigger=PropertyChanged}" />

I can imagine that there is a good reason why my converter.Convert() method is not called. I would like to have the following behaviour:

1. When the users enters text, the source is updated immediately
2. When the TextBox loses focus, the + is added.

Now I'm looking for a nice, but especially GENERIC way of doing this (as I have a lot of these TextBoxes in my app). So far I haven't been able to come up with a proper solution.

Answer 1

Agree w/Kent B, you need to post your Converter code.

I've been able to get part 1 to work with a simple converter (I'm binding a second unconverted TextBlock to show that the value is indeed getting updated).

However, if I understand your part 2, you're trying to get the TextBox's text to update with a "+" after it loses focus. This is a little trickier and I don't think you'll be able to do it with just an IConverter. If it can be done, I'd love to know the answer as well.

What you're essentially asking for is watermarked input behavior e.g. allow a user to enter some data, and have it get formatted correctly (both in the underlying DataBinding and in the UI). The quickest/dirtiest solution to this is to handle the TextBoxes' LostFocus but since you're using that all over your app, this may not be feasible.

You could also consider wrapping the TextBox in your own UserControl. If you look at WPFToolkit's implementation of a DatePicker it has similar behavior: allow the user to enter free form text, then auto-convert the value to a DateTime (if valid) and show the DateTime in a localized format.

HTH.

Answer 2

The other thing you might want to do, is edit the template for TextBox and move the actual PART_ContentHost to the right a bit, then have a TextBlock indicate the +/- part; i.e. change the template of the TextBox from:

Control
- Border
-- PART_ContentHost (the actual editing part)

into:

Control
- Border
-- Horizontal StackPanel
--- TextBlock (contains +/- sign, has 2px right margin)
--- PART_ContentHost (actual editable section)

Then, bind the TextBlock's content to the text, but with a converter that either writes a '+' or '-'. This way, the user can't delete the +/- part, and you don't have to worry about parsing it; this also makes it easier if you want to do something like make the negative sign red or something.

Answer 3

Thanks for your answers! I looked into this issue myself a bit futher and came up with the following solution (which I'm not entirely satisfied with, but it works fine)

I've created a CustomControl that adds functionality to the TextBox.

It provided an event handler for the LostFocus event When this event occurs, my converter is called.

However, the way I resolve the Converter is not very satisfying (I take it from the Style that is associated with my TextBox). The Style has a Setter for the Text property. From that setter I can access my Converter.

Of course I could also make a "PlusTextBox", but I use different converters and I wanted a generic solution.

public class TextBoxEx : TextBox
{
    public TextBoxEx()
    {
        AddHandler(LostFocusEvent,
          new RoutedEventHandler(CallConverter), true);
    }

    public Type SourceType
    {
        get { return (Type)GetValue(SourceTypeProperty); }
        set { SetValue(SourceTypeProperty, value); }
    }

    // Using a DependencyProperty as the backing store for SourceType.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SourceTypeProperty =
        DependencyProperty.Register("SourceType", typeof(Type), typeof(TextBoxEx), new UIPropertyMetadata(null));

    private static void CallConverter(object sender, RoutedEventArgs e)
    {
        TextBoxEx textBoxEx = sender as TextBoxEx;
        if (textBoxEx.Style == null) {
            return;
        }
        if (textBoxEx.SourceType == null) {
        }
        foreach (Setter setter in textBoxEx.Style.Setters) {
            if (setter.Property.ToString() == "Text") {
                if (! (setter.Value is Binding) ) {
                    return;
                }
                Binding binding = setter.Value as Binding;
                if (binding.Converter == null) {
                    return;
                }
                object value = binding.Converter.ConvertBack(textBoxEx.Text, textBoxEx.SourceType, binding.ConverterParameter, System.Globalization.CultureInfo.CurrentCulture);
                value = binding.Converter.Convert(value, typeof(string), binding.ConverterParameter, System.Globalization.CultureInfo.CurrentCulture);
                if (!(value is string)) {
                    return;
                }
                textBoxEx.Text = (string)value;
            }
        }
    }
}

Answer 4

In your converter, couldn't you just check the keyboard's current focus? Something like:

TextBox focusedTextBox = Keyboard.FocusedElement as TextBox;
if (focusedTextBox == null || focusedTextBox != senderOrWhatever)
{
 ' this is where you'd add the +, because the textbox doesn't have focus.
}