Can I bind to a utility class?

Question

I have a common class filled with generic functions like the one below to parse a text box:

public static void DoubleParse_KeyDown(object sender, KeyEventArgs e)
{
    if (e.Key == Key.Decimal)
    {
        var textBox = sender as TextBox;
        if (textBox != null)
            textBox.Text += Convert.ToChar(CultureInfo.CurrentCulture.NumberFormat.CurrencyDecimalSeparator);
    }
    else
    {
        e.Handled = (e.Key >= Key.D0 && e.Key <= Key.D9) ||
                    (e.Key >= Key.NumPad0 && e.Key <= Key.NumPad9) || 
                    e.Key == Key.Back || e.Key == Key.Delete ||
                    e.Key == Key.Left || e.Key == Key.Right || e.Key == Key.Unknown;
    }
}

I thought I could use this everywhere across my pages as a single source for TextBox keydown events. New to MVVM implementation in WP8, curious if there's a way to achieve this?

In the spirit of MVVM (though I'm not a purist), I understand it doesn't need to be in the viewmodel specifically, but I'd still like it centralized.

A quick note:

• The class isn't static, I understand I can't use that directly in xaml.
• I thought to make the class some kind of StaticResource and reference the functions in xaml. But that doesn't seem to work.
• I'm currently just using pass through functions in the code-behind and passing the sender on to the static functions.

Answer 1

You want an Attached Behavior.

public static class TextBoxBehavior
{
    public static bool GetAllowOnlyDecimalInput(TextBox texbox)
    {
        return (bool)texbox.GetValue(AllowOnlyDecimalInputProperty);
    }

    public static void SetAllowOnlyDecimalInput(
      TextBox texbox, bool value)
    {
        texbox.SetValue(AllowOnlyDecimalInputProperty, value);
    }

    public static readonly DependencyProperty AllowOnlyDecimalInputProperty =
        DependencyProperty.RegisterAttached(
        "AllowOnlyDecimalInput",
        typeof(bool),
        typeof(TextBox),
        new PropertyMetadata(false, OnAllowOnlyDecimalInputChanged));

    static void OnAllowOnlyDecimalInputChanged(
      DependencyObject depObj, DependencyPropertyChangedEventArgs e)
    {
        TextBox item = depObj as TextBox;
        if (item == null)
            return;

        if (e.NewValue is bool == false)
            return;

        if ((bool)e.NewValue)
            item.KeyDown += OnTextBoxDoubleParse_KeyDown;
        else
            item.KeyDown -= OnTextBoxDoubleParse_KeyDown;
    }

    static void OnTextBoxDoubleParse_KeyDown(object sender, KeyEventArgs e)
    {
        if (!Object.ReferenceEquals(sender, e.OriginalSource))
            return;

        TextBox item = e.OriginalSource as TextBox;
        if (item != null) {
            if (e.Key == Key.Decimal)
            {
                var textBox = sender as TextBox;
                if (textBox != null)
                    textBox.Text += Convert.ToChar(CultureInfo.CurrentCulture.NumberFormat.CurrencyDecimalSeparator);
            }
            else
            {
                e.Handled = (e.Key >= Key.D0 && e.Key <= Key.D9) ||
                            (e.Key >= Key.NumPad0 && e.Key <= Key.NumPad9) || 
                            e.Key == Key.Back || e.Key == Key.Delete ||
                            e.Key == Key.Left || e.Key == Key.Right || e.Key == Key.Unknown;
            }
        }
    }

    #endregion // AllowOnlyDecimalInput
}

In XAML use it with

<TextBox my:TextBoxBehavior.AllowOnlyDecimalInput="True" />

You can also set this in an WPF Style and have it reusable within all or many controls rather than adding the property each time manually.