Access complete bound value of a textbox in attached property

Question

With an attached property, I want to de-/increase the value of a textbox by that value when hitting Key.Up or Key.Down.

I defined the following property:

public static readonly DependencyProperty SmallFloatIncrementProperty = DependencyProperty.RegisterAttached(
      "SmallFloatIncrement",
      typeof(double),
      typeof(InputService),
      new UIPropertyMetadata(0.1));

    public static double GetSmallFloatIncrement(DependencyObject d)
    {
        return (double)d.GetValue(SmallFloatIncrementProperty);
    }

    public static void SetSmallFloatIncrement(DependencyObject d, double value)
    {
        d.SetValue(SmallFloatIncrementProperty, value);
    }

I have also registered for PreviewKeyDown on my TextBox and handle it like this:

    if(e.Key == Key.Up)
    {
      IncrementOrDecrementValue((DependencyObject)sender, true);
    }
    else if (e.Key == Key.Down)
    {
      IncrementOrDecrementValue((DependencyObject)sender, false);              
    }

...

    private static void IncrementOrDecrementValue(DependencyObject sender, bool doIncrement)
    {
        if (sender is TextBox)
        {
          TextBox tb = (TextBox)sender;
          var increment = GetSmallFloatIncrement((DependencyObject)sender);

          var text = tb.Text;
          double textBoxValue = 0.0;
          if (!string.IsNullOrEmpty(text))
          {
             try{
               textBoxValue = Convert.ToDouble(text, FormatProvider);
             }
             catch (FormatException)   { }
          }

          tb.Text = Convert.ToString((doIncrement) ? textBoxValue + increment : textBoxValue - increment, FormatProvider);
          ValidateResult(sender);        
        }
    }

I run into propblems when retrieving the value of the textbox. If for example the bound property is 1.234 but there is a StringFormat = N1 defined in the binding, the call tb.Text delivers 1.2 instead of the entire value. But I need to increment the original value by the specified amount (the value must be a multiple of the increment). Is there a way to retrieve the complete bound value of the textbox?

Thanks for your help!

Answer 1

Essentially you don't want to change the textbox's value, you want to alter the view-model's value by the keys. Therefore the correct solution is to bind this behavior directly to the view-model. So, here's the attached property holding the value itself.

public static double GetSmallFloatIncrement(DependencyObject obj)
{
    return (double)obj.GetValue(SmallFloatIncrementProperty);
}

public static void SetSmallFloatIncrement(DependencyObject obj, double value)
{
    obj.SetValue(SmallFloatIncrementProperty, value);
}

public static readonly DependencyProperty SmallFloatIncrementProperty =
    DependencyProperty.RegisterAttached("SmallFloatIncrement", typeof(double), typeof(TextBoxExtensions),
        new PropertyMetadata(0.0));

Another attached property to implement this functionality and toggle it on/off:

public static bool GetSmallFloatIncrementOn(DependencyObject obj)
{
    return (bool)obj.GetValue(SmallFloatIncrementOnProperty);
}

public static void SetSmallFloatIncrementOn(DependencyObject obj, bool value)
{
    obj.SetValue(SmallFloatIncrementOnProperty, value);
}

public static readonly DependencyProperty SmallFloatIncrementOnProperty =
    DependencyProperty.RegisterAttached("SmallFloatIncrementOn", typeof(bool), typeof(TextBoxExtensions),
        new PropertyMetadata(false, OnSmallFloatIncrementOn));

static void OnSmallFloatIncrementOn(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var textBox = (TextBox)d;

    void OnKeyDown(object sender, KeyEventArgs keyArgs)
    {
        if (keyArgs.Key == Key.Up)
            IncrementOrDecrementValue(textBox, .01);
        else if (keyArgs.Key == Key.Down)
            IncrementOrDecrementValue(textBox, -.01);
    }

    if ((bool)e.NewValue)
        textBox.PreviewKeyDown += OnKeyDown;
    else
        textBox.PreviewKeyDown -= OnKeyDown;
}

The function to increment gets simple now:

static void IncrementOrDecrementValue(TextBox textBox, double inc)
{
    var cur = GetSmallFloatIncrement(textBox);
    SetSmallFloatIncrement(textBox, cur + inc);
}

And now we can use it:

<TextBox Text="{Binding VMValue, StringFormat=F1}" 
             local:TextBoxExtensions.SmallFloatIncrement="{Binding VMValue, Mode=TwoWay}"
             local:TextBoxExtensions.SmallFloatIncrementOn="True"/>

Note: with these increment and string format values you'll have to push the key 10 times until you see a value change in the TextBox. But this is something you can adjust to your taste.