Changing a bound value in xaml

Question

I'm wondering if there's a way to bind the property of one element to another's but modify the data in between. For example could I have a textblock's FontSize be bound to the Window's width/20 or something similar? I've come across areas where this would be useful a few times now, but have always found workarounds (usually involving adding fields to my viewModel). A completely xaml solution is preferred.

Answer 1

Yes, by implementing IValueConverter .

Your scenario would look something like this for the converter:

[ValueConversion(typeof(double), typeof(double))]
public class DivideBy20Converter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var f = (double) value;
        return f/20.0;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var f = (double)value;
        return f * 20.0;
    }
}

... and something like this in XAML:

<Window x:Class="WpfApplication3.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
        xmlns:wpfApplication3="clr-namespace:WpfApplication3"
        Title="MainWindow" Height="350" Width="525"
        x:Name="Window">
    <Window.Resources>
        <wpfApplication3:DivideBy20Converter x:Key="converter"></wpfApplication3:DivideBy20Converter>        
    </Window.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition />
        </Grid.RowDefinitions>
        <TextBox FontSize="{Binding ElementName=Window, Path=Width, Converter={StaticResource converter}}"></TextBox>
    </Grid>
</Window>

Answer 2

You can use IValueConverters to handle logic like this.

Here is a example with the scenario you mentioned, You can bind to the window width and use the Converte r to divide the width by the value supplied in the ConverterParameter

public class MyConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value != null && parameter != null)
        {
            double divisor = 0.0;
            double _val = 0.0;
            if (double.TryParse(value.ToString(), out _val) && double.TryParse(parameter.ToString(), out divisor))
            {
                return _val / divisor;
            }
        }
        return value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return null;
    }
}

Xaml:

<Window x:Class="WpfApplication7.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:converters="clr-namespace:WpfApplication7"
        Title="MainWindow" Height="124" Width="464" Name="YourWindow" >

    <Window.Resources>
        <converters:MyConverter x:Key="MyConverter" />
    </Window.Resources>

    <StackPanel>
        <TextBlock FontSize="{Binding ElementName=YourWindow, Path=ActualWidth, Converter={StaticResource MyConverter}, ConverterParameter=20}" />
    </StackPanel>
</Window>