Specify relative FontSize in Run

Question

given a TextBlock with multiple Runs, how can I specify a relative FontSize? For example, if the TextBlock has FontSize 10, I would like one of the Runs to be 15. But if the FontSize becomes 20, I would like the Run to become 30.

Answer 1

You didn't say whether this was for WPF or other XAML framework. This sample works in WPF.

This is typically done with binding and a Value Converter.

ValueConverter Code

class FontSizeConverter : IValueConverter {
  public object Convert(object value, Type targetType, object parameter, 
         System.Globalization.CultureInfo culture) {
    // add input parameter testing as needed.
    var originalFontSize = (double)value;
    double alteredFontSize = originalFontSize * Ratio; ;   

    return alteredFontSize;
  }

  public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
    throw new NotImplementedException();
  }

  // Create a ratio property 
  // allows the converter to be used for different font ratios
  public double Ratio { get; set; }
}

Instantiate the Converter in the XAML resources and set the Ratio property.

XAML

<Window.Resources>
    <local:FontSizeConverter x:Key='FontSizeConverter'
                             Ratio='1.5' />
 </Window.Resources>

Then use a relative binding to get the parent TextBlock FontSize. Pass the value parent value to a ValueConverter.

XAML

<TextBlock FontSize='20'>
  <Run Text='The first bit.'
       FontSize='{Binding FontSize, Converter={StaticResource FontSizeConverter}, RelativeSource={RelativeSource FindAncestor, AncestorType=TextBlock}}' />
  <Run Text='The second bit' />          
  <Run Text='The third bit' />
</TextBlock>

If you want to apply the binding to all the Runs in the TextBlock you can repeat this binding for each Run or create a Style which uses the binding.

XAML

<Style TargetType='Run'>
      <Setter Property='FontSize'
              Value='{Binding FontSize, Converter={StaticResource FontSizeConverter}, RelativeSource={RelativeSource FindAncestor, AncestorType=TextBlock}}' />
    </Style>

Full XAML

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local='clr-namespace:SO_Questions'
        x:Class="SO_Questions.TextRunWindow"
        Title="TextRunWindow"
        Height="600"
        Width="600">
  <Window.Resources>
    <local:FontSizeConverter x:Key='FontSizeConverter'
                             Ratio='2.5' />
    <Style TargetType='Run'>
      <Setter Property='FontSize'
              Value='{Binding FontSize, Converter={StaticResource FontSizeConverter}, RelativeSource={RelativeSource FindAncestor, AncestorType=TextBlock}}' />
    </Style>
  </Window.Resources>
  <Grid>
<TextBlock FontSize='20'>
  <Run Text='The first bit.'
       FontSize='{Binding FontSize, Converter={StaticResource FontSizeConverter}, RelativeSource={RelativeSource FindAncestor, AncestorType=TextBlock}}' /><Run Text=' ' />
<Run Text='The second bit' />            
<Run Text='The third bit' />
</TextBlock>
  </Grid>
</Window>