How to stretch text in a square button?

Question

In my application, I have a grid of square buttons. The text content for each button is set at runtime. In most cases, the text is just one character long, but sometimes it's longer. I need to make the whole text to be always visible, i.e. stretch it (change the font size) to fit inside the button's border. How do I do it?

I tried to use a Viewbox, but it doesn't help.

A simplified version of my XAML:

<Viewbox Stretch="Uniform">
    <Button Content="Text" 
            Width="{Binding ActualHeight, RelativeSource={RelativeSource Self}}"/>
</Viewbox>

Any ideas on how can I achieve what I need (i.e. square buttons + text that always fits in)?

Answer 1

Your original suggestion was nearly correct, try this:

    <Button>
        <Viewbox Stretch="Fill">
            <TextBlock Text="Test"/>
        </Viewbox>
    </Button>

And to apply this to multiple buttons:

    <Style x:Key="StretchedButtonContent" TargetType="{x:Type Button}">
        <Setter Property="ContentTemplate">
            <Setter.Value>
                <DataTemplate>
                    <Viewbox Stretch="Fill">
                        <ContentPresenter Content="{TemplateBinding Content}"/>
                    </Viewbox>
                </DataTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Button Style="{StaticResource StretchedButtonContent}" Content="Test" />

My first thought was to use a RenderTransform and a converter. This gives the same result but is more complicated:

    <Converters:ScaleConverter x:Key="ScaleConverter" />

    <Button>
        <TextBlock Text="Test" RenderTransformOrigin="0.5,0.5">
            <TextBlock.RenderTransform>
                <ScaleTransform>
                  <ScaleTransform.ScaleX>
                    <MultiBinding Converter="{StaticResource ScaleConverter}">
                        <Binding RelativeSource="{RelativeSource AncestorType={x:Type Button}}" Path="ActualWidth" />
                        <Binding RelativeSource="{RelativeSource AncestorType={x:Type TextBlock}}" Path="ActualWidth" />
                    </MultiBinding>
                  </ScaleTransform.ScaleX>
                  <ScaleTransform.ScaleY>
                    <MultiBinding Converter="{StaticResource ScaleConverter}">
                        <Binding RelativeSource="{RelativeSource AncestorType={x:Type Button}}" Path="ActualHeight" />
                        <Binding RelativeSource="{RelativeSource AncestorType={x:Type TextBlock}}" Path="ActualHeight" />
                    </MultiBinding>
                  </ScaleTransform.ScaleY>
                </ScaleTransform>
            </TextBlock.RenderTransform>
        </TextBlock>
    </Button

and a converter

public class ScaleConverter : IMultiValueConverter
{
    #region Implementation of IMultiValueConverter

    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        return ((double) values[0])/((double) values[1]);
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }

    #endregion
}