How to automatically scale font size for a group of controls?

I wanted to edit the answer I had already offered, but then decided it makes more sense to post a new one, because it really depends on the requirements which one I’d prefer. This here probably fits Alan’s idea better, because

• The middle textblock stays in the middle of the window
• Fontsize adjustment due to height clipping is accomodated
• Quite a bit more generic
• No viewbox involved

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The other one has the advantage that

• Space for the textblocks is allocated more efficiently (no unnecessary margins)
• Textblocks may have different fontsizes

I tested this solution also in a top container of type StackPanel/DockPanel, behaved decently.

Note that by playing around with the column/row widths/heights (auto/starsized), you can get different behaviors. So it would also be possible to have all three textblock columns starsized, but that means width clipping does occur earlier and there is more margin. Or if the row the grid resides in is auto sized, height clipping will never occur.

Xaml:

<Window x:Class="WpfApplication1.MainWindow"
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
            xmlns:beh="clr-namespace:WpfApplication1.Behavior"
            Title="MainWindow" Height="350" Width="525">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="0.9*"/>
            <RowDefinition Height="0.1*" />
        </Grid.RowDefinitions>

        <Rectangle Fill="DarkOrange" />

        <Grid x:Name="TextBlockContainer" Grid.Row="1" >
            <i:Interaction.Behaviors>
                <beh:ScaleFontBehavior MaxFontSize="32" />
            </i:Interaction.Behaviors>
            <Grid.Resources>
                <Style TargetType="TextBlock" >
                    <Setter Property="Margin" Value="5" />
                    <Setter Property="VerticalAlignment" Value="Center" />
                </Style>
            </Grid.Resources>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"  />
                <ColumnDefinition Width="Auto"  />
                <ColumnDefinition Width="*"  />
            </Grid.ColumnDefinitions>

            <TextBlock Grid.Column="0" Text="SomeLongText" />
            <TextBlock Grid.Column="1" Text="TextA" HorizontalAlignment="Center"  />
            <TextBlock Grid.Column="2" Text="TextB" HorizontalAlignment="Right"  />
        </Grid>
    </Grid>
</Window>

ScaleFontBehavior:

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;
using System.Windows.Media;
using WpfApplication1.Helpers;

namespace WpfApplication1.Behavior
{
    public class ScaleFontBehavior : Behavior<Grid>
    {
        // MaxFontSize
        public double MaxFontSize { get { return (double)GetValue(MaxFontSizeProperty); } set { SetValue(MaxFontSizeProperty, value); } }
        public static readonly DependencyProperty MaxFontSizeProperty = DependencyProperty.Register("MaxFontSize", typeof(double), typeof(ScaleFontBehavior), new PropertyMetadata(20d));

        protected override void OnAttached()
        {
            this.AssociatedObject.SizeChanged += (s, e) => { CalculateFontSize(); };
        }

        private void CalculateFontSize()
        {
            double fontSize = this.MaxFontSize;

            List<TextBlock> tbs = VisualHelper.FindVisualChildren<TextBlock>(this.AssociatedObject);

            // get grid height (if limited)
            double gridHeight = double.MaxValue;
            Grid parentGrid = VisualHelper.FindUpVisualTree<Grid>(this.AssociatedObject.Parent);
            if (parentGrid != null)
            {
                RowDefinition row = parentGrid.RowDefinitions[Grid.GetRow(this.AssociatedObject)];
                gridHeight = row.Height == GridLength.Auto ? double.MaxValue : this.AssociatedObject.ActualHeight;
            }

            foreach (var tb in tbs)
            {
                // get desired size with fontsize = MaxFontSize
                Size desiredSize = MeasureText(tb);
                double widthMargins = tb.Margin.Left + tb.Margin.Right;
                double heightMargins = tb.Margin.Top + tb.Margin.Bottom; 

                double desiredHeight = desiredSize.Height + heightMargins;
                double desiredWidth = desiredSize.Width + widthMargins;

                // adjust fontsize if text would be clipped vertically
                if (gridHeight < desiredHeight)
                {
                    double factor = (desiredHeight - heightMargins) / (this.AssociatedObject.ActualHeight - heightMargins);
                    fontSize = Math.Min(fontSize, MaxFontSize / factor);
                }

                // get column width (if limited)
                ColumnDefinition col = this.AssociatedObject.ColumnDefinitions[Grid.GetColumn(tb)];
                double colWidth = col.Width == GridLength.Auto ? double.MaxValue : col.ActualWidth;

                // adjust fontsize if text would be clipped horizontally
                if (colWidth < desiredWidth)
                {
                    double factor = (desiredWidth - widthMargins) / (col.ActualWidth - widthMargins);
                    fontSize = Math.Min(fontSize, MaxFontSize / factor);
                }
            }

            // apply fontsize (always equal fontsizes)
            foreach (var tb in tbs)
            {
                tb.FontSize = fontSize;
            }
        }

        // Measures text size of textblock
        private Size MeasureText(TextBlock tb)
        {
            var formattedText = new FormattedText(tb.Text, CultureInfo.CurrentUICulture,
                FlowDirection.LeftToRight,
                new Typeface(tb.FontFamily, tb.FontStyle, tb.FontWeight, tb.FontStretch),
                this.MaxFontSize, Brushes.Black); // always uses MaxFontSize for desiredSize

            return new Size(formattedText.Width, formattedText.Height);
        }
    }
}

VisualHelper:

public static List<T> FindVisualChildren<T>(DependencyObject obj) where T : DependencyObject
{
    List<T> children = new List<T>();
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
    {
        var o = VisualTreeHelper.GetChild(obj, i);
        if (o != null)
        {
            if (o is T)
                children.Add((T)o);

            children.AddRange(FindVisualChildren<T>(o)); // recursive
        }
    }
    return children;
}

public static T FindUpVisualTree<T>(DependencyObject initial) where T : DependencyObject
{
    DependencyObject current = initial;

    while (current != null && current.GetType() != typeof(T))
    {
        current = VisualTreeHelper.GetParent(current);
    }
    return current as T;
}