Avalonia/samples/ControlCatalog/Pages/ViewboxPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.ViewboxPage">
    <UserControl.Resources>
        <StreamGeometry x:Key="Acorn">
            F1 M 16.6309,18.6563C 17.1309,
            8.15625 29.8809,14.1563 29.8809,
            14.1563C 30.8809,11.1563 34.1308,
            11.4063 34.1308,11.4063C 33.5,12
            34.6309,13.1563 34.6309,13.1563C
            32.1309,13.1562 31.1309,14.9062
            31.1309,14.9062C 41.1309,23.9062
            32.6309,27.9063 32.6309,27.9062C
            24.6309,24.9063 21.1309,22.1562
            16.6309,18.6563 Z M 16.6309,19.9063C
            21.6309,24.1563 25.1309,26.1562
            31.6309,28.6562C 31.6309,28.6562
            26.3809,39.1562 18.3809,36.1563C
            18.3809,36.1563 18,38 16.3809,36.9063C
            15,36 16.3809,34.9063 16.3809,34.9063C
            16.3809,34.9063 10.1309,30.9062 16.6309,19.9063 Z
        </StreamGeometry>
    </UserControl.Resources>

    <Grid RowDefinitions="Auto,*,*">
        <StackPanel Orientation="Vertical" Spacing="4">
            <TextBlock Classes="h1">Viewbox</TextBlock>
            <TextBlock Classes="h2">A control used to scale single child.</TextBlock>
        </StackPanel>

        <Grid Grid.Row="1" ColumnDefinitions="*,Auto" HorizontalAlignment="Center" Margin="0,10,0,0">

          <Border HorizontalAlignment="Center" Grid.Column="0" BorderThickness="1" BorderBrush="Orange" Width="200" Height="200">
            <Border VerticalAlignment="Center" HorizontalAlignment="Center" Grid.Column="0" BorderThickness="1" BorderBrush="CornflowerBlue" Width="{Binding #WidthSlider.Value}" Height="{Binding #HeightSlider.Value}" >
              <Viewbox x:Name="Viewbox"
                StretchDirection="{Binding #StretchDirectionSelector.SelectedItem}">
                <Ellipse Width="50" Height="50" Fill="CornflowerBlue" />
              </Viewbox>
            </Border>
          </Border>

          <StackPanel HorizontalAlignment="Left" Orientation="Vertical" Grid.Column="1" Margin="8,0,0,0" Width="150">
            <TextBlock Text="Width" />
            <Slider Minimum="10" Maximum="200" Value="100" x:Name="WidthSlider" TickFrequency="25" TickPlacement="TopLeft" />
            <TextBlock Text="Height" />
            <Slider Minimum="10" Maximum="200" Value="100" x:Name="HeightSlider" TickFrequency="25" TickPlacement="TopLeft" />
            <TextBlock Text="Stretch" />
            <ComboBox x:Name="StretchSelector" HorizontalAlignment="Stretch" Margin="0,0,0,2" SelectionChanged="StretchSelector_OnSelectionChanged" />
            <TextBlock Text="Stretch Direction" />
            <ComboBox x:Name="StretchDirectionSelector" HorizontalAlignment="Stretch" />
          </StackPanel>
        </Grid>

        <Grid ColumnDefinitions="Auto,*,*"
              RowDefinitions="*,*,*,*"
              Grid.Row="2" Margin="48"
              MaxWidth="400">
            <TextBlock Grid.Row="0" VerticalAlignment="Center">None</TextBlock>
            <TextBlock Grid.Row="1" VerticalAlignment="Center">Fill</TextBlock>
            <TextBlock Grid.Row="2" VerticalAlignment="Center">Uniform</TextBlock>
            <TextBlock Grid.Row="3" VerticalAlignment="Center">UniformToFill</TextBlock>

            <Viewbox Grid.Row="0" Grid.Column="1" Stretch="None">
                <TextBlock>Hello World!</TextBlock>
            </Viewbox>
            <Viewbox Grid.Row="1" Grid.Column="1" Stretch="Fill">
                <TextBlock>Hello World!</TextBlock>
            </Viewbox>
            <Viewbox Grid.Row="2" Grid.Column="1" Stretch="Uniform">
                <TextBlock>Hello World!</TextBlock>
            </Viewbox>
            <Viewbox Grid.Row="3" Grid.Column="1" Stretch="UniformToFill">
                <TextBlock>Hello World!</TextBlock>
            </Viewbox>

            <Viewbox Grid.Row="0" Grid.Column="2" Stretch="None">
                <Path Fill="Blue" Data="{StaticResource Acorn}"/>
            </Viewbox>
            <Viewbox Grid.Row="1" Grid.Column="2" Stretch="Fill">
                <Path Fill="Blue" Data="{StaticResource Acorn}"/>
            </Viewbox>
            <Viewbox Grid.Row="2" Grid.Column="2" Stretch="Uniform">
                <Path Fill="Blue" Data="{StaticResource Acorn}"/>
            </Viewbox>
            <Viewbox Grid.Row="3" Grid.Column="2" Stretch="UniformToFill">
                <Path Fill="Blue" Data="{StaticResource Acorn}"/>
            </Viewbox>
        </Grid>
    </Grid>
</UserControl>