Avalonia/samples/ReactiveUIDemo/Views/FooView.axaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="ReactiveUIDemo.Views.FooView"
             HorizontalAlignment="Stretch"
             VerticalAlignment="Stretch">
  <Border Background="Red">
    <TextBlock HorizontalAlignment="Center"
               VerticalAlignment="Center"
               FontSize="48">
      Foo!
    </TextBlock>
  </Border>
</UserControl>