Avalonia/samples/RenderDemo/Pages/CustomAnimatorPage.xaml

<UserControl 
  xmlns="https://github.com/avaloniaui" 
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
  xmlns:pages="clr-namespace:RenderDemo.Pages"
  x:Class="RenderDemo.Pages.CustomAnimatorPage"
  MaxWidth="600">
  <Grid>
    <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center">
      <TextBlock.Styles>
        <Style Selector="TextBlock">
          <Style.Animations>
            <Animation Duration="0:0:1" IterationCount="Infinite">
              <KeyFrame Cue="0%">
                <Setter Property="Text" Value="" Animation.Animator="{x:Type pages:CustomStringAnimator}"/>
              </KeyFrame>
              <KeyFrame Cue="100%">
                <Setter Property="Text" Value="0123456789" Animation.Animator="{x:Type pages:CustomStringAnimator}"/>
              </KeyFrame>
            </Animation>
          </Style.Animations>
        </Style>
      </TextBlock.Styles>
    </TextBlock>
  </Grid>
</UserControl>