Avalonia/src/Avalonia.Themes.Fluent/Controls/ButtonSpinner.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:sys="clr-namespace:System;assembly=netstandard"
                    xmlns:converters="clr-namespace:Avalonia.Controls.Converters;assembly=Avalonia.Controls"
                    x:CompileBindings="True">
  <Design.PreviewWith>
    <Border Padding="20">
      <StackPanel Spacing="20">
        <ButtonSpinner Content="Right spinner" />
        <ButtonSpinner ButtonSpinnerLocation="Left"
                       Content="Left spinner" />
        <ButtonSpinner BorderThickness="2"
                       BorderBrush="Blue"
                       Content="Right Border" />
        <ButtonSpinner ButtonSpinnerLocation="Left"
                       BorderThickness="2"
                       BorderBrush="Blue"
                       Content="Left Border" />
        <ButtonSpinner Content="Right disabled"
                       AllowSpin="False"/>
        <ButtonSpinner ButtonSpinnerLocation="Left"
                       Content="Left disabled"
                       AllowSpin="False"/>
        <ButtonSpinner ShowButtonSpinner="False"
                       Content="Hide spinner" />
        <ButtonSpinner Content="Error">
          <DataValidationErrors.Error>
            <sys:Exception>
              <x:Arguments>
                <x:String>Error</x:String>
              </x:Arguments>
            </sys:Exception>
          </DataValidationErrors.Error>
        </ButtonSpinner>
      </StackPanel>
    </Border>
  </Design.PreviewWith>

  <converters:MarginMultiplierConverter x:Key="ButtonSpinnerLeftThickness" Indent="1" Left="True" />
  <converters:MarginMultiplierConverter x:Key="ButtonSpinnerRightThickness" Indent="1" Right="True" />
  <StreamGeometry x:Key="ButtonSpinnerIncreaseButtonIcon">M0,9 L10,0 20,9 19,10 10,2 1,10 z</StreamGeometry>
  <StreamGeometry x:Key="ButtonSpinnerDecreaseButtonIcon">M0,1 L10,10 20,1 19,0 10,8 1,0 z</StreamGeometry>

  <ControlTheme x:Key="FluentButtonSpinnerRepeatButton" TargetType="RepeatButton" BasedOn="{StaticResource {x:Type RepeatButton}}">
    <Setter Property="CornerRadius" Value="0"/>
    <Setter Property="MinWidth" Value="34" />
    <Setter Property="VerticalAlignment" Value="Stretch" />
    <Style Selector="^:disabled /template/ ContentPresenter#PART_ContentPresenter">
      <Setter Property="BorderBrush" Value="{TemplateBinding BorderBrush}" />
    </Style>
  </ControlTheme>

  <ControlTheme x:Key="{x:Type ButtonSpinner}" TargetType="ButtonSpinner">
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="Foreground" Value="{DynamicResource TextControlForeground}" />
    <Setter Property="Padding" Value="10, 0" />
    <Setter Property="Background" Value="{DynamicResource TextControlBackground}" />
    <Setter Property="BorderBrush" Value="{DynamicResource TextControlBorderBrush}" />
    <Setter Property="BorderThickness" Value="{DynamicResource TextControlBorderThemeThickness}" />
    <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
    <Setter Property="MinHeight" Value="{DynamicResource TextControlThemeMinHeight}" />
    <Setter Property="MinWidth" Value="{DynamicResource TextControlThemeMinWidth}" />
    <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}" />
    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
    <Setter Property="VerticalContentAlignment" Value="Center" />
    <Setter Property="Focusable" Value="True"/>
    <Setter Property="Template">
      <ControlTemplate>
        <DataValidationErrors>
          <Border Background="{TemplateBinding Background}"
                  BorderBrush="{TemplateBinding BorderBrush}"
                  BorderThickness="{TemplateBinding BorderThickness}"
                  CornerRadius="{TemplateBinding CornerRadius}"
                  MinHeight="{TemplateBinding MinHeight}">
            <DockPanel>
              <StackPanel Name="PART_SpinnerPanel"
                          DockPanel.Dock="Right"
                          Orientation="Horizontal"
                          IsVisible="{TemplateBinding ShowButtonSpinner}">
                <RepeatButton Name="PART_IncreaseButton"
                              Theme="{StaticResource FluentButtonSpinnerRepeatButton}"
                              Background="{TemplateBinding Background}"
                              BorderBrush="{TemplateBinding BorderBrush}"
                              BorderThickness="{TemplateBinding BorderThickness, Converter={StaticResource ButtonSpinnerLeftThickness}}"
                              CornerRadius="0"
                              VerticalAlignment="Stretch"
                              VerticalContentAlignment="Center"
                              Foreground="{TemplateBinding Foreground}"
                              FontSize="{TemplateBinding FontSize}"
                              MinWidth="34">
                  <Path Fill="{TemplateBinding Foreground}"
                        Width="16"
                        Height="8"
                        Stretch="Uniform"
                        HorizontalAlignment="Center"
                        VerticalAlignment="Center"
                        Data="{StaticResource ButtonSpinnerIncreaseButtonIcon}" />
                </RepeatButton>

                <RepeatButton Name="PART_DecreaseButton"
                              Theme="{StaticResource FluentButtonSpinnerRepeatButton}"
                              Background="{TemplateBinding Background}"
                              BorderBrush="{TemplateBinding BorderBrush}"
                              BorderThickness="{TemplateBinding BorderThickness, Converter={StaticResource ButtonSpinnerLeftThickness}}"
                              CornerRadius="0"
                              VerticalAlignment="Stretch"
                              VerticalContentAlignment="Center"
                              Foreground="{TemplateBinding Foreground}"
                              FontSize="{TemplateBinding FontSize}"
                              MinWidth="34">
                  <Path Fill="{TemplateBinding Foreground}"
                        Width="16"
                        Height="8"
                        Stretch="Uniform"
                        HorizontalAlignment="Center"
                        VerticalAlignment="Center"
                        Data="{StaticResource ButtonSpinnerDecreaseButtonIcon}" />
                </RepeatButton>
              </StackPanel>

              <ContentPresenter Name="PART_ContentPresenter"
                                Grid.Column="1"
                                ContentTemplate="{TemplateBinding ContentTemplate}"
                                Content="{TemplateBinding Content}"
                                HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                                VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                                Padding="{TemplateBinding Padding}" />
            </DockPanel>
          </Border>
        </DataValidationErrors>
      </ControlTemplate>
    </Setter>
    <Style Selector="^:left">
      <Style Selector="^ /template/ StackPanel#PART_SpinnerPanel">
        <Setter Property="DockPanel.Dock" Value="Left"/>
      </Style>
      <Style Selector="^ /template/ RepeatButton">
        <Setter Property="BorderThickness" Value="{TemplateBinding BorderThickness, Converter={StaticResource ButtonSpinnerRightThickness}}"/>
      </Style>
    </Style>
    <Style Selector="^:error">
      <Setter Property="BorderBrush" Value="{DynamicResource SystemErrorTextColor}" />
    </Style>
  </ControlTheme>

</ResourceDictionary>