Add IsDirectionReversed to Slider.

Fixes #3617.

src/Avalonia.Controls/Slider.cs

@@ -49,6 +49,12 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<Orientation> OrientationProperty =
             ScrollBar.OrientationProperty.AddOwner<Slider>();
 
+        /// <summary>
+        /// Defines the <see cref="IsDirectionReversed"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsDirectionReversedProperty =
+            Track.IsDirectionReversedProperty.AddOwner<Slider>();
+
         /// <summary>
         /// Defines the <see cref="IsSnapToTickEnabled"/> property.
         /// </summary>
@@ -127,6 +133,19 @@ namespace Avalonia.Controls
             set { SetValue(OrientationProperty, value); }
         }
 
+        /// <summary>
+        /// Gets or sets the direction of increasing value.
+        /// </summary>
+        /// <value>
+        /// true if the direction of increasing value is to the left for a horizontal slider or
+        /// down for a vertical slider; otherwise, false. The default is false.
+        /// </value>
+        public bool IsDirectionReversed
+        {
+            get { return GetValue(IsDirectionReversedProperty); }
+            set { SetValue(IsDirectionReversedProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets a value that indicates whether the <see cref="Slider"/> automatically moves the <see cref="Thumb"/> to the closest tick mark.
         /// </summary>
@@ -202,20 +221,20 @@ namespace Avalonia.Controls
             {
                 case Key.Down:
                 case Key.Left:
-                    MoveToNextTick(-SmallChange);
+                    MoveToNextTick(IsDirectionReversed ? SmallChange : -SmallChange);
                     break;
 
                 case Key.Up:
                 case Key.Right:
-                    MoveToNextTick(SmallChange);
+                    MoveToNextTick(IsDirectionReversed ? -SmallChange : SmallChange);
                     break;
 
                 case Key.PageUp:
-                    MoveToNextTick(LargeChange);
+                    MoveToNextTick(IsDirectionReversed ? -LargeChange : LargeChange);
                     break;
 
                 case Key.PageDown:
-                    MoveToNextTick(-LargeChange);
+                    MoveToNextTick(IsDirectionReversed ? LargeChange : -LargeChange);
                     break;
 
                 case Key.Home:

src/Avalonia.Themes.Default/Slider.xaml

@@ -11,7 +11,7 @@
             <RowDefinition Height="Auto"/>
           </Grid.RowDefinitions>
           <Border Name="TrackBackground" Grid.Row="1" Height="4" Margin="6,0" VerticalAlignment="Center"/>
-          <Track Name="PART_Track" Grid.Row="1" Orientation="Horizontal">
+          <Track Name="PART_Track" Grid.Row="1" IsDirectionReversed="{TemplateBinding IsDirectionReversed}" Orientation="Horizontal">
             <Track.DecreaseButton>
                <RepeatButton Name="PART_DecreaseButton"
                              Classes="repeattrack" />
@@ -46,7 +46,7 @@
             <ColumnDefinition Width="Auto"/>
           </Grid.ColumnDefinitions>
           <Border Name="TrackBackground" Grid.Column="1" Width="4" Margin="0,6" HorizontalAlignment="Center"/>
-          <Track Name="PART_Track" Grid.Column="1" Orientation="Vertical">
+          <Track Name="PART_Track" Grid.Column="1" IsDirectionReversed="{TemplateBinding IsDirectionReversed}" Orientation="Vertical">
             <Track.DecreaseButton>
                <RepeatButton Name="PART_DecreaseButton"
                              Classes="repeattrack" />

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

@@ -62,7 +62,7 @@
                   <TickBar Name="TopTickBar" Placement="Top" Height="{DynamicResource SliderOutsideTickBarThemeHeight}" VerticalAlignment="Bottom" Margin="0,0,0,4" Grid.ColumnSpan="3" />
                   <!-- <TickBar Name="HorizontalInlineTickBar" Placement="Top" Fill="{DynamicResource SliderInlineTickBarFill}" Height="{DynamicResource SliderTrackThemeHeight}" Grid.Row="1" Grid.ColumnSpan="3" /> -->
                   <TickBar Name="BottomTickBar" Placement="Bottom" Height="{DynamicResource SliderOutsideTickBarThemeHeight}" VerticalAlignment="Top" Margin="0,4,0,0" Grid.Row="2" Grid.ColumnSpan="3" />
-                  <Track Name="PART_Track" Grid.Row="1" Grid.ColumnSpan="3" Orientation="Horizontal">
+                  <Track Name="PART_Track" Grid.Row="1" Grid.ColumnSpan="3" IsDirectionReversed="{TemplateBinding IsDirectionReversed}" Orientation="Horizontal">
                     <Track.DecreaseButton>
                       <RepeatButton Name="PART_DecreaseButton" Background="{TemplateBinding Foreground}" Focusable="False" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
                         <RepeatButton.Template>
@@ -124,7 +124,7 @@
                   <TickBar Name="LeftTickBar" Placement="Left" Width="{DynamicResource SliderOutsideTickBarThemeHeight}" HorizontalAlignment="Right" Margin="0,0,4,0" Grid.RowSpan="3" />
                   <!-- <TickBar Name="VerticalInlineTickBar" Placement="Inline" Fill="{DynamicResource SliderInlineTickBarFill}" Width="{DynamicResource SliderTrackThemeHeight}" Grid.Column="1" Grid.RowSpan="3" /> -->
                   <TickBar Name="RightTickBar" Placement="Right" Width="{DynamicResource SliderOutsideTickBarThemeHeight}" HorizontalAlignment="Left" Margin="4,0,0,0" Grid.Column="2" Grid.RowSpan="3" />
-                  <Track Name="PART_Track" Grid.Column="1" Grid.ColumnSpan="1" Grid.RowSpan="3" Orientation="Vertical">
+                  <Track Name="PART_Track" Grid.Column="1" Grid.ColumnSpan="1" Grid.RowSpan="3" IsDirectionReversed="{TemplateBinding IsDirectionReversed}" Orientation="Vertical">
                     <Track.DecreaseButton>
                       <RepeatButton Name="PART_DecreaseButton" Background="{TemplateBinding Foreground}" Focusable="False" VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
                         <RepeatButton.Template>