feat: Add GotoTop control, style and demo.

src/Avalonia/HandyControlDemo_Avalonia/Data/DemoInfo.json

@@ -46,6 +46,7 @@
       [ "ButtonGroup", "ButtonGroupDemo", "Brush.ButtonGroup", "", "Button" ],
       [ "ProgressButton", "ProgressButtonDemo", "Brush.ProgressBarStatusBar8", "", "Button" ],
       [ "Card", "CardDemo", "Brush.ContactCard", "", "DataDisplay" ],
+      [ "GotoTop", "GotoTopDemo", "Brush.GoToTop", "", "Interacting" ],
       [ "PinBox", "PinBoxDemo", "Brush.Ellipsis", "", "DataEntry" ],
       [ "SearchBar", "SearchBarDemo", "Brush.Search", "", "DataEntry" ],
       [ "ElementGroup", "ElementGroupDemo", "Brush.ButtonGroup", "", "Layout" ],

src/Avalonia/HandyControlDemo_Avalonia/UserControl/Controls/GotoTopDemo.axaml

@@ -0,0 +1,34 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:hc="https://handyorg.github.io/handycontrol"
+             x:Class="HandyControlDemo.UserControl.GotoTopDemo"
+             Background="{DynamicResource RegionBrush}">
+    <TransitioningContentControl>
+        <Panel Height="400">
+            <ScrollViewer x:Name="ScrollViewerDemo"
+                          Height="390"
+                          VerticalAlignment="Top"
+                          Margin="0,10,0,0">
+                <Border Height="2000"
+                        Margin="8,0">
+                    <Border.Background>
+                        <LinearGradientBrush StartPoint="50%,0%"
+                                             EndPoint="50%,100%">
+                            <GradientStop Offset="0"
+                                          Color="White" />
+                            <GradientStop Offset="1"
+                                          Color="Black" />
+                        </LinearGradientBrush>
+                    </Border.Background>
+                </Border>
+            </ScrollViewer>
+            <hc:GotoTop Animated="True"
+                        AutoHiding="True"
+                        AnimationTime="500"
+                        Target="{Binding #ScrollViewerDemo}"
+                        HorizontalAlignment="Right"
+                        VerticalAlignment="Bottom"
+                        Margin="0,0,20,20" />
+        </Panel>
+    </TransitioningContentControl>
+</UserControl>

src/Avalonia/HandyControlDemo_Avalonia/UserControl/Controls/GotoTopDemo.axaml.cs

@@ -0,0 +1,9 @@
+namespace HandyControlDemo.UserControl;
+
+public partial class GotoTopDemo : Avalonia.Controls.UserControl
+{
+    public GotoTopDemo()
+    {
+        InitializeComponent();
+    }
+}

src/Avalonia/HandyControl_Avalonia/Controls/Other/GotoTop.cs

@@ -0,0 +1,193 @@
+using System;
+using System.Linq;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
+
+namespace HandyControl.Controls;
+
+public class GotoTop : Button
+{
+    public static readonly StyledProperty<Control?> TargetProperty =
+        AvaloniaProperty.Register<GotoTop, Control?>(nameof(Target));
+
+    public static readonly StyledProperty<bool> AnimatedProperty =
+        AvaloniaProperty.Register<GotoTop, bool>(nameof(Animated), true);
+
+    public static readonly StyledProperty<double> AnimationTimeProperty =
+        AvaloniaProperty.Register<GotoTop, double>(nameof(AnimationTime), 200d);
+
+    public static readonly StyledProperty<double> HidingHeightProperty =
+        AvaloniaProperty.Register<GotoTop, double>(nameof(HidingHeight));
+
+    public static readonly StyledProperty<bool> AutoHidingProperty =
+        AvaloniaProperty.Register<GotoTop, bool>(nameof(AutoHiding), true);
+
+    private ScrollViewer? _scrollViewer;
+    private DispatcherTimer? _animationTimer;
+    private DateTime _animationStartTime;
+    private double _animationStartOffset;
+
+    static GotoTop()
+    {
+        TargetProperty.Changed.AddClassHandler<GotoTop>((gotoTop, e) => gotoTop.CreateGotoAction(e.GetNewValue<Control?>()));
+        AutoHidingProperty.Changed.AddClassHandler<GotoTop>((gotoTop, _) => gotoTop.UpdateVisibility());
+        HidingHeightProperty.Changed.AddClassHandler<GotoTop>((gotoTop, _) => gotoTop.UpdateVisibility());
+    }
+
+    public GotoTop()
+    {
+        Loaded += (_, _) => CreateGotoAction(Target);
+    }
+
+    public Control? Target
+    {
+        get => GetValue(TargetProperty);
+        set => SetValue(TargetProperty, value);
+    }
+
+    public bool Animated
+    {
+        get => GetValue(AnimatedProperty);
+        set => SetValue(AnimatedProperty, value);
+    }
+
+    public double AnimationTime
+    {
+        get => GetValue(AnimationTimeProperty);
+        set => SetValue(AnimationTimeProperty, value);
+    }
+
+    public double HidingHeight
+    {
+        get => GetValue(HidingHeightProperty);
+        set => SetValue(HidingHeightProperty, value);
+    }
+
+    public bool AutoHiding
+    {
+        get => GetValue(AutoHidingProperty);
+        set => SetValue(AutoHidingProperty, value);
+    }
+
+    protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnDetachedFromVisualTree(e);
+
+        if (_scrollViewer != null)
+        {
+            _scrollViewer.ScrollChanged -= ScrollViewerOnScrollChanged;
+        }
+
+        if (_animationTimer != null)
+        {
+            _animationTimer.Stop();
+        }
+    }
+
+    public virtual void CreateGotoAction(Control? obj)
+    {
+        if (_scrollViewer != null)
+        {
+            _scrollViewer.ScrollChanged -= ScrollViewerOnScrollChanged;
+        }
+
+        _scrollViewer = null;
+        var target = obj ?? this;
+
+        if (target is ScrollViewer directScrollViewer)
+        {
+            _scrollViewer = directScrollViewer;
+        }
+        else
+        {
+            _scrollViewer = target.GetVisualDescendants().OfType<ScrollViewer>().FirstOrDefault();
+        }
+
+        if (_scrollViewer != null)
+        {
+            _scrollViewer.ScrollChanged += ScrollViewerOnScrollChanged;
+        }
+
+        UpdateVisibility();
+    }
+
+    private void ScrollViewerOnScrollChanged(object? sender, ScrollChangedEventArgs e)
+    {
+        UpdateVisibility();
+    }
+
+    private void UpdateVisibility()
+    {
+        if (!AutoHiding)
+        {
+            IsVisible = true;
+            return;
+        }
+
+        IsVisible = (_scrollViewer?.Offset.Y ?? 0d) >= HidingHeight;
+    }
+
+    protected override void OnClick()
+    {
+        base.OnClick();
+
+        if (_scrollViewer == null)
+        {
+            return;
+        }
+
+        if (!Animated || AnimationTime <= 0)
+        {
+            _scrollViewer.Offset = new Vector(_scrollViewer.Offset.X, 0);
+            return;
+        }
+
+        StartAnimatedScroll();
+    }
+
+    private void StartAnimatedScroll()
+    {
+        if (_scrollViewer == null)
+        {
+            return;
+        }
+
+        _animationStartOffset = _scrollViewer.Offset.Y;
+        if (_animationStartOffset <= 0)
+        {
+            return;
+        }
+
+        _animationStartTime = DateTime.Now;
+
+        _animationTimer ??= new DispatcherTimer { Interval = TimeSpan.FromMilliseconds(16) };
+        _animationTimer.Tick -= AnimationTimerOnTick;
+        _animationTimer.Tick += AnimationTimerOnTick;
+        _animationTimer.Start();
+    }
+
+    private void AnimationTimerOnTick(object? sender, EventArgs e)
+    {
+        if (_scrollViewer == null)
+        {
+            _animationTimer?.Stop();
+            return;
+        }
+
+        var total = Math.Max(AnimationTime, 1d);
+        var elapsed = (DateTime.Now - _animationStartTime).TotalMilliseconds;
+        var progress = Math.Clamp(elapsed / total, 0d, 1d);
+
+        var current = _animationStartOffset * (1d - progress);
+        _scrollViewer.Offset = new Vector(_scrollViewer.Offset.X, current);
+
+        if (progress >= 1d)
+        {
+            _animationTimer?.Stop();
+            _scrollViewer.Offset = new Vector(_scrollViewer.Offset.X, 0d);
+        }
+    }
+}

src/Avalonia/HandyControl_Avalonia/Themes/Styles/GotoTop.axaml

@@ -0,0 +1,45 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal"
+                    xmlns:hc="clr-namespace:HandyControl.Controls">
+
+    <ControlTheme x:Key="GotoTopBaseStyle"
+                  BasedOn="{StaticResource ButtonBaseStyle}"
+                  TargetType="hc:GotoTop">
+        <Setter Property="Width"
+                Value="50" />
+        <Setter Property="Height"
+                Value="50" />
+        <Setter Property="hc:BorderElement.CornerRadius"
+                Value="25" />
+        <Setter Property="CornerRadius"
+                Value="{Binding $self.(hc:BorderElement.CornerRadius)}" />
+        <Setter Property="hc:IconElement.Geometry"
+                Value="{StaticResource UpGeometry}" />
+        <Setter Property="hc:IconElement.Width"
+                Value="14" />
+        <Setter Property="hc:IconElement.Height"
+                Value="14" />
+        <Setter Property="Foreground"
+                Value="{DynamicResource PrimaryTextBrush}" />
+        <Setter Property="Padding"
+                Value="16" />
+        <Setter Property="BorderThickness"
+                Value="1" />
+
+        <Style Selector="^:pointerover">
+            <Setter Property="Background"
+                    Value="{DynamicResource SecondaryRegionBrush}" />
+        </Style>
+
+        <Style Selector="^:pressed">
+            <Setter Property="Background"
+                    Value="{DynamicResource BorderBrush}" />
+        </Style>
+    </ControlTheme>
+
+    <ControlTheme x:Key="{x:Type hc:GotoTop}"
+                  BasedOn="{StaticResource GotoTopBaseStyle}"
+                  TargetType="hc:GotoTop" />
+
+</ResourceDictionary>

src/Avalonia/HandyControl_Avalonia/Themes/Styles/MergedStyles.axaml

@@ -75,6 +75,7 @@
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/Gravatar.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/Tag.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/Badge.axaml" />
+                    <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/GotoTop.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/Shield.axaml" />
             </ResourceDictionary.MergedDictionaries>
         </ResourceDictionary>