feat: Add ElementGroup control, styles and demo.

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

@@ -0,0 +1,45 @@
+<UserControl x:Class="HandyControlDemo.UserControl.ElementGroupDemo"
+             xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:hc="https://handyorg.github.io/handycontrol"
+             xmlns:langs="clr-namespace:HandyControlDemo.Properties.Langs"
+             Background="{DynamicResource RegionBrush}">
+    <TransitioningContentControl>
+        <hc:UniformSpacingPanel Margin="32" VerticalAlignment="Center" Orientation="Vertical" Spacing="16">
+            <hc:ElementGroup Orientation="Horizontal" Layout="Stack">
+                <Button HorizontalAlignment="Stretch" Width="NaN" Content="{x:Static langs:Lang.Button}"/>
+                <Button HorizontalAlignment="Stretch" Width="NaN" Content="{x:Static langs:Lang.Button}"/>
+                <Button HorizontalAlignment="Stretch" Width="NaN" Content="{x:Static langs:Lang.Button}"/>
+                <Button HorizontalAlignment="Stretch" Width="NaN" Content="{x:Static langs:Lang.Button}"/>
+            </hc:ElementGroup>
+            <hc:ElementGroup Orientation="Horizontal" MaxWidth="500">
+                <ToggleButton HorizontalAlignment="Stretch" Width="NaN" Content="{x:Static langs:Lang.ToggleButton}"/>
+                <Button HorizontalAlignment="Stretch" Width="NaN" Content="{x:Static langs:Lang.Button}"/>
+                <ToggleButton HorizontalAlignment="Stretch" Width="NaN" Content="{x:Static langs:Lang.ToggleButton}"/>
+                <Button HorizontalAlignment="Stretch" Width="NaN" Content="{x:Static langs:Lang.Button}"/>
+            </hc:ElementGroup>
+            <hc:ElementGroup Orientation="Horizontal" Layout="Stack">
+                <Border Theme="{StaticResource BorderRegion}" Padding="6,0">
+                    <Path Data="{StaticResource ClockGeometry}" Width="16" Height="16" Stretch="Uniform" Fill="{DynamicResource BorderBrush}"/>
+                </Border>
+                <TextBox MinWidth="200"/>
+                <Button Content="{x:Static langs:Lang.Button}"/>
+                <ToggleButton Content="{x:Static langs:Lang.ToggleButton}"/>
+            </hc:ElementGroup>
+            <hc:ElementGroup>
+                <hc:ElementGroup.ItemsPanel>
+                    <ItemsPanelTemplate>
+                        <Grid>
+                            <Grid.ColumnDefinitions>
+                                <ColumnDefinition/>
+                                <ColumnDefinition Width="Auto"/>
+                            </Grid.ColumnDefinitions>
+                        </Grid>
+                    </ItemsPanelTemplate>
+                </hc:ElementGroup.ItemsPanel>
+                <TextBox HorizontalAlignment="Stretch"/>
+                <Button Grid.Column="1" Width="100" Content="{x:Static langs:Lang.Button}" HorizontalAlignment="Right" Theme="{StaticResource ButtonPrimary}"/>
+            </hc:ElementGroup>
+        </hc:UniformSpacingPanel>
+    </TransitioningContentControl>
+</UserControl>

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

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

src/Avalonia/HandyControl_Avalonia/Controls/Panel/ElementGroup.cs

@@ -0,0 +1,170 @@
+using System.Collections.Generic;
+using System.Linq;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+using Avalonia.Layout;
+using HandyControl.Data;
+using HandyControl.Tools;
+
+namespace HandyControl.Controls;
+
+public class ElementGroup : ItemsControl
+{
+    private static readonly Dictionary<Orientation, Dictionary<ChildLocation, CornerRadius>> CornerRadiusDict;
+
+    public static readonly StyledProperty<Orientation> OrientationProperty =
+        StackPanel.OrientationProperty.AddOwner<ElementGroup>(
+            new StyledPropertyMetadata<Orientation>(defaultValue: Orientation.Horizontal));
+
+    public Orientation Orientation
+    {
+        get => GetValue(OrientationProperty);
+        set => SetValue(OrientationProperty, value);
+    }
+
+    public static readonly StyledProperty<LinearLayout> LayoutProperty =
+        AvaloniaProperty.Register<ElementGroup, LinearLayout>(nameof(Layout), defaultValue: LinearLayout.Uniform);
+
+    public LinearLayout Layout
+    {
+        get => GetValue(LayoutProperty);
+        set => SetValue(LayoutProperty, value);
+    }
+
+    static ElementGroup()
+    {
+        AffectsMeasure<ElementGroup>(OrientationProperty, LayoutProperty);
+
+        var defaultCornerRadius = ResourceHelper.GetResource<CornerRadius>("DefaultCornerRadius");
+
+        CornerRadiusDict = new Dictionary<Orientation, Dictionary<ChildLocation, CornerRadius>>
+        {
+            [Orientation.Horizontal] = new()
+            {
+                [ChildLocation.Single] = defaultCornerRadius,
+                [ChildLocation.First] = new CornerRadius(defaultCornerRadius.TopLeft, 0, 0, defaultCornerRadius.BottomLeft),
+                [ChildLocation.Middle] = new CornerRadius(),
+                [ChildLocation.Last] = new CornerRadius(0, defaultCornerRadius.TopRight, defaultCornerRadius.BottomRight, 0)
+            },
+            [Orientation.Vertical] = new()
+            {
+                [ChildLocation.Single] = defaultCornerRadius,
+                [ChildLocation.First] = new CornerRadius(defaultCornerRadius.TopLeft, defaultCornerRadius.TopRight, 0, 0),
+                [ChildLocation.Middle] = new CornerRadius(),
+                [ChildLocation.Last] = new CornerRadius(0, 0, defaultCornerRadius.BottomRight, defaultCornerRadius.BottomLeft)
+            }
+        };
+    }
+
+    protected override void PrepareContainerForItemOverride(Control container, object? item, int index)
+    {
+        base.PrepareContainerForItemOverride(container, item, index);
+
+        container.GotFocus += ElementAdded_GotFocus;
+        container.LostFocus += ElementAdded_LostFocus;
+        container.PropertyChanged += Container_PropertyChanged;
+
+        UpdateChildrenCornerRadius();
+    }
+
+    protected override void ClearContainerForItemOverride(Control container)
+    {
+        base.ClearContainerForItemOverride(container);
+
+        container.GotFocus -= ElementAdded_GotFocus;
+        container.LostFocus -= ElementAdded_LostFocus;
+        container.PropertyChanged -= Container_PropertyChanged;
+
+        UpdateChildrenCornerRadius();
+    }
+
+    private void Container_PropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
+    {
+        if (e.Property == IsVisibleProperty)
+        {
+            UpdateChildrenCornerRadius();
+        }
+    }
+
+    private void UpdateChildrenCornerRadius()
+    {
+        var visibleChildren = GetVisibleChildren();
+        var count = visibleChildren.Count;
+        if (count == 0)
+        {
+            return;
+        }
+
+        var orientation = Orientation;
+
+        if (count == 1)
+        {
+            UpdateChildCornerRadius(visibleChildren[0], CornerRadiusDict[orientation][ChildLocation.Single]);
+        }
+        else
+        {
+            UpdateChildCornerRadius(visibleChildren[0], CornerRadiusDict[orientation][ChildLocation.First]);
+
+            var childMargin = orientation == Orientation.Horizontal
+                ? new Thickness(-1, 0, 0, 0)
+                : new Thickness(0, -1, 0, 0);
+
+            for (var childIndex = 1; childIndex < count; childIndex++)
+            {
+                var child = visibleChildren[childIndex];
+                child.SetCurrentValue(MarginProperty, childMargin);
+                UpdateChildCornerRadius(child, CornerRadiusDict[orientation][ChildLocation.Middle]);
+            }
+
+            var lastChild = visibleChildren[count - 1];
+            lastChild.SetCurrentValue(MarginProperty, childMargin);
+            UpdateChildCornerRadius(lastChild, CornerRadiusDict[orientation][ChildLocation.Last]);
+        }
+    }
+
+    private List<Control> GetVisibleChildren()
+    {
+        return Items.OfType<Control>().Where(element => element.IsVisible).ToList();
+    }
+
+    private static void UpdateChildCornerRadius(AvaloniaObject child, CornerRadius cornerRadius)
+    {
+        if (child is Border border)
+        {
+            border.SetCurrentValue(Border.CornerRadiusProperty, cornerRadius);
+        }
+        else
+        {
+            BorderElement.SetCornerRadius(child, cornerRadius);
+        }
+    }
+
+    private void ElementAdded_LostFocus(object? sender, RoutedEventArgs e) => ResetElementZIndex(e.Source);
+
+    private void ElementAdded_GotFocus(object? sender, RoutedEventArgs e) => MaximizeElementZIndex(e.Source);
+
+    private static void ResetElementZIndex(object? element)
+    {
+        if (element is Control control)
+        {
+            control.ZIndex = 0;
+        }
+    }
+
+    private static void MaximizeElementZIndex(object? element)
+    {
+        if (element is Control control)
+        {
+            control.ZIndex = int.MaxValue;
+        }
+    }
+
+    private enum ChildLocation
+    {
+        Single,
+        First,
+        Middle,
+        Last
+    }
+}

src/Avalonia/HandyControl_Avalonia/Data/Enum/LinearLayout.cs

@@ -0,0 +1,8 @@
+namespace HandyControl.Data;
+
+public enum LinearLayout
+{
+    Stack,
+
+    Uniform
+}

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

@@ -0,0 +1,56 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal"
+                    xmlns:hc="clr-namespace:HandyControl.Controls">
+
+    <ItemsPanelTemplate x:Key="UniformHorizontalItemsPanelTemplate">
+        <UniformGrid Rows="1" />
+    </ItemsPanelTemplate>
+
+    <ItemsPanelTemplate x:Key="UniformVerticalItemsPanelTemplate">
+        <UniformGrid Columns="1" />
+    </ItemsPanelTemplate>
+
+    <ItemsPanelTemplate x:Key="StackHorizontalItemsPanelTemplate">
+        <StackPanel Orientation="Horizontal" />
+    </ItemsPanelTemplate>
+
+    <ItemsPanelTemplate x:Key="StackVerticalItemsPanelTemplate">
+        <StackPanel Orientation="Vertical" />
+    </ItemsPanelTemplate>
+
+    <ControlTheme x:Key="ElementGroupBaseStyle"
+                  TargetType="hc:ElementGroup">
+        <Setter Property="VerticalAlignment" Value="Top" />
+        <Setter Property="Layout" Value="Uniform" />
+        <Setter Property="Focusable" Value="False" />
+        <Setter Property="ItemsPanel" Value="{StaticResource UniformHorizontalItemsPanelTemplate}" />
+        <Setter Property="Template">
+            <ControlTemplate TargetType="hc:ElementGroup">
+                <Border Padding="{TemplateBinding Padding}"
+                        Background="{TemplateBinding Background}"
+                        BorderBrush="{TemplateBinding BorderBrush}"
+                        BorderThickness="{TemplateBinding BorderThickness}"
+                        CornerRadius="{TemplateBinding CornerRadius}">
+                    <ItemsPresenter Name="PART_ItemsPresenter"
+                                    ItemsPanel="{TemplateBinding ItemsPanel}" />
+                </Border>
+            </ControlTemplate>
+        </Setter>
+
+        <Style Selector="^[Layout=Uniform][Orientation=Vertical]">
+            <Setter Property="ItemsPanel" Value="{StaticResource UniformVerticalItemsPanelTemplate}" />
+        </Style>
+        <Style Selector="^[Layout=Stack][Orientation=Horizontal]">
+            <Setter Property="ItemsPanel" Value="{StaticResource StackHorizontalItemsPanelTemplate}" />
+        </Style>
+        <Style Selector="^[Layout=Stack][Orientation=Vertical]">
+            <Setter Property="ItemsPanel" Value="{StaticResource StackVerticalItemsPanelTemplate}" />
+        </Style>
+    </ControlTheme>
+
+    <ControlTheme x:Key="{x:Type hc:ElementGroup}"
+                  BasedOn="{StaticResource ElementGroupBaseStyle}"
+                  TargetType="hc:ElementGroup" />
+
+</ResourceDictionary>

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

@@ -58,6 +58,7 @@
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/SelectableTextBlock.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/HeaderedContentControl.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/Card.axaml" />
+                <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/ElementGroup.axaml" />
             </ResourceDictionary.MergedDictionaries>
         </ResourceDictionary>
     </Styles.Resources>