feat: initialize control.

src/Ursa/Controls/Timeline/Timeline.cs

@@ -0,0 +1,50 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Generators;
+using Avalonia.Controls.Templates;
+
+namespace Ursa.Controls;
+
+public class Timeline: ItemsControl
+{
+    
+    public static readonly StyledProperty<IDataTemplate?> ItemDescriptionTemplateProperty = AvaloniaProperty.Register<Timeline, IDataTemplate?>(
+        nameof(ItemDescriptionTemplate));
+
+    public IDataTemplate? ItemDescriptionTemplate
+    {
+        get => GetValue(ItemDescriptionTemplateProperty);
+        set => SetValue(ItemDescriptionTemplateProperty, value);
+    }
+
+    protected override bool IsItemItsOwnContainerOverride(Control item)
+    {
+        return item is TimelineItem;
+    }
+
+    protected override Control CreateContainerForItemOverride()
+    {
+        return new TimelineItem();
+    }
+    
+    protected override void PrepareContainerForItemOverride(Control container, object? item, int index)
+    {
+        base.PrepareContainerForItemOverride(container, item, index);
+        if (container is TimelineItem c )
+        {
+            if (item is ITimelineItemData data)
+            {
+                c[TimelineItem.TimeProperty] = data;
+                c[ContentControl.ContentProperty] = data.Content;
+                c[TimelineItem.DescriptionProperty] = data.Description;
+                if(ItemTemplate is {}) c[ContentControl.ContentTemplateProperty] = this.ItemTemplate;
+                if(ItemDescriptionTemplate is {}) c[TimelineItem.DescriptionTemplateProperty] = this.ItemDescriptionTemplate;
+            }
+            else
+            {
+                c.Content = item;
+                if (ItemTemplate is { }) c[ContentControl.ContentTemplateProperty] = this.ItemTemplate;
+            }
+        }
+    }
+}

src/Ursa/Controls/Timeline/TimelineItem.cs

@@ -0,0 +1,59 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
+using Avalonia.Controls.Templates;
+using Avalonia.Data;
+using Avalonia.Media;
+
+namespace Ursa.Controls;
+
+[PseudoClasses(PC_First, PC_Last)]
+public class TimelineItem: ContentControl
+{
+    public const string PC_First = ":first";
+    public const string PC_Last = ":last";
+    
+    public static readonly StyledProperty<IBrush> IconForegroundProperty =
+        AvaloniaProperty.Register<TimelineItem, IBrush>(nameof(IconForeground));
+
+    public IBrush IconForeground
+    {
+        get => GetValue(IconForegroundProperty);
+        set => SetValue(IconForegroundProperty, value);
+    }
+
+    public static readonly StyledProperty<object?> DescriptionProperty =
+        AvaloniaProperty.Register<TimelineItem, object?>(nameof(Description));
+
+    public object? Description
+    {
+        get => GetValue(DescriptionProperty);
+        set => SetValue(DescriptionProperty, value);
+    }
+
+    public static readonly StyledProperty<DateTime> TimeProperty = AvaloniaProperty.Register<TimelineItem, DateTime>(
+        nameof(Time));
+    public DateTime Time
+    {
+        get => GetValue(TimeProperty);
+        set => SetValue(TimeProperty, value);
+    }
+
+    public static readonly StyledProperty<string?> TimeFormatProperty = AvaloniaProperty.Register<TimelineItem, string?>(
+        nameof(TimeFormat));
+
+    public string? TimeFormat
+    {
+        get => GetValue(TimeFormatProperty);
+        set => SetValue(TimeFormatProperty, value);
+    }
+
+    public static readonly StyledProperty<IDataTemplate> DescriptionTemplateProperty = AvaloniaProperty.Register<TimelineItem, IDataTemplate>(
+        nameof(DescriptionTemplate));
+
+    public IDataTemplate DescriptionTemplate
+    {
+        get => GetValue(DescriptionTemplateProperty);
+        set => SetValue(DescriptionTemplateProperty, value);
+    }
+}

src/Ursa/Controls/Timeline/TimelineItemData.cs

@@ -0,0 +1,9 @@
+namespace Ursa.Controls;
+
+public interface ITimelineItemData
+{
+    public DateTime Time { get; set; }
+    public object Content { get; set; }
+    public object Description { get; set; }
+    public TimelineItemType ItemType { get; set; }
+}

src/Ursa/Controls/Timeline/TimelineItemType.cs

@@ -0,0 +1,11 @@
+namespace Ursa.Controls;
+
+public enum TimelineItemType
+{
+    None,
+    Default,
+    Ongoing,
+    Success,
+    Warning,
+    Error,
+}

src/Ursa/Controls/Timeline/TimelinePlacement.cs

@@ -0,0 +1,12 @@
+namespace Ursa.Controls;
+
+/// <summary>
+/// Describe how TimelineItem components should be placed around the line. 
+/// </summary>
+public enum TimelinePlacement
+{
+    Left,
+    Right,
+    Center,
+    Alternate,
+}