feat: add new Notification.

src/Ursa/Controls/Notification/INotification.cs

@@ -0,0 +1,17 @@
+namespace Ursa.Controls;
+
+/// <summary>
+/// Represents a notification that can be shown in a window or by the host operating system.
+/// </summary>
+public interface INotification : IMessage
+{
+    /// <summary>
+    /// Gets the Title of the notification.
+    /// </summary>
+    string? Title { get; }
+
+    /// <summary>
+    /// Gets the Content of the notification.
+    /// </summary>
+    string? Content { get; }
+}

src/Ursa/Controls/Notification/INotificationManager.cs

@@ -0,0 +1,14 @@
+namespace Ursa.Controls;
+
+/// <summary>
+/// Represents a notification manager that can be used to show notifications in a window or using
+/// the host operating system.
+/// </summary>
+public interface INotificationManager
+{
+    /// <summary>
+    /// Show a notification.
+    /// </summary>
+    /// <param name="notification">The notification to be displayed.</param>
+    void Show(INotification notification);
+}

src/Ursa/Controls/Notification/Notification.cs

@@ -0,0 +1,103 @@
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+using Avalonia.Controls.Notifications;
+
+namespace Ursa.Controls;
+
+/// <summary>
+/// A notification that can be shown in a window or by the host operating system.
+/// </summary>
+/// <remarks>
+/// This class represents a notification that can be displayed either in a window using
+/// <see cref="WindowNotificationManager"/> or by the host operating system (to be implemented).
+/// </remarks>
+public class Notification : INotification, INotifyPropertyChanged
+{
+    private string? _title, _content;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Notification"/> class.
+    /// </summary>
+    /// <param name="title">The title of the notification.</param>
+    /// <param name="content">The content to be displayed in the notification.</param>
+    /// <param name="type">The <see cref="NotificationType"/> of the notification.</param>
+    /// <param name="expiration">The expiry time at which the notification will close.
+    /// Use <see cref="TimeSpan.Zero"/> for notifications that will remain open.</param>
+    /// <param name="showClose">A value indicating whether the notification should show a close button.</param>
+    /// <param name="onClick">An Action to call when the notification is clicked.</param>
+    /// <param name="onClose">An Action to call when the notification is closed.</param>
+    public Notification(
+        string? title,
+        string? content,
+        NotificationType type = NotificationType.Information,
+        TimeSpan? expiration = null,
+        bool showClose = true,
+        Action? onClick = null,
+        Action? onClose = null)
+    {
+        Title = title;
+        Content = content;
+        Type = type;
+        Expiration = expiration ?? TimeSpan.FromSeconds(3);
+        ShowClose = showClose;
+        OnClick = onClick;
+        OnClose = onClose;
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Notification"/> class.
+    /// </summary>
+    public Notification() : this(null, null)
+    {
+    }
+
+    /// <inheritdoc/>
+    public string? Title
+    {
+        get => _title;
+        set
+        {
+            if (_title != value)
+            {
+                _title = value;
+                OnPropertyChanged();
+            }
+        }
+    }
+
+    /// <inheritdoc/>
+    public string? Content
+    {
+        get => _content;
+        set
+        {
+            if (_content != value)
+            {
+                _content = value;
+                OnPropertyChanged();
+            }
+        }
+    }
+
+    /// <inheritdoc/>
+    public NotificationType Type { get; set; }
+
+    /// <inheritdoc/>
+    public TimeSpan Expiration { get; set; }
+
+    /// <inheritdoc/>
+    public bool ShowClose { get; }
+
+    /// <inheritdoc/>
+    public Action? OnClick { get; set; }
+
+    /// <inheritdoc/>
+    public Action? OnClose { get; set; }
+
+    public event PropertyChangedEventHandler? PropertyChanged;
+
+    protected virtual void OnPropertyChanged([CallerMemberName] string? propertyName = null)
+    {
+        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+    }
+}

src/Ursa/Controls/Notification/NotificationCard.cs

@@ -0,0 +1,6 @@
+namespace Ursa.Controls;
+
+/// <summary>
+/// Control that represents and displays a notification.
+/// </summary>
+public class NotificationCard : MessageCard;

src/Ursa/Controls/Notification/WindowNotificationManager.cs

@@ -0,0 +1,171 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
+using Avalonia.Controls.Notifications;
+using Avalonia.Layout;
+using Avalonia.Threading;
+
+namespace Ursa.Controls;
+
+/// <summary>
+/// An <see cref="INotificationManager"/> that displays notifications in a <see cref="Window"/>.
+/// </summary>
+[PseudoClasses(PC_TopLeft, PC_TopRight, PC_BottomLeft, PC_BottomRight, PC_TopCenter, PC_BottomCenter)]
+public class WindowNotificationManager : WindowMessageManager, INotificationManager
+{
+    public const string PC_TopLeft = ":topleft";
+    public const string PC_TopRight = ":topright";
+    public const string PC_BottomLeft = ":bottomleft";
+    public const string PC_BottomRight = ":bottomright";
+    public const string PC_TopCenter = ":topcenter";
+    public const string PC_BottomCenter = ":bottomcenter";
+
+    /// <summary>
+    /// Defines the <see cref="Position"/> property.
+    /// </summary>
+    public static readonly StyledProperty<NotificationPosition> PositionProperty =
+        AvaloniaProperty.Register<WindowNotificationManager, NotificationPosition>(nameof(Position),
+            NotificationPosition.TopRight);
+
+    /// <summary>
+    /// Defines which corner of the screen notifications can be displayed in.
+    /// </summary>
+    /// <seealso cref="NotificationPosition"/>
+    public NotificationPosition Position
+    {
+        get => GetValue(PositionProperty);
+        set => SetValue(PositionProperty, value);
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="WindowNotificationManager"/> class.
+    /// </summary>
+    /// <param name="host">The TopLevel that will host the control.</param>
+    public WindowNotificationManager(TopLevel? host) : this()
+    {
+        if (host is not null)
+        {
+            InstallFromTopLevel(host);
+        }
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="WindowNotificationManager"/> class.
+    /// </summary>
+    public WindowNotificationManager()
+    {
+        UpdatePseudoClasses(Position);
+    }
+
+    static WindowNotificationManager()
+    {
+        HorizontalAlignmentProperty.OverrideDefaultValue<WindowNotificationManager>(HorizontalAlignment.Stretch);
+        VerticalAlignmentProperty.OverrideDefaultValue<WindowNotificationManager>(VerticalAlignment.Stretch);
+    }
+
+    /// <inheritdoc/>
+    public void Show(INotification content)
+    {
+        Show(content, content.Type, content.Expiration, content.ShowClose, content.OnClick, content.OnClose);
+    }
+
+    /// <inheritdoc/>
+    public override void Show(object content)
+    {
+        if (content is INotification notification)
+        {
+            Show(notification, notification.Type, notification.Expiration, notification.ShowClose, notification.OnClick,
+                notification.OnClose);
+        }
+        else
+        {
+            Show(content, NotificationType.Information);
+        }
+    }
+
+    /// <summary>
+    /// Shows a Notification
+    /// </summary>
+    /// <param name="content">the content of the notification</param>
+    /// <param name="type">the type of the notification</param>
+    /// <param name="expiration">the expiration time of the notification after which it will automatically close. If the value is Zero then the notification will remain open until the user closes it</param>
+    /// <param name="showClose">whether to show the close button</param>
+    /// <param name="onClick">an Action to be run when the notification is clicked</param>
+    /// <param name="onClose">an Action to be run when the notification is closed</param>
+    /// <param name="classes">style classes to apply</param>
+    public async void Show(
+        object content,
+        NotificationType type,
+        TimeSpan? expiration = null,
+        bool showClose = true,
+        Action? onClick = null,
+        Action? onClose = null,
+        string[]? classes = null)
+    {
+        Dispatcher.UIThread.VerifyAccess();
+
+        var notificationControl = new NotificationCard
+        {
+            Content = content,
+            NotificationType = type,
+            ShowClose = showClose
+        };
+
+        // Add style classes if any
+        if (classes is not null)
+        {
+            foreach (var @class in classes)
+            {
+                notificationControl.Classes.Add(@class);
+            }
+        }
+
+        notificationControl.MessageClosed += (sender, _) =>
+        {
+            onClose?.Invoke();
+
+            _items?.Remove(sender);
+        };
+
+        notificationControl.PointerPressed += (_, _) => { onClick?.Invoke(); };
+
+        Dispatcher.UIThread.Post(() =>
+        {
+            _items?.Add(notificationControl);
+
+            if (_items?.OfType<NotificationCard>().Count(i => !i.IsClosing) > MaxItems)
+            {
+                _items.OfType<NotificationCard>().First(i => !i.IsClosing).Close();
+            }
+        });
+
+        if (expiration == TimeSpan.Zero)
+        {
+            return;
+        }
+
+        await Task.Delay(expiration ?? TimeSpan.FromSeconds(3));
+
+        notificationControl.Close();
+    }
+
+    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+    {
+        base.OnPropertyChanged(change);
+
+        if (change.Property == PositionProperty)
+        {
+            UpdatePseudoClasses(change.GetNewValue<NotificationPosition>());
+        }
+    }
+
+    private void UpdatePseudoClasses(NotificationPosition position)
+    {
+        PseudoClasses.Set(PC_TopLeft, position == NotificationPosition.TopLeft);
+        PseudoClasses.Set(PC_TopRight, position == NotificationPosition.TopRight);
+        PseudoClasses.Set(PC_BottomLeft, position == NotificationPosition.BottomLeft);
+        PseudoClasses.Set(PC_BottomRight, position == NotificationPosition.BottomRight);
+        PseudoClasses.Set(PC_TopCenter, position == NotificationPosition.TopCenter);
+        PseudoClasses.Set(PC_BottomCenter, position == NotificationPosition.BottomCenter);
+    }
+}