LehrerApp/LehrerApp.Desktop/Services/NotificationService.cs

using Avalonia.Threading;
using CommunityToolkit.Mvvm.ComponentModel;
using System.Collections.ObjectModel;

namespace LehrerApp.Desktop.Services;

public enum NotificationKind { Success, Error }

/// <summary>
/// Einheitliche Erfolgs-/Fehlermeldungen als Toast (13.2.3). Wird im MainWindow als
/// Overlay über der aktuellen Seite angezeigt und blendet sich nach kurzer Zeit selbst aus.
/// </summary>
public class NotificationService
{
    public ObservableCollection<ToastItem> Toasts { get; } = [];

    public void ShowSuccess(string message) => Show(message, NotificationKind.Success, TimeSpan.FromSeconds(3));
    public void ShowError(string message)   => Show(message, NotificationKind.Error,   TimeSpan.FromSeconds(6));

    private void Show(string message, NotificationKind kind, TimeSpan duration)
    {
        void AddAndScheduleRemoval()
        {
            var toast = new ToastItem(message, kind);
            Toasts.Add(toast);
            _ = RemoveAfter(toast, duration);
        }

        if (Dispatcher.UIThread.CheckAccess()) AddAndScheduleRemoval();
        else Dispatcher.UIThread.Post(AddAndScheduleRemoval);
    }

    private async Task RemoveAfter(ToastItem toast, TimeSpan duration)
    {
        await Task.Delay(duration);
        Toasts.Remove(toast);
    }
}

public partial class ToastItem : ObservableObject
{
    public string Message { get; }
    public NotificationKind Kind { get; }
    public bool IsError => Kind == NotificationKind.Error;

    public ToastItem(string message, NotificationKind kind)
    {
        Message = message;
        Kind = kind;
    }
}