Forward exceptions to root ViewModel for handling

src/BitzArt.Blazor.MVVM/Components/PageBase{TViewModel}.cs

@@ -21,6 +21,25 @@ public abstract class PageBase<TViewModel> : ComponentBase<TViewModel>, IStateCo
 
     private const string StateKey = "state";
 
+    public delegate Task PageExceptionHandler(object sender, Exception ex);
+
+    /// <summary>
+    /// Called when an exception is thrown within the scope of the page and needs to be handled.
+    /// </summary>
+    public PageExceptionHandler? ExceptionHandler { get; set; }
+
+    override protected void OnInitialized()
+    {
+        base.OnInitialized();
+        ViewModel.ExceptionHandler += (sender, ex) =>
+        {
+            if (ExceptionHandler is null)
+                return Task.CompletedTask;
+
+            return ExceptionHandler.Invoke(sender, ex);
+        };
+    }
+
     protected override async Task RestoreStateAsync()
     {
         var state = await Js.InvokeAsync<string?>("getInnerText", [StateKey]);

src/BitzArt.Blazor.MVVM/Services/ViewModelFactory.cs

@@ -75,6 +75,14 @@ public ViewModel Create(IServiceProvider serviceProvider, Type viewModelType, Co
                     if (injectedViewModel.OnComponentStateContainerWasSet is not null)
                         await injectedViewModel.OnComponentStateContainerWasSet.Invoke(container);
                 };
+
+                injectedViewModel.ExceptionHandler += async (sender, exception) =>
+                {
+                    if (viewModel.ExceptionHandler is null)
+                        return;
+
+                    await viewModel.ExceptionHandler.Invoke(sender, exception);
+                };
             }
 
             else if (injection.IsParentViewModelInjection)

tests/BitzArt.Blazor.MVVM.Tests/ExceptionHandlingTests.cs

@@ -0,0 +1,68 @@
+using BitzArt.Blazor.MVVM.Tests.ViewModels;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace BitzArt.Blazor.MVVM.Tests;
+
+public class ExceptionHandlingTests
+{
+    [Fact]
+    public async Task HandleAsync_WhenExceptionIsThrown_ShouldInvokeExceptionHandler()
+    {
+        // Arrange
+        var viewModel = new TestLayer1ViewModel();
+        var exceptionHandlerInvoked = false;
+
+        viewModel.ExceptionHandler = (sender, ex) =>
+        {
+            exceptionHandlerInvoked = true;
+            return Task.CompletedTask;
+        };
+
+        // Act
+        try
+        {
+            await viewModel.HandleAsync(() => throw new Exception());
+        }
+        catch (Exception)
+        {
+        }
+
+        // Assert
+        Assert.True(exceptionHandlerInvoked);
+    }
+
+    [Fact]
+    public async Task HandleAsync_HierarchicalStructure_ShouldForwardToRoot()
+    {
+        // Arrange
+
+        var serviceCollection = new ServiceCollection();
+        serviceCollection.AddBlazorMvvm()
+            .AddViewModel<TestLayer1ViewModel>()
+            .AddViewModel<TestLayer2ViewModel>();
+
+        var serviceProvider = serviceCollection.BuildServiceProvider();
+
+        var rootViewModel = serviceProvider.GetRequiredService<TestLayer1ViewModel>();
+        var childViewModel = rootViewModel.TestLayer2ViewModel;
+
+        var rootExceptionHandlerInvoked = false;
+        rootViewModel.ExceptionHandler += (sender, ex) =>
+        {
+            rootExceptionHandlerInvoked = true;
+            return Task.CompletedTask;
+        };
+
+        // Act
+        try
+        {
+            await childViewModel.HandleAsync(() => throw new Exception());
+        }
+        catch (Exception)
+        {
+        }
+
+        // Assert
+        Assert.True(rootExceptionHandlerInvoked);
+    }
+}