Add new ResourceStartedEvent that fires after resource starts but before health checks

src/Aspire.Hosting/ApplicationModel/ResourceStartedEvent.cs

@@ -0,0 +1,28 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Aspire.Hosting.Eventing;
+
+namespace Aspire.Hosting.ApplicationModel;
+
+/// <summary>
+/// Event that is raised when a resource has started and is in a running state, but before any health checks have run.
+/// </summary>
+/// <param name="resource">The resource that has started.</param>
+/// <param name="services">The service provider for the app host.</param>
+/// <remarks>
+/// This event is fired after a resource transitions to a running state but before health checks begin.
+/// It fills the gap between <see cref="BeforeResourceStartedEvent"/> and <see cref="ResourceReadyEvent"/>.
+/// </remarks>
+public class ResourceStartedEvent(IResource resource, IServiceProvider services) : IDistributedApplicationResourceEvent
+{
+    /// <summary>
+    /// The resource that has started.
+    /// </summary>
+    public IResource Resource => resource;
+
+    /// <summary>
+    /// The service provider for the app host.
+    /// </summary>
+    public IServiceProvider Services => services;
+}

src/Aspire.Hosting/DistributedApplicationEventingExtensions.cs

@@ -22,6 +22,21 @@ public static IResourceBuilder<T> OnBeforeResourceStarted<T>(this IResourceBuild
         where T : IResource
         => builder.OnEvent(callback);
 
+    /// <summary>
+    /// Subscribes a callback to the <see cref="ResourceStartedEvent"/> event within the AppHost.
+    /// </summary>
+    /// <typeparam name="T">The resource type.</typeparam>
+    /// <param name="builder">The resource builder.</param>
+    /// <param name="callback">A callback to handle the event.</param>
+    /// <returns>The <see cref="IResourceBuilder{T}"/>.</returns>
+    /// <remarks>
+    /// This event is fired after a resource has started and is in a running state, but before any health checks have run.
+    /// It fills the gap between <see cref="BeforeResourceStartedEvent"/> and <see cref="ResourceReadyEvent"/>.
+    /// </remarks>
+    public static IResourceBuilder<T> OnResourceStarted<T>(this IResourceBuilder<T> builder, Func<T, ResourceStartedEvent, CancellationToken, Task> callback)
+        where T : IResource
+        => builder.OnEvent(callback);
+
     /// <summary>
     /// Subscribes a callback to the <see cref="ResourceStoppedEvent"/> event within the AppHost.
     /// </summary>

src/Aspire.Hosting/Health/ResourceHealthCheckService.cs

@@ -105,6 +105,10 @@ private async Task MonitorResourceHealthAsync(ResourceMonitorState state)
         var cancellationToken = state.CancellationToken;
         var resource = state.LatestEvent.Resource;
 
+        // Fire the ResourceStartedEvent before health check monitoring begins
+        var resourceStartedEvent = new ResourceStartedEvent(resource, services);
+        await eventing.PublishAsync(resourceStartedEvent, cancellationToken).ConfigureAwait(false);
+
         if (!resource.TryGetAnnotationsIncludingAncestorsOfType<HealthCheckAnnotation>(out var annotations))
         {
             // NOTE: If there are no health check annotations then there

tests/Aspire.Hosting.Tests/Health/ResourceHealthCheckServiceTests.cs

@@ -651,6 +651,22 @@ await app.ResourceNotifications.PublishUpdateAsync(resource.Resource, s => s wit
         await app.StopAsync().TimeoutAfter(TestConstants.LongTimeoutTimeSpan);
     }
 
+    [Fact]
+    public void ResourceStartedEventHasCorrectProperties()
+    {
+        // Arrange
+        var resource = new ParentResource("test-resource");
+        var serviceProvider = new ServiceCollection().BuildServiceProvider();
+
+        // Act
+        var resourceStartedEvent = new ResourceStartedEvent(resource, serviceProvider);
+
+        // Assert
+        Assert.Equal(resource, resourceStartedEvent.Resource);
+        Assert.Equal(serviceProvider, resourceStartedEvent.Services);
+        Assert.Equal("test-resource", resourceStartedEvent.Resource.Name);
+    }
+
     private sealed class ParentResource(string name) : Resource(name)
     {
     }