Output notification handler messages as a header

src/Umbraco.Cms.Api.Management/Configuration/ConfigureUmbracoManagementApiSwaggerGenOptions.cs

@@ -60,7 +60,7 @@ public void Configure(SwaggerGenOptions swaggerGenOptions)
 
         // Sets Security requirement on backoffice apis
         swaggerGenOptions.OperationFilter<BackOfficeSecurityRequirementsOperationFilter>();
+        swaggerGenOptions.OperationFilter<NotificationHeaderFilter>();
         swaggerGenOptions.SchemaFilter<RequireNonNullablePropertiesSchemaFilter>();
     }
-
 }

src/Umbraco.Cms.Api.Management/Controllers/ManagementApiControllerBase.cs

@@ -7,6 +7,7 @@
 using Umbraco.Cms.Api.Common.Filters;
 using Umbraco.Cms.Api.Common.Mvc.ActionResults;
 using Umbraco.Cms.Api.Management.DependencyInjection;
+using Umbraco.Cms.Api.Management.Filters;
 using Umbraco.Cms.Core;
 using Umbraco.Cms.Core.Features;
 using Umbraco.Cms.Core.Models.Membership;
@@ -19,6 +20,7 @@ namespace Umbraco.Cms.Api.Management.Controllers;
 [Authorize(Policy = "New" + AuthorizationPolicies.UmbracoFeatureEnabled)]
 [MapToApi(ManagementApiConfiguration.ApiName)]
 [JsonOptionsName(Constants.JsonOptionsNames.BackOffice)]
+[AppendEventMessages]
 public abstract class ManagementApiControllerBase : Controller, IUmbracoFeature
 {
     protected IActionResult CreatedAtId<T>(Expression<Func<T, string>> action, Guid id)

src/Umbraco.Cms.Api.Management/Filters/AppendEventMessagesAttribute.cs

@@ -0,0 +1,67 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.Filters;
+using Umbraco.Cms.Api.Management.ViewModels;
+using Umbraco.Cms.Core;
+using Umbraco.Cms.Core.Events;
+using Umbraco.Cms.Core.Serialization;
+
+namespace Umbraco.Cms.Api.Management.Filters;
+
+/// <summary>
+/// When applied to a controller, this ensures that any event messages created during a request (e.g. by notification
+/// handlers) are appended to the response in a custom header.
+/// </summary>
+/// <remarks>
+/// GET operations are explicitly ignored by this, as they are not expected to generate event messages.
+/// </remarks>
+public sealed class AppendEventMessagesAttribute : TypeFilterAttribute
+{
+    public AppendEventMessagesAttribute()
+        : base(typeof(AppendEventMessagesFilter))
+    {
+    }
+
+    private class AppendEventMessagesFilter : IActionFilter
+    {
+        private readonly IEventMessagesFactory _eventMessagesFactory;
+        private readonly IJsonSerializer _jsonSerializer;
+
+        public AppendEventMessagesFilter(IEventMessagesFactory eventMessagesFactory, IJsonSerializer jsonSerializer)
+        {
+            _eventMessagesFactory = eventMessagesFactory;
+            _jsonSerializer = jsonSerializer;
+        }
+
+        public void OnActionExecuted(ActionExecutedContext context)
+        {
+            if (context.HttpContext.Response is null)
+            {
+                return;
+            }
+
+            if (context.HttpContext.Request.Method == HttpMethod.Get.Method)
+            {
+                return;
+            }
+
+            EventMessage[]? eventMessages = _eventMessagesFactory.GetOrDefault()?.GetAll()?.ToArray();
+            if (eventMessages is null || eventMessages.Any() is false)
+            {
+                return;
+            }
+
+            var headerContent = _jsonSerializer.Serialize(eventMessages.Select(message =>
+                new NotificationHeaderModel
+                {
+                    Message = message.Message, Category = message.Category, Type = message.MessageType
+                }));
+
+            context.HttpContext.Response.Headers[Constants.Headers.Notifications] = headerContent;
+        }
+
+        public void OnActionExecuting(ActionExecutingContext context)
+        {
+        }
+
+    }
+}