Umbraco-CMS/src/Umbraco.Core/Extensions/UmbracoBuilderExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Extensions.DependencyInjection;
using Umbraco.Cms.Core.DependencyInjection;
using Umbraco.Cms.Core.Events;

namespace Umbraco.Cms.Core.Extensions
{
    public static class UmbracoBuilderExtensions
    {
        /// <summary>
        /// Registers all <see cref="INotificationHandler{TNotification}"/> within an assembly
        /// </summary>
        /// <param name="self"><see cref="IUmbracoBuilder"/></param>
        /// <typeparam name="T">Type contained within the targeted assembly</typeparam>
        /// <returns></returns>
        public static IUmbracoBuilder AddNotificationsFromAssembly<T>(this IUmbracoBuilder self)
        {
            AddNotificationHandlers<T>(self);
            AddAsyncNotificationHandlers<T>(self);

            return self;
        }

        private static void AddNotificationHandlers<T>(IUmbracoBuilder self)
        {
            var notificationHandlers = GetNotificationHandlers<T>();
            foreach (var notificationHandler in notificationHandlers)
            {
                var handlerImplementations = GetNotificationHandlerImplementations<T>(notificationHandler);
                foreach (var implementation in handlerImplementations)
                {
                    RegisterNotificationHandler(self, implementation, notificationHandler);
                }
            }
        }

        private static List<Type> GetNotificationHandlers<T>() =>
            typeof(T).Assembly.GetTypes()
                .Where(x => x.IsAssignableToGenericType(typeof(INotificationHandler<>)))
                .ToList();

        private static List<Type> GetNotificationHandlerImplementations<T>(Type handlerType) =>
            handlerType
                .GetInterfaces()
                .Where(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(INotificationHandler<>))
                .ToList();

        private static void AddAsyncNotificationHandlers<T>(IUmbracoBuilder self)
        {
            var notificationHandlers = GetAsyncNotificationHandlers<T>();
            foreach (var notificationHandler in notificationHandlers)
            {
                var handlerImplementations = GetAsyncNotificationHandlerImplementations<T>(notificationHandler);
                foreach (var handler in handlerImplementations)
                {
                    RegisterNotificationHandler(self, handler, notificationHandler);
                }
            }
        }

        private static List<Type> GetAsyncNotificationHandlers<T>() =>
            typeof(T).Assembly.GetTypes()
                .Where(x => x.IsAssignableToGenericType(typeof(INotificationAsyncHandler<>)))
                .ToList();

        private static List<Type> GetAsyncNotificationHandlerImplementations<T>(Type handlerType) =>
            handlerType
                .GetInterfaces()
                .Where(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(INotificationAsyncHandler<>))
                .ToList();

        private static void RegisterNotificationHandler(IUmbracoBuilder self, Type notificationHandlerType, Type implementingHandlerType)
        {
            var descriptor = new UniqueServiceDescriptor(notificationHandlerType, implementingHandlerType, ServiceLifetime.Transient);
            if (!self.Services.Contains(descriptor))
            {
                self.Services.Add(descriptor);
            }
        }

        private static bool IsAssignableToGenericType(this Type givenType, Type genericType)
        {
            var interfaceTypes = givenType.GetInterfaces();

            foreach (var it in interfaceTypes)
            {
                if (it.IsGenericType && it.GetGenericTypeDefinition() == genericType)
                {
                    return true;
                }
            }

            if (givenType.IsGenericType && givenType.GetGenericTypeDefinition() == genericType)
                return true;

            var baseType = givenType.BaseType;
            return baseType != null && IsAssignableToGenericType(baseType, genericType);
        }
    }
}