NH-3935 - Proper Setup of Listeners For DI

[nhibernate-bot]

Sammi Maan Sinno created an issue — 17th January 2017, 16:53:57:

So I've been exploring how Event Listeners are set up with DI and it looks like they are instantiated when the session factory is first configured and they are persisted for the duration of the session factory. I feel like this takes a way from some of the features DI provides, like registering types per request which seems impossible if we are not resolving the EventListeners when we need them as appose to at application start.

I've made some adjustments to the code in order to resolve event listeners as needed:

When configuring:

   cfg.SetListenersDI(ListenerType.PostInsert, new[]
                {
                    typeof(FormGeneratorCreate).AssemblyQualifiedName
                });

In Cfg/Configuration.cs

		public void SetListenersDI(ListenerType type, string[] listenerClasses)
		{
			eventListeners.DIListenerRegistration.Add(type, listenerClasses);
		}

Event/EventListeners.cs

		public readonly IDictionary<ListenerType, string[]> DIListenerRegistration =
			new Dictionary<ListenerType, string[]>();

		public IPostUpdateEventListener[] PostCommitUpdateEventListeners
		{
			get
			{
				IPostUpdateEventListener[] eventListeners =
					GetListenersForDI(ListenerType.PostCommitUpdate) as IPostUpdateEventListener[];

				if (eventListeners != null)
				{
					return eventListeners;
				}

				return postCommitUpdateEventListeners;
			}
			set
			{
				if (value != null)
				{
					postCommitUpdateEventListeners = value;
				}
			}
		}


		private object[] GetListenersForDI(ListenerType listenerType)
		{
			string[] listenerClasses;
			if (DIListenerRegistration.TryGetValue(listenerType, out listenerClasses))
			{
				var listeners = (object[])Array.CreateInstance(GetListenerClassFor(listenerType), listenerClasses.Length);
				for (int i = 0; i < listeners.Length; i<ins></ins>)
				{
					try
					{
						listeners<i> = Cfg.Environment.BytecodeProvider.ObjectsFactory.CreateInstance(ReflectHelper.ClassForName(listenerClasses<i>));
					}
					catch (Exception e)
					{
						throw new MappingException(
							"Unable to instantiate specified event (" <ins> listenerType </ins> ") listener class: " + listenerClasses<i>, e);
					}
				}

				return listeners;
			}

			return null;
		}

I'm not too sure why it was done the other way (maybe for performance)? Even if we are resolving these types when needed I don't believe it would be that much of a performance hit and would allow more flexibility with DI.

---

Sammi Maan Sinno added a comment — 19th January 2017, 17:03:12:

any thoughts on this? if I fork and do a merge request is there a possibility it will get accepted?

[fairking]

Can't get the point how Di works with listeners at the moment. Any progress with this feature?
I got this:

The weird thing is the instance is created immediately in the same line.

[hazzik]

As far as I know, no one work on this. You are more than welcome to submit a PR. See contributing.

[hazzik]

Can't get the point how Di works with listeners at the moment.

It does not.

[fairking]

@hazzik Thanks. Will see what I can do.

[fairking]

Ok, I have found a temporary solution unless there is a better implementation. I created a custom interceptor to inject a ServiceProvider into the session. No need NH code changes as Sammi Maan Sinno proposed above. The only downside is you cannot use a constructor in event listener to inject services and event listener itself is not scoped but static (please be aware of it). So you use interceptor instead. Please find example bellow.

DependencyInjectionInterceptor.cs:

using NHibernate;
using System;
using Microsoft.Extensions.DependencyInjection;

namespace MyAmazingApplication
{
    public class DependencyInjectionInterceptor : EmptyInterceptor
    {
        private readonly IServiceProvider _serviceProvider;

        public DependencyInjectionInterceptor(IServiceProvider serviceProvider)
        {
            _serviceProvider = serviceProvider;
        }

        public T GetService<T>() => _serviceProvider.GetService<T>();

        public T GetRequiredService<T>() => _serviceProvider.GetRequiredService<T>();
    }
}

Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    ...
    var cfg = new Configuration();
    ... // your config setup
    cfg.SetListeners(NHibernate.Event.ListenerType.PreInsert, new[] { new AuditEventListener() });
    cfg.SetListeners(NHibernate.Event.ListenerType.PreUpdate, new[] { new AuditEventListener() });

    services.AddSingleton(cfg);
    services.AddSingleton(s => s.GetRequiredService<Configuration>().BuildSessionFactory());
    services.AddScoped(s => s.GetRequiredService<ISessionFactory>().WithOptions().Interceptor(new     DependencyInjectionInterceptor(s)).OpenSession());

    ... // you other services setup
}

AuditEventListener.cs:

public class AuditEventListener : IPreUpdateEventListener, IPreInsertEventListener
{
    public bool OnPreUpdate(PreUpdateEvent e)
        {
            var user = ((DependencyInjectionInterceptor)e.Session.Interceptor).GetService<ICurrentUser>();

            if (e.Entity is IEntity)
                UpdateAuditTrail(user, e.State, e.Persister.PropertyNames, (IEntity)e.Entity, false);

            return false;
        }
}

So you use interceptor to get your scoped or any other service:
var myService = ((DependencyInjectionInterceptor)e.Session.Interceptor).GetService<IService>();

I hope it might be helpful for everyone.