Are Decorators Supported? about mediatr

[jbogard]

I don't know, are there?

[jbogard]

I use decorators all the time with StructureMap using MediatR if that's what you're asking.

[mpaul31]

i have not had a chance to give it a spin but that makes me feel better knowing you use them all the time.

i have not decided upon a DI framework yet but was thinking StructureMap or SimpleInj. Wasn't looking forward to writing YetAnotherCommandHandler framework ;)

Thanks Jimmy

[bapti]

I use simple injector and have have used generic and specific decorators with MediatR without issue. Really gets rid of those cross cutting concerns like logging, metrics timing, transactions etc.

[maldworth]

A little late to the party :), but I'm using mediatr with AutoFac and generic decorators as well. Working like a charm. You can see a sample of it working here

[sstorie]

@maldworth Does this work with IAsyncNotification handlers as well? I'm trying to use this approach and can't get it working with notifications when I don't provide any decorators. If I include a decorator then my notifications handlers are called multiple times (twice in the case of adding a single decorator).

I've got a somewhat simpler implementation of your approach using this code:

        private static void RegisterHandlers(ContainerBuilder builder, Assembly[] assemblies, Type handlerType, params Type[] decorators)
        {
            builder.RegisterAssemblyTypes(assemblies)
                .As(t => t.GetInterfaces()
                          .Where(v => v.IsClosedTypeOf(handlerType))
                          .Select(v => new KeyedService(handlerType.Name, v)));

            for (int i = 0; i < decorators.Length; i++)
            {
                RegisterGenericDecorator(
                    builder,
                    decorators[i],
                    handlerType,
                    i == 0 ? handlerType : decorators[i - 1],
                    i != decorators.Length - 1);
            }
        }

        private static void RegisterGenericDecorator(
            ContainerBuilder builder,
            Type decoratorType,
            Type decoratedServiceType,
            Type fromKeyType,
            bool hasKey)
        {
            var result = builder.RegisterGenericDecorator(
               decoratorType,
               decoratedServiceType,
               fromKeyType.Name);

            if (hasKey)
            {
                result.Keyed(decoratorType.Name, decoratedServiceType);
            }
        }

Then the code calling this looks like this:

            RegisterHandlers(
                builder,
                assemblies,
                typeof(IAsyncRequestHandler<,>),
                typeof(AsyncValidatorHandler<,>),
                typeof(AsyncCommandPipeline<,>),
                typeof(AsyncAuthorizationHandler<,>));

            RegisterHandlers(
                builder,
                assemblies,
                typeof(IAsyncNotificationHandler<>));

However, in this case the notification handlers are never called. If I change that second statement to this instead:

            RegisterHandlers(
                builder,
                assemblies,
                typeof(IAsyncNotificationHandler<>),
                typeof(AsyncNotificationPipeline<>));

Then each notification handler is called twice, instead of a single time.

I think it's more of a problem with Autofac than anything to do with MediatR, but I started heading in the right direction based on the code you shared, so thank you for that :)

FWIW, I'm using Mediatr-2.0.0-beta-005 at the moment.

[maldworth]

@sstorie The first statement will not work, and I can quickly explain why. When you start registering decorators it takes advantage of the Named/Keyed autofac feature. But when using this feature, that means when resolving a named service, you have to pass in the name. But when we wire up the Mediator resolver:
eg.

// Sets the delegate resolver factories for Mediatr.
// These factories are used by Mediatr to find the appropriate Handlers
builder.Register<SingleInstanceFactory>(ctx =>
{
    var c = ctx.Resolve<IComponentContext>();
    return t => c.Resolve(t);
});
builder.Register<MultiInstanceFactory>(ctx =>
{
    var c = ctx.Resolve<IComponentContext>();
    return t => (IEnumerable<object>)c.Resolve(typeof(IEnumerable<>).MakeGenericType(t));
});

You'll notice that we don't resolve with passing in a name, and that is intentional. So to get around this you'll notice that the helper method has this if statement:

if (hasKey)
{
    result.Keyed(decoratorType.Name, decoratedServiceType);
}

Which means, only key this decorator if it is not the outer most decorator. So in the first statement, you only passed in the HandlerType, and no decorators. The HandlerType is always keyed in the helper method.

You can see I was doing this in an ugly way here, before I cleaned it up in a later commit. You'll notice that I commented the outermost decorator, and there is no toKey:.

Now, why your second one is getting two notification calls? I'm not sure, because I just tested it out using your static helper methods (instead of mine) to register a single notification handler. The notification handler was called once as expected when using the second register statement you provided. I also made sure I was using beta-005.

[sstorie]

@maldworth Thanks for the response. I started to better understand how the decorators in Autofac work as I continued to debug this yesterday, and it makes sense to me now why my first attempt failed. Your explanation helped confirm that I'm understanding this now.

I still don't understand why I get notification handlers called multiple times, but perhaps it's a problem with my decorator itself. I'm not sure though because the decorator is also called twice, so it's like the container is giving the mediator those multiple instances or something. I'll keep digging :)

Thanks again for responding to my question.