Resolving dependency based on custom criteria - autofac

Question

My app relies on multiple event bus objects which are basic publish/subscribe notification model (http://caliburn.codeplex.com/wikipage?title=The%20Event%20Aggregator).
What I want to do is share certain an instance of aggregators with a groups of components. Say component I have a single event bus that's shared between component A, B, and C, and then another event bus that's shared between D,E,F.
I essentially want to declare the event busses as singleton and inject them based on some criteria. I kinda wanna avoid subtyping the event busses just for the purposes of distinguishing resolution.
I've used Google Guice IoC in java which allows metadata resolution for a parameter. Aka in java it allowed me to something equivalent to this.
Example:
public A([SpecialUseAggregator]IEventAggregator something)
public B([SpecialUseAggregator]IEventAggregator something)
public E([AnotherUseAggregator]IEventAggregator something)
public F([AnotherUseAggregator]IEventAggregator something)
Any suggestions?

Answer 1

Autofac does not have/use attributes for the registration. One solution is to use the Named/Keyed registration feature.
So you need to need to register you two EventAggreator with different names/keys and when registering your consumer types A,B, etc you can use the WithParameter to tell Autofac which IEventAggreator it should use for the given instance:
var contianerBuilder = new ContainerBuilder();
contianerBuilder.Register(c => CreateAndConfigureSpecialEventAggregator())
.Named<IEventAggreator>("SpecialUseAggregator");
contianerBuilder.Register(c => CreateAndConfigureAnotherUseAggregator())
.Named<IEventAggreator>("AnotherUseAggregator");
contianerBuilder.RegisterType<A>).AsSelf()
.WithParameter(ResolvedParameter
.ForNamed<IEventAggreator>("SpecialUseAggregator"));
contianerBuilder.RegisterType<B>().AsSelf()
.WithParameter(ResolvedParameter
.ForNamed<IEventAggreator>("SpecialUseAggregator"));
contianerBuilder.RegisterType<C>).AsSelf()
.WithParameter(ResolvedParameter
.ForNamed<IEventAggreator>("AnotherUseAggregator"));
contianerBuilder.RegisterType<D>().AsSelf()
.WithParameter(ResolvedParameter
.ForNamed<IEventAggreator>("AnotherUseAggregator"));
var container = contianerBuilder.Build();
I you still would like to use attributes then you can do it with Autofac because it has all the required extension points it just requires some more code to teach Autofac about your attribute and use it correctly.
If you are registering your types with scanning you cannot use the easily use the WithParameter registration however you use the Metadata facility in Autofac:
Just create an attribute which will hold your EventAggreator key:
public class EventAggrAttribute : Attribute
{
public string Key { get; set; }
public EventAggrAttribute(string key)
{
Key = key;
}
}
And attribute your classes:
[EventAggrAttribute("SpecialUseAggregator")]
public class AViewModel
{
public AViewModel(IEventAggreator eventAggreator)
{
}
}
Then when you do the scanning you need to use the WithMetadataFrom to register the metadata:
contianerBuilder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
.Where(t => t.Name.EndsWith("ViewModel"))
.OnPreparing(Method)
.WithMetadataFrom<EventAggrAttribute>();
And finally you need the OnPreparing event where you do the metadata based resolution:
private void Method(PreparingEventArgs obj)
{
// Metadata["Key"] is coming from the EventAggrAttribute.Key
var key = obj.Component.Metadata["Key"].ToString();
ResolvedParameter resolvedParameter =
ResolvedParameter.ForNamed<IEventAggreator>();
obj.Parameters = new List<Parameter>() { resolvedParameter};
}
Here is gist of a working unit test.