ViewModels as Handlers with MediatR, StructureMap, Caliburn.Micro - structuremap

Question

We are using Caliburn.Micro for our MVVM framework, StructureMap for our IoC container, and MediatR for our mediator implementation. This is all working fine, except the recommended way to register the MediatR event handlers doesn't play nicely with Caliburn.Micro's recommended approach with using the ViewModels as their own handlers.
Caliburn.Micro implements the mediator pattern via the EventAggregator, which requires that you inject the IEventAggregator into your ViewModel and Subscribe to itself (or something implements the IHandle<> interface). MediatR takes a more decoupled approach, recommending you reflectively scan assemblies for types that close the IRequestHandler<,> and other types.
I believe it's my lack of experience with StructureMap that is my issue.
What I'd like to do is be able to implement the Handler functionality on the ViewModels themselves (like Caliburn.Micro suggests) but also ensure the ViewModels are registered as Singletons for Caliburn.Micro.
public class RibbonMenuViewModel : PropertyChangedBase, INotificationHandler<SomethingSelectedEvent> { }
When StructureMap processes the following Registry, there will be 2 instances of RibbonMenuViewModel: one singleton version for Caliburn.Micro and one transient version that closes the MediatR INotificationHandler<> generic type.
StructureMap Registry
public class ViewModelsRegistry : Registry
{
public ViewModelsRegistry()
{
// ensure registration for the ViewModel for Caliburn.Micro
this.ForConcreteType<RibbonMenuViewModel>().Configure.Singleton();
// MediatR handler registrations
this.Scan(s =>
{
s.Assembly(this.GetType().Assembly);
s.ConnectImplementationsToTypesClosing(typeof (IRequestHandler<,>));
s.ConnectImplementationsToTypesClosing(typeof (IAsyncRequestHandler<,>));
s.ConnectImplementationsToTypesClosing(typeof (INotificationHandler<>));
s.ConnectImplementationsToTypesClosing(typeof (IAsyncNotificationHandler<>));
});
}
}
I would like advice on the best way to use the Singleton ViewModel registration as the INotificationHandler instance for MediatR
Here is the Caliburn.Micro configuration for reference:
Caliburn Bootstrapper Configuration
protected override void Configure()
{
this.configureTypeMappings();
if (!Execute.InDesignMode)
{
this.configureIocContainer();
}
}
private void configureIocContainer()
{
this.container = new Container(this.getStructureMapConfig);
}
private void getStructureMapConfig(ConfigurationExpression cfg)
{
cfg.For<IWindowManager>().Use<WindowManager>().Singleton();
cfg.Scan(s =>
{
s.AssemblyContainingType<ViewModelsRegistry>();
s.LookForRegistries();
});
}
protected override IEnumerable<object> GetAllInstances(Type serviceType)
{
return this.container.GetAllInstances(serviceType).OfType<object>();
}
protected override object GetInstance(Type serviceType, string key)
{
if (serviceType == null) serviceType = typeof(object);
var returnValue = key == null
? this.container.GetInstance(serviceType) : this.container.GetInstance(serviceType, key);
return returnValue;
}
protected override void BuildUp(object instance) { this.container.BuildUp(instance); }

Answer 1

I had a similar problem.
Here is how I fixed it.
public class ViewModelsRegistry : Registry
{
public ViewModelsRegistry()
{
// MediatR handler registrations
this.Scan(s =>
{
s.Assembly(this.GetType().Assembly);
s.ConnectImplementationsToTypesClosing(typeof (IRequestHandler<,>));
s.ConnectImplementationsToTypesClosing(typeof (IAsyncRequestHandler<,>));
s.ConnectImplementationsToTypesClosing(typeof (INotificationHandler<>));
s.ConnectImplementationsToTypesClosing(typeof (IAsyncNotificationHandler<>));
s.ExcludeType<RibbonMenuViewModel>();
});
// ensure registration for the ViewModel for Caliburn.Micro
For(typeof(INotificationHandler<>)).Singleton().Add(typeof(RibbonMenuViewModel));
}
}
I hope this helps.