Calling a server method that is outside of the Hub class from the client in SignalR

Question

Consider the class below:

using Microsoft.AspNet.SignalR;
public class TwitterStream
    {
        // Hub Context
        IHubContext context = GlobalHost.ConnectionManager.GetHubContext<GeoFeedHub>();

        public void ChangeStreamBounds(double latitude, double longitude)
        {
            Debug.WriteLine(latitude + "-" + longitude);
        }

        // Lots of other interesting code redacted
   }

Is it possible to call the ChangeStreamBounds method from the client even though it is outside of the Hub class? It's possible to call client functions from the server (and from outside of the Hub class) but is it possible to do it the other way round?

Unfortunately I've backed myself into a bit of a corner and the code must be executed from the class that I've written (not from the Hub itself - unless of course you can run a SignalR Hub as a Task factory)

Answer 1

There may be an answer to your question that involves HubConnections and IHubProxys (which let you bind to hub method calls) or the lower-level API, but I think you may be going about this the wrong way.

Conceptually, you want the GeoFeedHub to handle client requests, and the TwitterStream class to handle interacting with the Twitter API. Thus, your GeoFeedHub class has a dependency on TwitterStream.

It's good that your TwitterStream class has async methods, and this is fully supported in SignalR. You can have async Hub methods that call into TwitterStream, which removes the need for your TaskFactory usage in Global.asax.

Instead of creating your TwitterStream at application start and trying to find a way to bind Hub calls to it (a backwards dependency and violation of the Single Responsibility Principle), it would be cleaner to let your Hub stand as the point of contact between your realtime clients, and inject an instance of TwitterStream into the GeoFeedHub so the Hub can access the Twitter API.

---

Here's some sample code that should illustrate this idea:

public class GeoFeedHub : Hub
{
    // Declare dependency on TwitterStream class
    private readonly TwitterStream _twitterStream;

    // Use constructor injection to get an instance of TwitterStream
    public GeoFeedHub(TwitterStream _twitterStream)
    {
        _twitterStream = _twitterStream;
    }

    // Clients can call this method, which uses the instance of TwitterStream
    public async Task SetStreamBounds(double latitude, double longitude)
    {
        await _twitterStream.SetStreamBoundsAsync(latitude, longitude);
    }
}


public class TwitterStream
{
    public TwitterStream() 
    {
    }

    public async Task SetStreamBoundsAsync(double latitude, double longitude)
    {
        // Do something with Twitter here maybe?
        await SomeComponent.SetStreamBoundsAsync(latitude, longitude);
    }

    // More awesome code here
}

---

I used DI in the example, so here's some of the glue code you'd need to hook that up. This code would go into your App_Start folder:

// Configure Unity as our DI container
public class UnityConfig
{

    private static readonly Lazy<IUnityContainer> Container = new Lazy<IUnityContainer>(() =>
    {
        var container = new UnityContainer();
        RegisterTypes(container);
        return container;
    });

    public static IUnityContainer GetConfiguredContainer()
    {
        return Container.Value;
    }


    private static void RegisterTypes(IUnityContainer container)
    {
        var twitterService = new TwitterService();
        container.RegisterInstance(twitterService);

        /*
         * Using RegisterInstance effectively makes a Singleton instance of 
         * the object accessible throughout the application.  If you don't need 
         * (or want) the class to be shared among all clients, then use 
         * container.RegisterType<TwitterService, TwitterService>();
         * which will create a new instance for each client (i.e. each time a Hub
         * is created, you'll get a brand new TwitterService object)
         */
    }
}


// If you're using ASP.NET, this can be used to set the DependencyResolver for SignalR 
// so it uses your configured container

[assembly: WebActivatorEx.PreApplicationStartMethod(typeof(UnitySignalRActivator), "Start")]
[assembly: WebActivatorEx.ApplicationShutdownMethod(typeof(UnitySignalRActivator), "Shutdown")]

public static class UnitySignalRActivator
{
    public static void Start() 
    {
        var container = UnityConfig.GetConfiguredContainer();
        GlobalHost.DependencyResolver = new SignalRUnityDependencyResolver(container);
    }

    public static void Shutdown()
    {
        var container = UnityConfig.GetConfiguredContainer();
        container.Dispose();
    }
}

public class SignalRUnityDependencyResolver : DefaultDependencyResolver
{
    private readonly IUnityContainer _container;

    public SignalRUnityDependencyResolver(IUnityContainer container)
    {
        _container = container;
    }

    public override object GetService(Type serviceType)
    {
        return _container.IsRegistered(serviceType) 
            ? _container.Resolve(serviceType) 
            : base.GetService(serviceType);
    }

    public override IEnumerable<object> GetServices(Type serviceType)
    {
        return _container.IsRegistered(serviceType) 
            ? _container.ResolveAll(serviceType) 
            : base.GetServices(serviceType);
    }
}