避免依赖注入中的依赖周期

Question

C# Windows Form application. I have an Hub and a class. Both should reference each other. This is because:

1. from the hub I need to call the class' methods
2. from the class I need to retrieve my Hub

Right now I'm able to do the first point using Autofac:

using Autofac;
using Autofac.Integration.SignalR;
using Microsoft.AspNet.SignalR;
using Microsoft.Owin.Cors;
using Microsoft.Owin.Hosting;
using Owin;
using MyProject.Classes;
using System;
using System.Reflection;
using System.Windows.Forms;

namespace MyProject
{
    static class Program
    {
        static IDisposable webApp;

        [STAThread]
        static void Main()
        {
            string url = "http://localhost:8080";
            webApp = WebApp.Start(url);
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Engine());
        }
    }

    class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            app.UseCors(CorsOptions.AllowAll);

            var builder = new ContainerBuilder();
            var config = new HubConfiguration();
            builder.RegisterHubs(Assembly.GetExecutingAssembly()).PropertiesAutowired();
            builder.RegisterType<Erp>().PropertiesAutowired().InstancePerLifetimeScope();
            var container = builder.Build();
            config.Resolver = new AutofacDependencyResolver(container);
            app.UseAutofacMiddleware(container);
            app.MapSignalR(config);
        }
    }
}

here the Hub:

using Microsoft.AspNet.SignalR;
using MyProject.Classes;
using System;
using System.Threading.Tasks;

namespace MyProject.Hubs
{
    public class LiveHub : Hub
    {
        private readonly Erp _erp;

        public LiveHub(Erp erp)
        {
            _erp = erp;
        }

        public override Task OnConnected()
        {
            _erp.someMethod();
            return base.OnConnected();
        }
    }
}

and here Erp.cs:

public class Erp
{
    public Erp()
    {
    }

    public void SomeMethod()
    {
        // do something
    }

    public void SomeOtherMethod()
    {
        // usually I do:
        var hub = GlobalHost.ConnectionManager.GetHubContext<LiveHub>();
        hub.Clients.All.foo();
    }
}

but here I read:

A common error in OWIN integration is use of the GlobalHost. In OWIN you create the configuration from scratch. You should not reference GlobalHost anywhere when using the OWIN integration. Microsoft has documentation about this and other IoC integration concerns here.

If I cannot use the "old" method, how should I retrieve my Hub? I tried to add another DI in Erp for LiveHub but it doesn't work. In my form I create an instance of Erp:

public partial class Engine : Form
{
    private Erp _erp = new Erp();

    public Engine()
    {
        InitializeComponent();
    }
}

if I add that DI the declaration will be impossible because I need to define the LiveHub in constructor, that requires itself the Erp parameter...

What am I not seeing?

Answer 1

You can decouple the Hub from the object ( Erp in your case) by emitting events.

namespace MyProject.Hubs
{
    public class LiveHub : Hub
    {    
        public event Action SomethingHappened;

        public override Task OnConnected()
        {
            SomethingHappened?.Invoke();
            return base.OnConnected();
        }
    }
}

Now you can connect the Erp without the Hub having to know it. You will have to subscribe to the event somewhere else in your code. But the circular reference is broken.

To decouple Engine from Form you could do something like this:

public partial class EngineForm : Form
{    
    public EngineForm()
    {
        InitializeComponent();
    }
}

public class Engine
{
    public Engine(EngineForm form, Erp erp)
    {
        this.form = form;
        this.erp = erp;
    }

    // Here is where you'll write some code to coordinate
    // communication between the Erp and the EngineForm. 
    //
    // The main advantage is that you can inject the Erp 
    // and have it preconfigured.
}