Understanding SignalR- Hubs, Dependency Injection, and Controllers

Question

I've been pouring through documentation and forums trying to understand SignalR but I'm pretty stuck.

What I'm trying to achieve is, in a chat application: store messages outside of the Hub so that each time a user joins the chat, they can see all messages that had been sent before they joined.

So it seemed like an external class was the way to do that so I got that working with dependency injection

In ChatHub.cs

namespace SignalRChat.Hubs
{
    public class ChatHub: Hub
    {

        public IChatStorage _chatStorage;

        public ChatHub(IChatStorage chatStorage)
        {
            _chatStorage = chatStorage;
        }
    // and so on

And I have a method in ChatHub to send a message to chatStorage, but I'm confused on how to send back a list of all messages from chatStorage to ChatHub, or even to JavaScript. It seemed like a Controller was the way to do that but I'm not sure how to call the controller's methods:

namespace SignalRChat.Controllers
{
    public class ChatController: Controller
    {
        private IHubContext<ChatHub> _hubContext;

        public ChatController(IHubContext<ChatHub> hubContext)
        {
            _hubContext = hubContext;
        }

        public void Send(List<Message> messages)
        {
            // to do: something where chatStorage calls this method, then this
            // method uses _hubContext.Clients.All.SendAsync 
            // But, how do I even call Send()???
        }
    }
}

Fundamentally I just don't understand how to wire everything up. SignalR is just really confusing.

I get how the simple server Hub and client JavaScript relationship works. But, then with dependency injection I don't get why

public ChatHub(IChatStorage chatStorage)
        {
            _chatStorage = chatStorage;
        }

works. I didn't change any code to say like new ChatHub(new IChatStorage). Microsoft's docs even say that SignalR only calls default Hub constructors.

In Startup.cs nothing seems to specify that I want to call ChatHub with a new chatStorage:

public void ConfigureServices(IServiceCollection services)
    {
        services.AddRazorPages();
        services.AddSignalR();
        services.AddSingleton<IChatStorage, ChatStorage>();
        services.AddSingleton<ChatController>();
    }

  public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        // omitted some default code

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapRazorPages();
            endpoints.MapHub<ChatHub>("/chatHub");
        });
    }

So first question, how does that work? How does it know to pass an argument to the ChatHub constructor? I understand the services.AddSingleton part, just not how that gets "wired up".

Same thing with a controller class. How does the program know to pass an IHubContext object into its constructor? Where do you specify that?

Finally, how would you go about making this setup work? Current I'm trying to communicate from ChatHub->chatStorage->ChatController->ChatHub. And to do that I'm trying to pass a reference to chatController in chatStorage.

Not sure if it's clear what I'm asking. If anything I'm looking for a clear explanation on how these concepts all work together, rather than a specific solution to my code.

Thank you!

Answer 1

So first question, how does that work? How does it know to pass an argument to the ChatHub constructor? I understand the services.AddSingleton part, just not how that gets "wired up".

When SignalR instantiates your hub instance (the framework controls this, you don't), it will resolve any dependencies specified in your Hub's constructor. This is part of the dependency injection system that's part of .NET (as mentioned in the comments).

Same thing with a controller class. How does the program know to pass an IHubContext object into its constructor? Where do you specify that?

Same idea, but you didn't have to wire up IHubContext, that's something that AddSignalR does.

Finally, how would you go about making this setup work? Current I'm trying to communicate from ChatHub->chatStorage->ChatController->ChatHub. And to do that I'm trying to pass a reference to chatController in chatStorage.

It's not exactly clear to me what you want the interaction to be between the client and the server to both the hub and the controller.

What I'm trying to achieve is, in a chat application: store messages outside of the Hub so that each time a user joins the chat, they can see all messages that had been sent before they joined.

Going back to this original statement I can ask a few clarifying questions:

• When the user writes a message in the chat room, are you calling the hub or a controller action (REST API)? That will determine where you need to inject the IChatStorage type. Once you receive a message, you'll stash the message in IChatStorage.
• When the user joins the chat, they'll make a call to the server to retrieve all messages. This call will return messages stored in IChatStorage.

Assuming you want to use the hub for everything, you would expose methods on the hub do accomplish this. If you wanted to use REST API calls from the client, then you would use the controller.