SignalR call to specific client from controller ASP.NET Core 2.1

Question

I need to send an instant message from the server to the client after the user has submitted a form in a browser.

I followed the Microsoft steps here to set up a signalR connection, created a Hub class, signalr.js etc.

The problem is that I can only invoke a message to all clients, but I need to invoke the message to the specific caller who initiated the request (otherwise everyone will get the message).

This is my POST Action in the HomeController.cs:

[HttpPost]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Submit(string signalRconnectionId, Dictionary<string, string> inputs)
    {
        //Invoke signal to all clients sending a message to initSignal WORKS FINE
        await _signalHubContext.Clients.All.SendAsync("initSignal", "This is a message from the server!");

        //Invoke signal to specified client where signalRConnectionId = connection.id DOES NOT WORK
        await _signalHubContext.Clients.Client(signalRconnectionId).SendAsync("initSignal", "This is a message from server to this client: " + signalRconnectionId);

        return RedirectToAction("Success", inputs);
    }

my client javascript file:

    //Create connection and start it
const connection = new signalR.HubConnectionBuilder()
    .withUrl("/signalHub")
    .configureLogging(signalR.LogLevel.Information)
    .build();
connection.start().catch(err => console.error(err.toString()));

console.log("connectionID: " + connection.id);
$("#signalRconnectionId").attr("value", connection.id);

//Signal method invoked from server
connection.on("initSignal", (message) => {

    console.log("We got signal! and the message is: " + message);


});

I have tried debugging the action method and I get correctly passed in the connectionId which is "0" (incrementing by 1 per connection)

On the page you linked, there is another link that explains `Clients` : https://learn.microsoft.com/en-us/aspnet/core/signalr/hubs?view=aspnetcore-2.1 — Neil, Aug 13 '18 at 11:26
`Clients.Caller.SendAsync("ReceiveMessage", message);` - This will invoke the client side function only for the client who initiated the request. — Thangadurai, Aug 13 '18 at 13:16
Have you tried it yourself? When I use _signalHubContext.Clients there is no caller method available. If I do it from the Hub class there is, but then I need to put business logic in there, I would like to do it from the hubcontext I got through DI in the controller — Morten_564834, Aug 13 '18 at 16:25
Where is this assumption from: `connectionId which is "0" (incrementing by 1 per connection)` — aaron, Aug 15 '18 at 06:00
Possible duplicate of [Connection ID when calling SignalR Core Hub method from Controller](https://stackoverflow.com/questions/50367586/connection-id-when-calling-signalr-core-hub-method-from-controller) — aaron, Aug 15 '18 at 06:00
@aaron you're right this is just my assumption based on my own observations; if I start connection the first time it has Id of 0, second time it has 1 and so forth. But I can't find any documentation on what the expected Id should look like and how it works. — Morten_564834, Aug 16 '18 at 13:59
the context.Id is not the right Id to get, the actual ConnectionId looks like this: at4Hldd4GnWiSaO1Cit-mQ — Morten_564834, Aug 24 '18 at 12:45

score 8 · Accepted Answer · edited Jul 11 '21 at 10:04

So here's the final solution I came up with inspired by the answer from this thread

I got the connectionId by calling the Hub class from client and then from client calling the controller passing in the connectionId.

Hub class:

public class SignalHub : Hub
{
    

    public string GetConnectionId()
    {
        return Context.ConnectionId;
    }
}

client-side javascript code executed at startup:

connection.start().catch(err => console.error(err.toString())).then(function(){
connection.invoke('getConnectionId')
    .then(function (connectionId) {
        // Send the connectionId to controller
        console.log("connectionID: " + connectionId);
        $("#signalRconnectionId").attr("value", connectionId);
    });
});

HomeController.cs:

public async Task<IActionResult> Submit(string signalRconnectionId, Dictionary<string, string> inputs)
    {
        
        //Invoke signal to specified client WORKS NOW
        await _signalHubContext.Clients.Client(signalRconnectionId).SendAsync("initSignal", "This is a message from server to this client: " + signalRconnectionId);

        return RedirectToAction("Success", inputs);
    }

It works fine, but still feels a little like a roundtrip, it would have been easier if we didn't have to go through the hub class to make this happen. Maybe just having the connectionId from the client-side to begin with, but maybe there is a good reason for the design :)

I was not able to invoke the hub method, where I have business logic within the method too. But it seems like it was only sending the message directly to the front end user. Oh, and I was able to user _hubContext.Clients.User(userId).SendAsync("method1", message) — Alan Chang, Jun 19 '19 at 01:49

score 0 · Answer 2 · answered Aug 15 '18 at 06:05

0

According to Microsoft, you can not access to the ConnectionId and Caller from outside a hub https://learn.microsoft.com/en-us/aspnet/core/signalr/hubcontext?view=aspnetcore-2.1

When hub methods are called from outside of the Hub class, there's no caller associated with the invocation. Therefore, there's no access to the ConnectionId, Caller, and Others properties.

answered Aug 15 '18 at 06:05

user8013875

92
1
1
9

Yes thanks - bummer :/. Wonder if this is a technical boundary or why they would do this, it kind of ruins the flexibility of instant communication from server. I would then need to call a Hub method from client when form is submitted and then inside this method call a controller action or another repository method, hence mixing business logic within a hub class hmm – Morten_564834 Aug 16 '18 at 08:03
Just use the UserId instead – davidfowl Aug 17 '18 at 04:34
1

But UserId would require the site to use authentication right? – Morten_564834 Aug 17 '18 at 13:26

SignalR call to specific client from controller ASP.NET Core 2.1

2 Answers2

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