How to send a periodic message from the server in SignalR

Question

I've created a new ASP.NET 6 web app. I want to periodically broadcast a message through a SignalR hub, from the server. How can I access the hub from the server? Other answers suggest using GlobalHost but it belongs to a deprecated version of SignalR

Example code from web app Program.cs:

app.MapHub<SiteLayoutHub>("hubs/site");

app.Run();

Task.Factory.StartNew(async () =>
{
    var hub = GetSiteLayoutHub(); // How can I get this hub?
    while (true)
    {
        var uiState = GetUIState();
        await hub.SendUIUpdateMessage(uiState);
        Thread.Sleep(500);
    }
});

SiteLayoutHub.cs:

public class SiteLayoutHub : Hub
{
    public async Task SendUIUpdateMessage(UIState uiState)
    {
        await Clients.All.SendAsync("UIUpdateMessage", uiState);
    }
}

You'll need to create a .Net client to run on the server side. I'd close it after you're finished. Or map another hub endpoint dedicated only to this client. — GH DevOps
– GH DevOps, Commented Jan 30, 2023 at 16:54
learn.microsoft.com/aspnet/core/signalr/… or learn.microsoft.com/aspnet/core/signalr/… Note: You do not (and should not) get access to a Hub in background tasks, you get access to an interface that lets you send messages to clients. — Brennan
– Brennan, Commented Jan 31, 2023 at 0:39

davidfowl · Accepted Answer · 2023-01-31 05:05:43Z

5

These are all of the pieces required:

using Microsoft.AspNetCore.SignalR;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddSignalR();
builder.Services.AddHostedService<UIUpdateService>();

var app = builder.Build();

app.MapHub<SiteLayoutHub>("hubs/site");

app.Run();

public class SiteLayoutHub : Hub { }

public class UIUpdateService : BackgroundService
{
    private readonly IHubContext<SiteLayoutHub> _hubContext;
    public UIUpdateService(IHubContext<SiteLayoutHub> hubContext)
    {
        _hubContext = hubContext;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        var timer = new PeriodicTimer(TimeSpan.FromSeconds(5));

        while (await timer.WaitForNextTickAsync(stoppingToken))
        {
            var uiState = GetUiState();
            await _hubContext.Clients.All.SendAsync("UIState", uiState);
        }
    }

    private object GetUiState()
    {
        throw new NotImplementedException();
    }
}

answered Jan 31, 2023 at 5:05

davidfowl

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1 Comment

Efi Z Over a year ago

Works perfectly, much appreciated. For anyone else looking for this answer, you might find this MSDN page useful: learn.microsoft.com/en-us/aspnet/core/fundamentals/host/…

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