Run BackgroundService with parameter from code

Question

How I can run BackgroundService by parameter from code?

I have SignalR, user join to group (example group:a, group:b, group:c, group:d). I want run CatService with current join group:

public class CatHub : Hub
{
    public async Task JoinToGroup(string group)
    {
        // how i can run new CatService with current group ??? 
        CORE_RUN_NEW_SERVICE<CatService>(_, group);
    }
}

public class CatService : BackgroundService
{
    private readonly IHubContext<CatHub> _hub;
    private readonly string _group;  
    public CandleSig(IHubContext<CatHub> hub, string group)
    {
        _hub = hub;
        _group = group
    }

    protected override Task ExecuteAsync(CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            _hub.SendToGroup(_group, "test")
            await Task.Delay(TimeSpan.FromMinutes(1), ct);
        }
    }
}

Answer 1

You cannot modify the number of background services once the application is running.

However, you can create a single background service that has a queue. First define the queue:

public sealed class MyBackgroundQueue
{
  private readonly Channel<string> _channel = Channel.CreateUnbounded<string>();

  public ValueTask QueueAsync(string item) => _channel.WriteAsync(item);

  public IAsyncEnumerable<T> DequeueAllAsync(CancellationToken ct) =>
      _channel.ReadAllAsync(ct);
}

inject it into your background service:

public class CatService : BackgroundService
{
  private readonly IHubContext<CatHub> _hub;
  private readonly MyBackgroundQueue _queue;
  public CatService(IHubContext<CatHub> hub, MyBackgroundQueue queue)
  {
    _hub = hub;
    _queue = queue;
  }

  protected override Task ExecuteAsync(CancellationToken ct)
  {
    await foreach (var item in _queue.DequeueAllAsync(ct);
      _hub.SendToGroup(_group, "test")
  }
}

and also into your hub:

public class CatHub : Hub
{
  private readonly MyBackgroundQueue _queue;
  public CatHub(MyBackgroundQueue queue) => _queue = queue;

  public async Task JoinToGroup(string group)
  {
    await _queue.EnqueueAsync(group);
  }
}