Groepen, gebruikers en verbindingsbeheer

public class ChatHub : Hub
{
    public async Task JoinRoom(string roomName)
    {
        await Groups.AddToGroupAsync(Context.ConnectionId, roomName);
        await Clients.Group(roomName)
                     .SendAsync("UserJoined", Context.User!.Identity!.Name);
    }

    public async Task LeaveRoom(string roomName)
    {
        await Groups.RemoveFromGroupAsync(Context.ConnectionId, roomName);
        await Clients.Group(roomName)
                     .SendAsync("UserLeft", Context.User!.Identity!.Name);
    }
}

public async Task SendToRoom(string roomName, string message)
{
    var sender = Context.User!.Identity!.Name;

    // Everyone in the room
    await Clients.Group(roomName)
                 .SendAsync("ReceiveMessage", sender, message);

    // Everyone in the room except the sender
    await Clients.GroupExcept(roomName, Context.ConnectionId)
                 .SendAsync("ReceiveMessage", sender, message);
}

// Send to all connections of user with ID "user-123"
await Clients.User("user-123")
             .SendAsync("Notification", "Your order has shipped!");

// SignalR uses the NameIdentifier claim by default.
// Context.UserIdentifier returns the current user's ID.
var myId = Context.UserIdentifier;
await Clients.User(myId!).SendAsync("Ack", "Server received your message");

public class EmailUserIdProvider : IUserIdProvider
{
    public string? GetUserId(HubConnectionContext connection)
    {
        return connection.User?.FindFirst(ClaimTypes.Email)?.Value;
    }
}

// Register:
builder.Services.AddSingleton<IUserIdProvider, EmailUserIdProvider>();

public class ConnectionTracker
{
    private readonly ConcurrentDictionary<string, HashSet<string>> _userConnections
        = new();

    public void AddConnection(string userId, string connectionId)
        => _userConnections.GetOrAdd(userId, _ => new()).Add(connectionId);

    public void RemoveConnection(string userId, string connectionId)
    {
        if (_userConnections.TryGetValue(userId, out var conns))
        {
            conns.Remove(connectionId);
            if (conns.Count == 0) _userConnections.TryRemove(userId, out _);
        }
    }

    public IEnumerable<string> GetConnections(string userId)
        => _userConnections.TryGetValue(userId, out var c) ? c : Enumerable.Empty<string>();
}

public override async Task OnConnectedAsync()
{
    // Add to a personal group named by user ID
    var userId = Context.UserIdentifier;
    if (userId is not null)
        await Groups.AddToGroupAsync(Context.ConnectionId, userId);
    await base.OnConnectedAsync();
}

// Now send to a user by their personal group:
await _hub.Clients.Group(userId)
           .SendAsync("Update", payload);

public class OrderShippedHandler : INotificationHandler<OrderShipped>
{
    private readonly IHubContext<OrderHub> _hub;
    public OrderShippedHandler(IHubContext<OrderHub> hub) => _hub = hub;

    public async Task Handle(OrderShipped notification, CancellationToken ct)
    {
        // Push to the customer's personal group
        await _hub.Clients.Group(notification.CustomerId.ToString())
                  .SendAsync("OrderShipped",
                      new { notification.OrderId, notification.TrackingNumber },
                      ct);
    }
}

// JavaScript client: re-join groups after reconnect
connection.onreconnected(async connectionId => {
    console.log(`Reconnected as ${connectionId}`);
    // Re-join previously joined rooms
    for (const room of joinedRooms) {
        await connection.invoke("JoinRoom", room);
    }
});

public override async Task OnConnectedAsync()
{
    var userId = Context.UserIdentifier!;
    var count = _tracker.AddConnection(userId, Context.ConnectionId);
    if (count == 1) // first connection for this user
        await Clients.Others.SendAsync("UserOnline", userId);
    await base.OnConnectedAsync();
}

public override async Task OnDisconnectedAsync(Exception? ex)
{
    var userId = Context.UserIdentifier!;
    var count = _tracker.RemoveConnection(userId, Context.ConnectionId);
    if (count == 0) // no more connections
        await Clients.Others.SendAsync("UserOffline", userId);
    await base.OnDisconnectedAsync(ex);
}