.NET 8中如何通过SignalR从ASP.NET Core后台服务向Blazor WASM特定用户发实时通知

我是SignalR新手，首次学习实践该技术。在.NET 8环境下，我尝试通过ASP.NET Core Web API的BackgroundService，借助SignalR向Blazor WASM应用中的特定用户发送实时通知。当前设置能正常向所有客户端发送通知，但想改成定向发送，不清楚怎么在BackgroundService里获取目标用户ID，也不确定现有思路是否正确。

客户端代码 #

private HubConnection? _hubConnection;
private readonly List<string> _notifications = new();

[Inject] IAccessTokenProvider TokenProvider { get; set; }

[Inject] IConfiguration Configuration { get; set; }

[CascadingParameter] private Task<AuthenticationState> authenticationStateTask { get; set; }

protected override async Task OnInitializedAsync()
{
    var authState = await authenticationStateTask;
    var user = authState.User;

    if (user.Identity.IsAuthenticated)
    {
        _hubConnection = new HubConnectionBuilder()
                        .WithUrl($"{Configuration.GetValue<string>("ApiBaseAddress")}notifications",
                        options =>
                        {
                            options.AccessTokenProvider = async () =>
                            {
                                var accessTokenResult = await TokenProvider.RequestAccessToken();
                                accessTokenResult.TryGetToken(out var token);
                                return token.Value;
                            };
                        })
                        .Build();

        _hubConnection.On<string>("ReceiveNotification", notification =>
        {
            _notifications.Add(notification);
            InvokeAsync(StateHasChanged);
        });

        await _hubConnection.StartAsync();
    }
}

服务端代码 #

Hub定义 #

[Authorize]
public class NotificationsHub : Hub<INotificationClient>
{
}

public interface INotificationClient
{
    Task ReceiveNotification(string message);
}

Program.cs配置 #

...
builder.Services.AddSignalR();
...
app.MapHub<NotificationsHub>("notifications");

现有BackgroundService代码 #

public class InventoryNotifier(
    IHubContext<NotificationsHub, INotificationClient> hubContext,
    ILogger<InventoryNotifier> logger) : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        using var timer = new PeriodicTimer(TimeSpan.FromSeconds(5));

        while (!stoppingToken.IsCancellationRequested && await timer.WaitForNextTickAsync(stoppingToken))
        {
            await hubContext.Clients
                .All
                .ReceiveNotification($"InventoryNotifier is starting... {DateTime.Now}");
        }

        stoppingToken.Register(() => logger.LogWarning($"{nameof(InventoryNotifier)} is stopping due to host shut down."));
    }
}

当前问题 #

现在使用Clients.All可以实现群发通知，但要改为Clients.User(userId)定向发送时，不清楚如何在BackgroundService中获取目标用户ID。尝试使用IUserIdProvider.GetUserId()方法，但该方法需要HubConnectionContext，而BackgroundService中无法获取这个上下文。

我曾尝试的错误逻辑：

public class InventoryNotifier(
    IHubContext<NotificationsHub, INotificationClient> hubContext,
    IUserIdProvider userIdProvider,
    ILogger<InventoryNotifier> logger) : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        using var timer = new PeriodicTimer(TimeSpan.FromSeconds(5));
        
        while (!stoppingToken.IsCancellationRequested && await timer.WaitForNextTickAsync(stoppingToken))
        {
            await hubContext.Clients
                .User(userIdProvider.GetUserId()) // 此处无法获取HubConnectionContext，逻辑错误
                //.All
                .ReceiveNotification($"InventoryNotifier is starting... {DateTime.Now}");
        }

        stoppingToken.Register(() => logger.LogWarning($"{nameof(InventoryNotifier)} is stopping due to host shut down."));
    }
}

解决方案 #

核心思路纠正 #

IUserIdProvider.GetUserId()是在SignalR连接建立时，从当前活跃的连接上下文（HubConnectionContext）中提取用户ID的方法，它不能直接在BackgroundService中调用——因为BackgroundService是后台运行的服务，没有绑定任何活跃的SignalR连接上下文。

正确的做法是：直接使用业务场景中已知的目标用户ID，或者从数据库、业务服务等渠道动态获取用户ID，再通过Clients.User(targetUserId)发送定向通知。

具体实现步骤 #

1. 确认SignalR能正确识别用户ID

默认情况下，SignalR会从认证用户的ClaimTypes.NameIdentifier Claim中获取用户ID。你可以在Hub的OnConnectedAsync方法中验证当前连接的用户ID是否正确：

[Authorize]
public class NotificationsHub : Hub<INotificationClient>
{
    public override async Task OnConnectedAsync()
    {
        // 获取当前连接的用户ID，可用于调试验证
        var currentUserId = Context.UserIdentifier;
        await base.OnConnectedAsync();
    }
}

如果你的用户ID来自其他Claim（比如邮箱），可以自定义IUserIdProvider：

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

然后在Program.cs中注册这个自定义提供器：

builder.Services.AddSingleton<IUserIdProvider, EmailBasedUserIdProvider>();

2. 在BackgroundService中使用目标用户ID发送通知

根据业务场景，目标用户ID的获取方式分两种：

方式一：固定目标用户ID

如果是给特定固定用户发通知，直接在BackgroundService中定义目标ID即可：

public class InventoryNotifier(
    IHubContext<NotificationsHub, INotificationClient> hubContext,
    ILogger<InventoryNotifier> logger) : BackgroundService
{
    // 实际业务中可从配置、数据库等地方获取
    private readonly string _targetUserId = "user_12345";

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        using var timer = new PeriodicTimer(TimeSpan.FromSeconds(5));
        
        while (!stoppingToken.IsCancellationRequested && await timer.WaitForNextTickAsync(stoppingToken))
        {
            await hubContext.Clients
                .User(_targetUserId)
                .ReceiveNotification($"InventoryNotifier is starting... {DateTime.Now}");
        }

        stoppingToken.Register(() => logger.LogWarning($"{nameof(InventoryNotifier)} is stopping due to host shut down."));
    }
}

方式二：动态获取目标用户ID

如果需要根据业务逻辑动态获取用户ID（比如监控到某个用户的库存不足，就通知该用户），可以注入对应的业务服务或仓储来获取：

// 假设这是你的用户仓储接口
public interface IUserRepository
{
    Task<string?> GetTargetInventoryUserIdAsync(CancellationToken cancellationToken);
}

public class InventoryNotifier(
    IHubContext<NotificationsHub, INotificationClient> hubContext,
    IUserRepository userRepository,
    ILogger<InventoryNotifier> logger) : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        using var timer = new PeriodicTimer(TimeSpan.FromSeconds(5));
        
        while (!stoppingToken.IsCancellationRequested && await timer.WaitForNextTickAsync(stoppingToken))
        {
            // 从业务仓储获取需要通知的用户ID
            var targetUserId = await userRepository.GetTargetInventoryUserIdAsync(stoppingToken);
            if (!string.IsNullOrEmpty(targetUserId))
            {
                await hubContext.Clients
                    .User(targetUserId)
                    .ReceiveNotification($"InventoryNotifier is starting... {DateTime.Now}");
            }
        }

        stoppingToken.Register(() => logger.LogWarning($"{nameof(InventoryNotifier)} is stopping due to host shut down."));
    }
}

内容的提问来源于stack exchange，提问作者fingers10