如何在没有客户端请求的情况下在signalR中向客户端发送消息

using Microsoft.AspNetCore.SignalR;
using System.Threading.Tasks;

public class MyHub : Hub
{
    public async Task SendMessage(string message)
    {
        await Clients.All.SendAsync("ReceiveMessage", message);
    }
}

// 在Startup.cs文件中的ConfigureServices方法中注册Hub类
public void ConfigureServices(IServiceCollection services)
{
    services.AddSignalR();
}

// 在Startup.cs文件中的Configure方法中配置SignalR路由
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseRouting();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapHub<MyHub>("/myhub");
    });
}

public class MyBackgroundTask : BackgroundService
{
    private readonly IHubContext<MyHub> _hubContext;

    public MyBackgroundTask(IHubContext<MyHub> hubContext)
    {
        _hubContext = hubContext;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            // 发送消息给所有客户端
            await _hubContext.Clients.All.SendAsync("ReceiveMessage", "Hello from background task");

            // 等待一段时间
            await Task.Delay(TimeSpan.FromSeconds(10), stoppingToken);
        }
    }
}