要实现在ASP.NET Core 3.1中使用Worker Service模板和SignalR，您可以按照以下步骤进行操作：

1. 创建一个新的ASP.NET Core Worker Service项目。您可以使用Visual Studio或者通过命令行运行dotnet new worker -n MyWorkerService来创建项目。

2. 安装必需的NuGet包。在项目的.csproj文件中，添加以下NuGet包引用：

3. 在Worker.cs文件中添加SignalR的代码示例：

using Microsoft.AspNetCore.SignalR.Client;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using System;
using System.Threading;
using System.Threading.Tasks;

namespace MyWorkerService
{
    public class Worker : BackgroundService
    {
        private readonly ILogger _logger;
        private HubConnection _hubConnection;

        public Worker(ILogger logger)
        {
            _logger = logger;
        }

        public override async Task StartAsync(CancellationToken cancellationToken)
        {
            _hubConnection = new HubConnectionBuilder()
                .WithUrl("https://localhost:5001/chatHub") // Replace with your SignalR Hub URL
                .Build();

            _hubConnection.Closed += async (exception) =>
            {
                await Task.Delay(new Random().Next(0, 5) * 1000);
                await _hubConnection.StartAsync();
            };

            await _hubConnection.StartAsync();

            await base.StartAsync(cancellationToken);
        }

        public override async Task StopAsync(CancellationToken cancellationToken)
        {
            await _hubConnection.StopAsync();
            await base.StopAsync(cancellationToken);
        }

        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
        {
            while (!stoppingToken.IsCancellationRequested)
            {
                _logger.LogInformation("Worker running at: {time}", DateTimeOffset.Now);

                // Call SignalR hub method
                await _hubConnection.SendAsync("SendMessage", "Worker Service", "Hello from Worker Service!");

                await Task.Delay(10000, stoppingToken);
            }
        }
    }
}

4. 创建SignalR Hub。在项目中创建一个名为ChatHub.cs的新类文件，并添加以下代码：

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

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

5. 在Program.cs文件中添加SignalR的相关配置。打开Program.cs文件，并将CreateHostBuilder方法中的代码替换为以下内容：

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureServices((hostContext, services) =>
        {
            services.AddHostedService();
        })
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.UseStartup();
        });

6. 配置SignalR路由。在项目中创建一个名为Startup.cs的新类文件，并添加以下代码：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

namespace MyWorkerService
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddSignalR();
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapHub("/chatHub"); // Replace with your desired SignalR Hub URL
            });
        }
    }
}

7. 运行项目。您可以通过使用Visual Studio的调试功能或者通过运行dotnet run命令来运行项目。

现在，您的ASP.NET Core Worker Service应该能够与SignalR Hub进行通信，并发送消息到SignalR客户端。