Demonstrate sending message to specific client(s) according group.

EventReceiverService.cs

using Microsoft.AspNetCore.SignalR;
using Newtonsoft.Json;
using SignalR.Lab.Web.Hubs;
using SignalR.Lab.Web.Models;
using System;
using System.Timers;

namespace SignalR.Lab.Web.Services
{
    public class EventReceiverService
    {
        // .....

        private void MockEventGenTimer_Elapsed(object sender, ElapsedEventArgs e)
        {
            _mockEventGenTimer.Enabled = false;

            var mockEvent = new EventModel()
            {
                Content = $"{new Random().Next()}"
            };
            
            // target to line1 or line2 or line3
            var target = new Random().Next(1, 4);
           
　　　　　　 // implement and invoke send message to group 　　
            _eventHub.Clients.Group($"Line{target}").SendAsync(nameof(IEventHub.SendMessageToGroup),
                $"Line{target}", JsonConvert.SerializeObject(mockEvent)).GetAwaiter().GetResult();
                
            _mockEventGenTimer.Enabled = true;
        }
    }
}