Real-Time Communication with SignalR

using Microsoft.AspNetCore.SignalR;

// ChatHub.cs — a Hub is the server class clients connect to.
// Each PUBLIC method here is something a client can call over the wire.
// NOTE: SignalR needs an ASP.NET Core host, so this won't run in the fiddle —
// study it, then practise the same PUSH idea in plain C# further down.
public class ChatHub : Hub
{
    // A client calls this; the server then PUSHES to every connected client.
    public async Task SendMessage(string user, string message)
    {
 
...

using System;
using System.Collections.Generic;

// 🎯 YOUR TURN — SignalR can't run here, so we model the SAME idea in
// plain C#. A "hub" keeps a list of subscriber handlers and broadcasts
// a message to every one of them — exactly like Clients.All.SendAsync.

class MiniHub
{
    // Every subscriber is an Action<string> — a handler that takes a message.
    private readonly List<Action<string>> _subscribers = new();

    // Subscribe = the client "tunes in".
    public void Subscribe(Action<
...

using Microsoft.AspNetCore.SignalR;

// Groups are PRIVATE channels — only members of a group receive a
// Group broadcast. Think of a conference call inside the radio station.
public class RoomHub : Hub
{
    // Add THIS connection to a named group (e.g. a chat room).
    public async Task JoinRoom(string room)
    {
        await Groups.AddToGroupAsync(Context.ConnectionId, room);
        // Tell only this room that someone arrived.
        await Clients.Group(room).SendAsync("ReceiveMessage",
...

using System;
using System.Collections.Generic;

// 🎯 YOUR TURN — add GROUPS to the hub: a Dictionary mapping a
// group name to the list of subscribers in it. Sending to a group
// reaches only that group (the Clients.Group idea).

class GroupHub
{
    // group name -> the handlers subscribed to that group
    private readonly Dictionary<string, List<Action<string>>> _groups = new();

    // Add a handler to a named group (creating the group if it's new).
    public void JoinGroup(string group
...

// JavaScript client — the browser end of the connection.
// Install:  npm install @microsoft/signalr
import * as signalR from "@microsoft/signalr";

const connection = new signalR.HubConnectionBuilder()
    .withUrl("/hubs/chat")                 // must match app.MapHub(...)
    .withAutomaticReconnect()              // auto-retry if the link drops
    .build();

// Register handlers BEFORE start() — the name must match the server's
// SendAsync("ReceiveMessage", ...) exactly, or the message is
...

using Microsoft.AspNetCore.SignalR;

// A Hub instance is created PER CALL and thrown away — never store state
// in fields. Track connections with the lifecycle overrides instead.
public class PresenceHub : Hub
{
    // Runs when a client opens the connection.
    public override async Task OnConnectedAsync()
    {
        // Context.ConnectionId is a fresh id for THIS connection only.
        await Clients.Others.SendAsync("UserJoined", Context.ConnectionId);
        await base.OnConnectedAsyn
...