What is SignalR?

SignalR is a library for ASP.NET Core that enables adding WebSocket and Long Polling functionality to web applications for creating real-time interactive scenarios.

It provides two-way communication between server and client.

Main use cases:

• Chat applications.
• Online games.
• Real-time data updates (e.g., charts, notifications).
• User activity monitoring.

Main components:

• Hubs: Classes that handle incoming client calls and send messages to clients.
• Connections: Represent client connections. They can be established via WebSocket, Server-Sent Events, or Long Polling (depending on client and server support).

Example of creating a hub:

// MyHub.cs
using Microsoft.AspNetCore.SignalR;
using System.Threading.Tasks;

public class MyHub : Hub
{
    // Method that can be called by the client
    public async Task SendMessage(string user, string message)
    {
        // Sending message to all connected clients
        await Clients.All.SendAsync("ReceiveMessage", user, message);
    }
}

Configuration in Startup.cs:

// In ConfigureServices
services.AddSignalR();

// In Configure
app.UseRouting();
app.UseEndpoints(endpoints =>
{
    endpoints.MapHub<MyHub>("/myhub"); // Mapping hub to URL
});

On the client side (JavaScript):

// Using npm
// npm install @microsoft/signalr

const hubConnection = new signalR.HubConnectionBuilder()
    .withUrl("/myhub") // Hub URL
    .build();

// Handling incoming messages from server
hubConnection.on("ReceiveMessage", (user, message) => {
    // Logic to process received message
    console.log(`${user}: ${message}`);
});

// Opening connection
hubConnection.start()
    .then(() => console.log("Connection started"))
    .catch(err => console.error(err.toString()));

// Sending message to server
// hubConnection.invoke("SendMessage", "Client", "Hello from client!");

SignalR automatically chooses the optimal transport depending on client and server capabilities, starting with WebSocket and falling back to older protocols if necessary. This ensures broad compatibility.