ASP.NET Core SignalR

https://github.com/mkader/SignalRExample

The ASP.NET Core SignalR repository on GitHub.

MicroSoft SingalR Docs

SignalR Samples

https://github.com/dotnet/aspnetcore/tree/master/src/SignalR

https://stackoverflow.com/questions/16983630/how-does-signalr-decide-which-transport-method-to-be-used

SignalR is an open source library and allows developers to simplify adding real-time web functionality to applications. 
"real-time web" functionality means that the server-side code can instantly push content to connected clients.

Examples

• Chat applications
• Notification applications (Team meet, share ideas)
• Live dashboards (display instantly updated sales information).

The SignalR API allows server-side code to call JavaScript functions on connected clients via remote procedure calls (RPCs).

Feature of SignalR

• It automatically takes care of connection management. 
• It can send messages to all connected clients at the same time or  to a specific client or group of clients. (Chat room).
• It can automatically scales to handle increasing traffic.

Transports 

To handle real-time communication, SignalR supports (These are called transports): 

• WebSockets 
• Server-Sent Events 
• Long Polling

SignalR lies on top of the lower-level transports. 

SignalR automatically chooses the best transport method that is within the capabilities of the server and client. 

• If WebSockets (the most efficient of the transports) are not supported by the server or browser, SignalR will fall back on Server-Sent Events. 
• If Server-Sent Events are not supported, SignalR will fall back on Long Polling. 
• When SignalR establishes a connection, it will start sending keep-alive messages to verify that the connection is still alive. If the connection is not alive, SignalR will throw an exception.
• The WebSocket protocol enables two-way, real-time communication between clients and servers in web-based applications.

Hubs 

Hubs are used by SignalR to communicate between servers and clients. The hub allows a client and server to call methods on each other. 

Configure the SignalR middleware by registering the service in the Startup.ConfigureServices() {.. services.AddSignalR(); } 

Set up the SignalR routes by calling endpoints.MapHub in the Startup.Configure()  

app.UseEndpoints(endpoints => {     

...   

endpoints.MapHub<ChatHub>("/WhatsupChatHub"); 

}); 

The Hub class (HubCallerContext) property provide information about the connection: 

ConnectionAborted, ConnectionId, Features, Items, User,  UserIdentifier 

The Hub class methods: an Abort method and GetHttpContext extension method. 

The hubs are impermanent (last for a short time) and transient, should not store state in a property on the hub class, because every hub method call is executed on a new instance of the hub.

IHubCallerClients

The hub will handle client-server communication. Create a whatsupchat\Hubs\WhatsupChatHub.cs class 

using System.Threading.Tasks;
using Microsoft.AspNetCore.SignalR;
namespace whatsupchat.Hubs;
{
    public class WhatsupChatHub : Hub
    {
     public async Task MessageSender(string user, string message)
     {
        await Clients.All.SendAsync("MessageReceiver", user, message);
    }
   }
}

"Clients.All" property that invokes a method on all the clients connected to this hub.

The Clients object implements the IHubCallerClients interface (public IHubCallerClients Clients { get; set; }). This interface has the following properties: 

• All — All clients connected to the hub. 
• Caller — Gets a caller to the connection that triggered the current invocation. 
• Others — Gets a caller to all connections except the one that triggered the current invocation. 

It also includes methods such as: 

• AllExcept — All clients connected to the hub, but excludes the specified connections passed as a read-only list of strings. 
• Client — A specific client connection. 
• Clients — The specified read-only list of client connections. 
• Group — All connections in the specified group. 
• GroupExcept — All connections in the specified group, but excludes the specified connections passed as a read-only list of strings. 
• Groups — All connections in the specified read-only list of groups.
• OthersInGroup — Gets a caller to all connections in the specified group, except the one which triggered the current invocation.  
• User — All connections associated with the specified user. 
• Users — All connections associated with the users specified in the read-only list of strings.

The WhatsupChatHub class inherits from the SignalR Hub class. This Hub base class is responsible for managing connections, groups, and messaging. The following properties: 

• Clients — Gets/sets an object on the clients that are connected to this hub.
• Context — Get/set the hub caller context.
• Groups — Get/set the group manager. 

The Hub class also has the following properties: 

• OnConnectedAsync — Called when a new connection is established with the hub and returns a task representing the asynchronous connect. 
• OnDisconnectedAsync — Called when the connection to the hub is terminated and returns a task representing the asynchronous disconnect. 
• Dispose — Dispose of all the resources currently in use by this hub instance.