The Simplest Thing Possible: Creating Push Notifications with SignalR
The ASP.NET team has created lots of new goodies. One of the most useful, SignalR, is an async library for .NET to help build real-time, multi-user interactive web applications. Imagine this scenario: you have a web application and would like a simple way to push notifications to any number of clients. Perhaps you want to inform the client when something happens. You could write a polling mechanism, but that is inefficient. An event-based approach is a much more efficient approach. Event-based systems that can publish and subscribe to events are typically loosely coupled systems that easily adapt to change and are highly scalable. The SignalR library makes it very easy to build loosely coupled scalable applications that can send real-time updates to specified clients. In this article, I will take you through the some basic steps to get up and running with SignalR. The Example In this brief example I’ll demonstrate how to instrument an ASP.NET MVC 4 application. In other words, certain application components will be monitored when their services are invoked. This means that whenever a controller action is called, a message will be sent to clients that are equipped to respond to the message. The monitor in this case will be a simple HTML page with some JavaScript. SignalR Resources Before getting started, I want to cover some fundamentals about what you’ll use SignalR for, how it works, and how to get to other resources. You can find the home page for SignalR at www.signalr.net. The home page itself does not contain much in the way of direct information but it does contain some important links to the SignalR GitHub source code repository at the following URL: https://github.com/SignalR/SignalR/. Within the GitHub Repository you will find all of the source code, issue list, code samples and wiki. The canonical use for SignalR are those cases where messaging is required. For example, consider the Jabbr application (jabbr.net). Jabbr is a SignalR-based group chat system. You can view and download the code on GitHub (https://github.com/davidfowl/JabbR). SignalR is an abstraction over a standard Internet connection. The two primary components in SignalR are hubs and persistent connections. Connection persistence is how the server can communicate with one or more clients. The vehicle to communicate with clients is a hub. The best way to demonstrate how SignalR works is to create an example project which is illustrated in the next section. Step 1 - Create a New Project The hypothetical is simple: broadcast a message to an HTML page when a controller action is invoked. To make this happen, the example will use SignalR’s Hub class. Per the documentation, a SignalR Hub provides an RPC framework over a persistent connection. Think of a SignalR Hub as a switchboard of sorts that takes care of communicating with a client. I’ll discuss the specifics of the Hub class in the next section. The MVC app for this example is a stock Internet-based MVC application. Step 2 - Install the SignalR NuGet Package With the MVC application in place, the next step is to install the SignalR NuGet Package. NuGet makes quick work of installing SignalR and all of its dependencies. Figure 1 illustrates the process. 
Figure 1: SignalR NuGet Package installation using the NuGet Package Manager Console.Step 3 - Create the Hub Class and Server Implementation Code With SignalR installed, the next step is to create the NotificationHub class (based on the Hub base class). using System;
using System.Linq;
using SignalR.Hubs;
namespace signalRExample
{
public class NotificationHub : Hub
{
public string Activate()
{
return "Monitor Activated";
}
}
}
Next you want to create some server-side code that will send the message to whatever clients may be listening to this particular event. To facilitate, add the following code as a private method to the Home Controller class: void SendMessage(string message)
{
GlobalHost
.ConnectionManager
.GetHubContext<NotificationHub>().Clients.sendMessage(
message);
}
With the SendMessage method in place, you just need to add code that calls the method. The following example calls the SendMessage method from the Index Action: SendMessage("Index action invoked.");
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By: John V. Petersen John Petersen has been developing software for over 20 years. It all started when, as a staff accountant, he was asked to get involved in a system upgrade to replace an old IBM Series 1 computer (about the size of a large refrigerator!). Those first programs were written in Clipper, Summer 87. Since that time, John’s tools included dBase, FoxBase, Visual FoxPro and Visual Basic. An early adopter of .NET, he then decided to go to law school. After practicing law for a few years, John realized that technology was a lot more interesting than the law. Today, John focuses on ASP.NET development and is having more fun than ever solving business problems for clients. John is a Practice Director for Custom Application Development at Neudesic, a Microsoft Gold Partner and the Trusted Technology Partner in Business Innovation. A 9-time recipient of Microsoft’s Most Valuable Professional Award, John is a current ASP.NET/IIS MVP. John is also an ASP Insider and is the INETA Mentor for PA and WV. John is the author of several books and is a frequent contributor to CODE Magazine and DevPro magazine. John holds a BS in Business Administration from Mansfield University, an MBA in Information Systems from St. Joseph’s University and a JD from the Rutgers School of Law – Camden.
email: johnvpetersen@gmail.com
blog: codebetter.com/johnvpetersen
twitter: @johnvpetersen
john.v.petersen@comcast.net |