I told you, Magic!

No, seriously, just SignalR (and a bit of magic).

After installing SignalR via NuGet using Install-Package Microsoft.AspNet.SignalR you just have to follow a few steps.

First, you have to add the OWIN Startup Class and write one line of code:

    using Microsoft.Owin;
    using Owin;
    [assembly: OwinStartup(typeof(SignalRChat.Startup))]
    namespace SignalRChat
    {
        public class Startup
        {
            public void Configuration(IAppBuilder app)
            {
                // The LINE OF CODE
                app.MapSignalR();               
            }
        }
    }

The next step is to create our SignalR Hub with the broadcast method:

    using Microsoft.AspNet.SignalR;
    public class DrawHub : Hub
    {
        public void DrawMove(int[] p)
        {
            // Call the client method to draw the line.
            Clients.Others.DrawMove(p);
        }
    }

Wait, what???

Yes, you can call javascript methods from the server!

Setting up our html:

    <script src="Scripts/jquery-2.1.3.js"></script>
    <script src="Scripts/jquery.signalR-2.2.0.js"></script>
    <!--Reference the autogenerated SignalR hub script. -->
    <script src="signalr/hubs"></script>

Now, you need to initialize the hub by name in the client side:

    // Declare a proxy to reference the hub by class name. 
    var drawHub = $.connection.drawHub;

I told you, Magic!

Just get your server hub via the $.connection object. SignalR does all the work.

Next step, write the client method:

    // Create client functions that the hub can call to broadcast messages.
    drawHub.client.DrawMove = function (p) {
        // Call to canvas paint function
        DrawMove(p);
    };

And finally, call the server method from the client side (again, Magic!)

    // oX = original X
    // oY = original Y
    // x = current X
    // y = current Y
    // w = canvas.lineWidth
    // c = canvas.strokeStyle
    drawHub.server.drawMove([oX, oY, x, y, w, c]);

Wow... That was a lot of work... Nah, just kidding!

If you want to know more about SignalR, you can visit their website at http://signalr.net/ or the ASP.NET dedicated site at http://www.asp.net/signalr with lots of docs, samples & tutorials.

Also, you can contact me on Twitter