"Pushing" Data to a Silverlight Application

The Web Service consumption feature in Silverlight 2 Beta1, based on the WCF technology and accessible through the "Add Service Refernce" dialog, makes it really easy for a Silverlight application to request data from a remote service, or to upload some data to a remote location.

But what if you want the service to initiate the conversation? Consider these examples:

• An Instant Messaging / Chat application. When a user types a message into your Silverlight app, you use WCF to make, for example, a SendChatMessage() call. But what about receiving messages that others type back? Sure, you could call something like ReceiveChatMessage() continuously in a loop, and it could return null if there are no messages waiting. But this is a lot of work, not to mention the severe implications this would have for the performance of your app. Wouldn't it be nice if it "just worked"?
  
• A Monitoring application. A lot of people are writing applications that monitor something that can be changed at any time on the server. Maybe it's a stock ticker / charting app, or maybe it's monitoring some physical sensor... Again, you could continuously poll the server to see if anything changed, but this is inefficient.

To solve problems like this, we are trying to ship a "duplex" feature in Silverlight 2 Beta2. (There is a very good chance that the feature will make it in, but of course no guarantees, and anything I say about the design is subject to change by the time we ship). I'll first give you a quick overview and then the details:

Quick Overview

Let's say you want to build a chat application. You'll start by defining a chat message such as the following:

[DataContract]
public class ChatMessage {
[DataMember] public string messageText;
[DataMember] public int textColor;
[DataMember] public bool isSmileyFace;
}

You would then create a "duplex service" on the server-side the way you would normally create a duplex service in WCF (more info below). The last step would be to configure the WCF service with a special "Silverlight duplex binding" by making an appropriate modification to Web.config.

In Silverlight, you will not be able to do an "Add Service Reference" to this service. Instead, you will manually copy the ChatMessage class to your Silverlight project. Then, after adding a new Extension assembly to your Silverlight project as well as some sample code we'll provide, you'll be able to do something like this:

var r = new DuplexReceiver<ChatMessage>("http://example.com/myService.svc");
r.OnMessageReceived += new OnMessageReceivedEventHandler<ChatMessage>(r_OnMessageReceived);

And that's all! Whenever the server chooses to send a message, you will need to handle this event by providing the following callback method:

static void r_OnMessageReceived(object sender, OnMessageReceivedEventArgs<ChatMessage> e)
{
   //Handle a chat message received from the server
   //E.g.:
   chatBox.Text = chatBox.Text + e.Result.messageText;
   //...
}

Details

For those of you familiar with WCF: What we're actually aiming to ship in Beta2 are two things:
- A new binding for WCF on the server side
- The client-side equivalent of this binding for WCF-in-Silverlight
What is not included in the plan (for Silverlight 2, not just for Beta2), is any Service Model support (like callback contracts and the like), meaning no "Add Service Reference" support.

Normally, this would mean that you need to be a WCF guru to use this (create a channel manually, receive a WCF Message, deserialize). Fortunately, we are hoping to ship sample code for DuplexReceiver < T > , that will make simple scenarios really easy (like you saw above).

For an overview of building the server side of duplex services with WCF, a good starting point is http://msdn2.microsoft.com/en-us/library/ms731064.aspx and http://msdn2.microsoft.com/en-us/library/ms731184.aspx- but here too, we may provide some sample server-side code to make this easier.

"Under the hood", the duplex mechanism works by polling the server using HTTP requests. However, this is "smart polling": the polling request to the server will not immediately return if there are no messages, but will linger around and return only when a message actually becomes available. There is a lot more to this but I can't talk about it yet (except for referring to Clarke's Third Law :)

This post is mostly to get the basic information out there - there are a lot more details to cover, which I may do in future posts.