If you've upgraded to Flash MX 2004 Professional or you purchased the FireFly Data Connection Kit for Flash MX, then you've probably read a lot about how easy it is to have Flash pull data into a Flash Movie. The only problem is that this is not how applications work in the real world.

In the real world there has to be a constant conversation between the client and the server application. I am going to use e-mail as an example. If you have a traditional POP3 e-mail client (one you get through your ISP or Web hosting company) you have to actively connect to it and check if you have e-mail. You can do this by either hitting the “send and receive” button or setting your e-mail recheck setting to an arbitrary time. A more efficient model would have your email client only check and download email when it's available. Enterprise e-mail systems, such as Microsoft Exchange, leverage both the e-mail client on your desktop and the e-mail server. When email arrives at the server a message is sent to the e-mail client that a new message has been delivered. The server, in this case, pushed data out to the client.

Other examples can include realtime data transfer for information such as stock numbers, sales figures, and supply chain management. These areas require a tool that can push data out to a client.

Using XMLSocket Connections
As part of the evolution of XML, a subset of XML technology called XMLSockets has emerged. XMLSockets are great for solutions such as e-mail systems, systems, chat rooms, and low-latency applications in which the user opens up a connection with the server and requires that the server and the client pass data back and forth. The standard passes full and complete XML documents. The impact enables applications to “chat” over network lines.

You can dig through the specifications of HTML and you will not find support for XMLSockets. But XMLSocket connection has been a standard feature within Flash since version 5, with only one security feature to worry about (see sidebar).

Through ActionScript you can connect to an XMLSocket server, send and receive XML-formatted data, and close connections to the server.

XMLSocket Server
For XMLSocket information to be passed back and forth between a client and server you need the client (which you know is Flash) and the server.

Although XMLSocket technology is relatively new, there are a number of notable XMLSocket servers on the market.

• UNITY 2.0: (www.moock.org/unity). Colin Moock, esteemed author of the ActionScript Definitive Guide series, has created an easy-to-use and powerful Java-based XMLSocket server. What I like is that you can download a free developer's version of the server before paying up for the full version. You can also buy a sample application that has over 10,000 lines of ActionScript 2.0 code.
• Shovemedia: (www.shovemedia.com). Shovemedia is another Java-based socket server. The Pong game in this article uses this server for multiple players to connect to.
• Swocket: (http://swocket.sourceforge.net). Swocket is an open source socket server built in Python.

You can find an even more comprehensive list of XMLSocket servers on the XML/XMLSocket server list over at the Flash Wiki (http://chattyfig.figleaf. com/flashcoderswiki/index.php?XML%2F XMLSocket%20Servers).

XMLSocket Class in Flash
Not a lot has changed since Flash 5 with the use and functionality of the XMLSocket class. As with other objects in Flash, before you can begin using the object you must set a variable that declares the new object. The following will declare the variable mySocket as a new XMLSocket class:

mySocket = new XMLSocket();

Once you have declared you are using an XMLSocket you can connect, send, and close the XMLSocket class. Table I shows methods and their descriptions.

There are also event handlers you can use to push and pull data to and from an XMLSocket server. These include:

• onConnect
• onClose
• onData
• onXML

Creating a Pong Game with an XMLSocket Server
One of the tried-and-true examples out there using XMLSocket servers is Pong. Pong is a simple game where two or more people connect in a game room and play against each other.

You can download the complete Flash source code to run this game from www.sys-con.com/mx/sourcec.cfm. You will need an XMLSocket server to run the program. The source code is Shovemedia's Pong but you can use any XMLSocket server, such as Unity.

I won't go through the mechanics of the game in detail - there are about 500 lines of code. What I want to direct your attention to is the connection to the XMLSocket server and how it is used to create XML data to pass the players' moves back and forth to each other over the Internet.

Download the files for the Pong Game. You will want to open pongGame.as. Go to line 287. You will see the the code shown in Code 1.

Essentially, you have two functions. The first connects to a server through an open port. The second function prints to the Flash Player whether a connection has been successfully made.

Summary
Pushing data to and from a Flash movie is essential for games, presentations, and Rich Internet Applications. XMLSocket connection gives you an alternative to competing solutions such as XUpdate, Flash Remoting, and traditional GET/PUT HTTP protocols.